Linux ( ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution (distro), which includes the kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses and recommends the name "GNU/Linux" to emphasize the use and importance of GNU software in many distributions, causing some controversy.

Popular Linux distributions include Debian, Fedora Linux, Arch Linux and Ubuntu. Commercial distributions include Red Hat Enterprise Linux and SUSE Linux Enterprise. Desktop Linux distributions include a windowing system such as X11 or Wayland, and a desktop environment such as GNOME or KDE Plasma. Distributions intended for servers may omit graphics altogether, or include a solution stack such as LAMP. Because Linux is freely redistributable, anyone may create a distribution for any purpose.

Linux was originally developed for personal computers based on the Intel x86 architecture, but has since been ported to more platforms than any other operating system. Because of the dominance of the Linux-based Android on smartphones, Linux, including Android, has the largest installed base of all general-purpose operating systems, . Although Linux is, , used by only around 2.6 percent of desktop computers, the Chromebook, which runs the Linux kernel-based ChromeOS, dominates the US K–12 education market and represents nearly 20 percent of sub-$300 notebook sales in the US. Linux is the leading operating system on servers (over 96.4% of the top 1 million web servers' operating systems are Linux), leads other big iron systems such as mainframe computers, and is used on all of the world's 500 fastest supercomputers (, having gradually displaced all competitors).

Linux also runs on embedded systems, i.e. devices whose operating system is typically built into the firmware and is highly tailored to the system. This includes routers, automation controls, smart home devices, video game consoles, televisions (Samsung and LG Smart TVs), automobiles (Tesla, Audi, Mercedes-Benz, Hyundai and Toyota), and spacecraft (Falcon 9 rocket, Dragon crew capsule and the Perseverance rover).

Linux is one of the most prominent examples of free and open-source software collaboration. The source code may be used, modified and distributed commercially or non-commercially by anyone under the terms of its respective licenses, such as the GNU General Public License (GPL). The Linux kernel, for example, is licensed under the GPLv2, with an exception for system calls that allows code that calls the kernel via system calls not to be licensed under the GPL.

History

Precursors

The Unix operating system was conceived and implemented in 1969, at AT&T's Bell Labs, in the United States by Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna. First released in 1971, Unix was written entirely in assembly language, as was common practice at the time. In 1973, in a key pioneering approach, it was rewritten in the C programming language by Dennis Ritchie (with the exception of some hardware and I/O routines). The availability of a high-level language implementation of Unix made its porting to different computer platforms easier.

Due to an earlier antitrust case forbidding it from entering the computer business, AT&T licensed the operating system's source code as a trade secret to anyone who asked. As a result, Unix grew quickly and became widely adopted by academic institutions and businesses. In 1984, AT&T divested itself of its regional operating companies, and was released from its obligation not to enter the computer business; freed of that obligation, Bell Labs began selling Unix as a proprietary product, where users were not legally allowed to modify it.

Onyx Systems began selling early microcomputer-based Unix workstations in 1980. Later, Sun Microsystems, founded as a spin-off of a student project at Stanford University, also began selling Unix-based desktop workstations in 1982. While Sun workstations did not utilize commodity PC hardware, for which Linux was later originally developed, it represented the first successful commercial attempt at distributing a primarily single-user microcomputer that ran a Unix operating system.

With Unix increasingly "locked in" as a proprietary product, the GNU Project, started in 1983 by Richard Stallman, had the goal of creating a "complete Unix-compatible software system" composed entirely of free software. Work began in 1984. Later, in 1985, Stallman started the Free Software Foundation and wrote the GNU General Public License (GNU GPL) in 1989. By the early 1990s, many of the programs required in an operating system (such as libraries, compilers, text editors, a command-line shell, and a windowing system) were completed, although low-level elements such as device drivers, daemons, and the kernel, called GNU Hurd, were stalled and incomplete.

MINIX was created by Andrew S. Tanenbaum, a computer science professor, and released in 1987 as a minimal Unix-like operating system targeted at students and others who wanted to learn operating system principles. Although the complete source code of MINIX was freely available, the licensing terms prevented it from being free software until the licensing changed in April 2000.

Although not released until 1992, due to legal complications, development of 386BSD, from which NetBSD, OpenBSD and FreeBSD descended, predated that of Linux.

Linus Torvalds has stated on separate occasions that if the GNU kernel or 386BSD had been available at the time (1991), he probably would not have created Linux.

Creation
While attending the University of Helsinki in the fall of 1990, Torvalds enrolled in a Unix course. The course utilized a MicroVAX minicomputer running Ultrix, and one of the required texts was Operating Systems: Design and Implementation by Andrew S. Tanenbaum. This textbook included a copy of Tanenbaum's MINIX operating system. It was with this course that Torvalds first became exposed to Unix. In 1991, he became curious about operating systems. Frustrated by the licensing of MINIX, which at the time limited it to educational use only, he began to work on his own operating system kernel, which eventually became the Linux kernel.

On July 3, 1991, in an effort to implement Unix system calls, Linus Torvalds attempted unsuccessfully to obtain a digital copy of the POSIX standards documentation with a request to the comp.os.minix newsgroup. After not finding the POSIX documentation, Torvalds initially resorted to determining system calls from SunOS documentation owned by the university for use in operating its Sun Microsystems server. He also learned some system calls from Tanenbaum's MINIX text.

Torvalds began the development of the Linux kernel on MINIX and applications written for MINIX were also used on Linux. Later, Linux matured and further Linux kernel development took place on Linux systems. GNU applications also replaced all MINIX components, because it was advantageous to use the freely available code from the GNU Project with the fledgling operating system; code licensed under the GNU GPL can be reused in other computer programs as long as they also are released under the same or a compatible license. Torvalds initiated a switch from his original license, which prohibited commercial redistribution, to the GNU GPL. Developers worked to integrate GNU components with the Linux kernel, creating a fully functional and free operating system.

Naming

Linus Torvalds had wanted to call his invention "Freax", a portmanteau of "free", "freak", and "x" (as an allusion to Unix). During the start of his work on the system, some of the project's makefiles included the name "Freax" for about half a year. Initially, Torvalds considered the name "Linux" but dismissed it as too egotistical.

To facilitate development, the files were uploaded to the FTP server (ftp.funet.fi) of FUNET in September 1991. Ari Lemmke, Torvalds' coworker at the Helsinki University of Technology (HUT) who was one of the volunteer administrators for the FTP server at the time, did not think that "Freax" was a good name, so he named the project "Linux" on the server without consulting Torvalds. Later, however, Torvalds consented to "Linux".

According to a newsgroup post by Torvalds, the word "Linux" should be pronounced ( ) with a short 'i' as in 'print' and 'u' as in 'put'. To further demonstrate how the word "Linux" should be pronounced, he included an audio guide with the kernel source code. However, in this recording, he pronounces Linux as  () with a short but close front unrounded vowel, instead of a near-close near-front unrounded vowel as in his newsgroup post.

Commercial and popular uptake

Adoption of Linux in production environments, rather than being used only by hobbyists, started to take off first in the mid-1990s in the supercomputing community, where organizations such as NASA started to replace their increasingly expensive machines with clusters of inexpensive commodity computers running Linux. Commercial use began when Dell and IBM, followed by Hewlett-Packard, started offering Linux support to escape Microsoft's monopoly in the desktop operating system market.

Today, Linux systems are used throughout computing, from embedded systems to virtually all supercomputers, and have secured a place in server installations such as the popular LAMP application stack. Use of Linux distributions in home and enterprise desktops has been growing. Linux distributions have also become popular in the netbook market, with many devices shipping with customized Linux distributions installed, and Google releasing their own ChromeOS designed for netbooks.

Linux's greatest success in the consumer market is perhaps the mobile device market, with Android being the dominant operating system on smartphones and very popular on tablets and, more recently, on wearables. Linux gaming is also on the rise with Valve showing its support for Linux and rolling out SteamOS, its own gaming-oriented Linux distribution, which was later implemented in their Steam Deck platform. Linux distributions have also gained popularity with various local and national governments, such as the federal government of Brazil.

Current development

Linus Torvalds is the lead maintainer for the Linux kernel and guides its development, while Greg Kroah-Hartman is the lead maintainer for the stable branch. William John Sullivan is the executive director of the Free Software Foundation, which in turn supports the GNU components. Finally, individuals and corporations develop third-party non-GNU components. These third-party components comprise a vast body of work and may include both kernel modules and user applications and libraries.

Linux vendors and communities combine and distribute the kernel, GNU components, and non-GNU components, with additional package management software in the form of Linux distributions.

Design

Many open source developers agree that the Linux kernel was not designed but rather evolved through natural selection. Torvalds considers that although the design of Unix served as a scaffolding, "Linux grew with a lot of mutations – and because the mutations were less than random, they were faster and more directed than alpha-particles in DNA." Eric S. Raymond considers Linux's revolutionary aspects to be social, not technical: before Linux, complex software was designed carefully by small groups, but "Linux evolved in a completely different way. From nearly the beginning, it was rather casually hacked on by huge numbers of volunteers coordinating only through the Internet. Quality was maintained not by rigid standards or autocracy but by the naively simple strategy of releasing every week and getting feedback from hundreds of users within days, creating a sort of rapid Darwinian selection on the mutations introduced by developers." Bryan Cantrill, an engineer of a competing OS, agrees that "Linux wasn't designed, it evolved", but considers this to be a limitation, proposing that some features, especially those related to security, cannot be evolved into, "this is not a biological system at the end of the day, it's a software system."

A Linux-based system is a modular Unix-like operating system, deriving much of its basic design from principles established in Unix during the 1970s and 1980s. Such a system uses a monolithic kernel, the Linux kernel, which handles process control, networking, access to the peripherals, and file systems. Device drivers are either integrated directly with the kernel, or added as modules that are loaded while the system is running.

The GNU userland is a key part of most systems based on the Linux kernel, with Android being the notable exception. The GNU C library, an implementation of the C standard library, works as a wrapper for the system calls of the Linux kernel necessary to the kernel-userspace interface, the toolchain is a broad collection of programming tools vital to Linux development (including the compilers used to build the Linux kernel itself), and the coreutils implement many basic Unix tools. The GNU Project also develops Bash, a popular CLI shell. The graphical user interface (or GUI) used by most Linux systems is built on top of an implementation of the X Window System. More recently, the Linux community seeks to advance to Wayland as the new display server protocol in place of X11. Many other open-source software projects contribute to Linux systems.

Installed components of a Linux system include the following:
 A bootloader, for example GNU GRUB, LILO, SYSLINUX or systemd-boot. This is a program that loads the Linux kernel into the computer's main memory, by being executed by the computer when it is turned on and after the firmware initialization is performed.
 An init program, such as the traditional sysvinit and the newer systemd, OpenRC and Upstart. This is the first process launched by the Linux kernel, and is at the root of the process tree. It starts processes such as system services and login prompts (whether graphical or in terminal mode).
 Software libraries, which contain code that can be used by running processes. On Linux systems using ELF-format executable files, the dynamic linker that manages the use of dynamic libraries is known as ld-linux.so. If the system is set up for the user to compile software themselves, header files will also be included to describe the programming interface of installed libraries. Besides the most commonly used software library on Linux systems, the GNU C Library (glibc), there are numerous other libraries, such as SDL and Mesa.
 The C standard library is the library necessary to run programs written in C on a computer system, with the GNU C Library being the standard. It provides an implementation of the POSIX API, as well as extensions to that API. For embedded systems, alternatives such as musl, EGLIBC (a glibc fork once used by Debian) and uClibc (which was designed for uClinux) have been developed, although the last two are no longer maintained. Android uses its own C library, Bionic. However, musl can additionally be used as a replacement for glibc on desktop and laptop systems, as seen on certain Linux distributions like Void Linux.
 Basic Unix commands, with GNU coreutils being the standard implementation. Alternatives exist for embedded systems, such as the copyleft BusyBox, and the BSD-licensed Toybox.
 Widget toolkits are the libraries used to build graphical user interfaces (GUIs) for software applications. Numerous widget toolkits are available, including GTK and Clutter developed by the GNOME Project, Qt developed by the Qt Project and led by The Qt Company, and Enlightenment Foundation Libraries (EFL) developed primarily by the Enlightenment team.
 A package management system, such as dpkg and RPM. Alternatively packages can be compiled from binary or source tarballs.
 User interface programs such as command shells or windowing environments.

User interface
The user interface, also known as the shell, is either a command-line interface (CLI), a graphical user interface (GUI), or controls attached to the associated hardware, which is common for embedded systems. For desktop systems, the default user interface is usually graphical, although the CLI is commonly available through terminal emulator windows or on a separate virtual console.

CLI shells are text-based user interfaces, which use text for both input and output. The dominant shell used in Linux is the Bourne-Again Shell (bash), originally developed for the GNU Project. Most low-level Linux components, including various parts of the userland, use the CLI exclusively. The CLI is particularly suited for automation of repetitive or delayed tasks and provides very simple inter-process communication.

On desktop systems, the most popular user interfaces are the GUI shells, packaged together with extensive desktop environments, such as KDE Plasma, GNOME, MATE, Cinnamon, LXDE, Pantheon and Xfce, though a variety of additional user interfaces exist. Most popular user interfaces are based on the X Window System, often simply called "X". It provides network transparency and permits a graphical application running on one system to be displayed on another where a user may interact with the application; however, certain extensions of the X Window System are not capable of working over the network. Several X display servers exist, with the reference implementation, X.Org Server, being the most popular.

Server distributions might provide a command-line interface for developers and administrators, but provide a custom interface towards end-users, designed for the use-case of the system. This custom interface is accessed through a client that resides on another system, not necessarily Linux based.

Several types of window managers exist for X11, including tiling, dynamic, stacking and compositing. Window managers provide means to control the placement and appearance of individual application windows, and interact with the X Window System. Simpler X window managers such as dwm, ratpoison, i3wm, or herbstluftwm provide a minimalist functionality, while more elaborate window managers such as FVWM, Enlightenment or Window Maker provide more features such as a built-in taskbar and themes, but are still lightweight when compared to desktop environments. Desktop environments include window managers as part of their standard installations, such as Mutter (GNOME), KWin (KDE) or Xfwm (xfce), although users may choose to use a different window manager if preferred.

Wayland is a display server protocol intended as a replacement for the X11 protocol; , it has received relatively wide adoption. Unlike X11, Wayland does not need an external window manager and compositing manager. Therefore, a Wayland compositor takes the role of the display server, window manager and compositing manager. Weston is the reference implementation of Wayland, while GNOME's Mutter and KDE's KWin are being ported to Wayland as standalone display servers. Enlightenment has already been successfully ported since version 19.

Video input infrastructure

Linux currently has two modern kernel-userspace APIs for handling video input devices: V4L2 API for video streams and radio, and DVB API for digital TV reception.

Due to the complexity and diversity of different devices, and due to the large number of formats and standards handled by those APIs, this infrastructure needs to evolve to better fit other devices. Also, a good userspace device library is the key of the success for having userspace applications to be able to work with all formats supported by those devices.

Development

The primary difference between Linux and many other popular contemporary operating systems is that the Linux kernel and other components are free and open-source software. Linux is not the only such operating system, although it is by far the most widely used. Some free and open-source software licenses are based on the principle of copyleft, a kind of reciprocity: any work derived from a copyleft piece of software must also be copyleft itself. The most common free software license, the GNU General Public License (GPL), is a form of copyleft, and is used for the Linux kernel and many of the components from the GNU Project.

Linux-based distributions are intended by developers for interoperability with other operating systems and established computing standards. Linux systems adhere to POSIX, SUS, LSB, ISO, and ANSI standards where possible, although to date only one Linux distribution has been POSIX.1 certified, Linux-FT.

Free software projects, although developed through collaboration, are often produced independently of each other. The fact that the software licenses explicitly permit redistribution, however, provides a basis for larger-scale projects that collect the software produced by stand-alone projects and make it available all at once in the form of a Linux distribution.

Many Linux distributions manage a remote collection of system software and application software packages available for download and installation through a network connection. This allows users to adapt the operating system to their specific needs. Distributions are maintained by individuals, loose-knit teams, volunteer organizations, and commercial entities. A distribution is responsible for the default configuration of the installed Linux kernel, general system security, and more generally integration of the different software packages into a coherent whole. Distributions typically use a package manager such as apt, yum, zypper, pacman or portage to install, remove, and update all of a system's software from one central location.

Community

A distribution is largely driven by its developer and user communities. Some vendors develop and fund their distributions on a volunteer basis, Debian being a well-known example. Others maintain a community version of their commercial distributions, as Red Hat does with Fedora, and SUSE does with openSUSE.

In many cities and regions, local associations known as Linux User Groups (LUGs) seek to promote their preferred distribution and by extension free software. They hold meetings and provide free demonstrations, training, technical support, and operating system installation to new users. Many Internet communities also provide support to Linux users and developers. Most distributions and free software / open-source projects have IRC chatrooms or newsgroups. Online forums are another means for support, with notable examples being LinuxQuestions.org and the various distribution specific support and community forums, such as ones for Ubuntu, Fedora, and Gentoo. Linux distributions host mailing lists; commonly there will be a specific topic such as usage or development for a given list.

There are several technology websites with a Linux focus. Print magazines on Linux often bundle cover disks that carry software or even complete Linux distributions.

Although Linux distributions are generally available without charge, several large corporations sell, support, and contribute to the development of the components of the system and of free software. An analysis of the Linux kernel in 2017 showed that well over 85% of the code developed by programmers who are being paid for their work, leaving about 8.2% to unpaid developers and 4.1% unclassified. Some of the major corporations that provide contributions include Intel, Samsung, Google, AMD, Oracle and Facebook. A number of corporations, notably Red Hat, Canonical and SUSE, have built a significant business around Linux distributions.

The free software licenses, on which the various software packages of a distribution built on the Linux kernel are based, explicitly accommodate and encourage commercialization; the relationship between a Linux distribution as a whole and individual vendors may be seen as symbiotic. One common business model of commercial suppliers is charging for support, especially for business users. A number of companies also offer a specialized business version of their distribution, which adds proprietary support packages and tools to administer higher numbers of installations or to simplify administrative tasks.

Another business model is to give away the software to sell hardware. This used to be the norm in the computer industry, with operating systems such as CP/M, Apple DOS and versions of the classic Mac OS prior to 7.6 freely copyable (but not modifiable). As computer hardware standardized throughout the 1980s, it became more difficult for hardware manufacturers to profit from this tactic, as the OS would run on any manufacturer's computer that shared the same architecture.

Programming on Linux
Most programming languages support Linux either directly or through third-party community based ports. The original development tools used for building both Linux applications and operating system programs are found within the GNU toolchain, which includes the GNU Compiler Collection (GCC) and the GNU Build System. Amongst others, GCC provides compilers for Ada, C, C++, Go and Fortran. Many programming languages have a cross-platform reference implementation that supports Linux, for example PHP, Perl, Ruby, Python, Java, Go, Rust and Haskell. First released in 2003, the LLVM project provides an alternative cross-platform open-source compiler for many languages. Proprietary compilers for Linux include the Intel C++ Compiler, Sun Studio, and IBM XL C/C++ Compiler. BASIC is available in procedural form from QB64, PureBasic, Yabasic, GLBasic, Basic4GL, XBasic, wxBasic, SdlBasic, and Basic-256, as well as object oriented through Gambas, FreeBASIC, B4X, Basic for Qt, Phoenix Object Basic, NS Basic, ProvideX, Chipmunk Basic, RapidQ and Xojo. Pascal is implemented through GNU Pascal, Free Pascal, and Virtual Pascal, as well as graphically via Lazarus, PascalABC.NET, or Delphi using FireMonkey (previously through Borland Kylix).

A common feature of Unix-like systems, Linux includes traditional specific-purpose programming languages targeted at scripting, text processing and system configuration and management in general. Linux distributions support shell scripts, awk, sed and make. Many programs also have an embedded programming language to support configuring or programming themselves. For example, regular expressions are supported in programs like grep and locate, the traditional Unix message transfer agent Sendmail contains its own Turing complete scripting system, and the advanced text editor GNU Emacs is built around a general purpose Lisp interpreter.

Most distributions also include support for PHP, Perl, Ruby, Python and other dynamic languages. While not as common, Linux also supports C# and other CLI languages (via Mono), Vala, and Scheme. Guile Scheme acts as an extension language targeting the GNU system utilities, seeking to make the conventionally small, static, compiled C programs of Unix design rapidly and dynamically extensible via an elegant, functional high-level scripting system; many GNU programs can be compiled with optional Guile bindings to this end. A number of Java virtual machines and development kits run on Linux, including the original Sun Microsystems JVM (HotSpot), and IBM's J2SE RE, as well as many open-source projects like Kaffe and Jikes RVM; Kotlin, Scala, Groovy and other JVM languages are also available.

GNOME and KDE are popular desktop environments and provide a framework for developing applications. These projects are based on the GTK and Qt widget toolkits, respectively, which can also be used independently of the larger framework. Both support a wide variety of languages. There are a number of Integrated development environments available including Anjuta, Code::Blocks, CodeLite, Eclipse, Geany, ActiveState Komodo, KDevelop, Lazarus, MonoDevelop, NetBeans, and Qt Creator, while the long-established editors Vim, nano and Emacs remain popular.

Hardware support

The Linux kernel is a widely ported operating system kernel, available for devices ranging from mobile phones to supercomputers; it runs on a highly diverse range of computer architectures, including ARM-based Android smartphones and the IBM Z mainframes. Specialized distributions and kernel forks exist for less mainstream architectures; for example, the ELKS kernel fork can run on Intel 8086 or Intel 80286 16-bit microprocessors, while the µClinux kernel fork may run on systems without a memory management unit. The kernel also runs on architectures that were only ever intended to use a manufacturer-created operating system, such as Macintosh computers (with PowerPC, Intel, and Apple silicon processors), PDAs, video game consoles, portable music players, and mobile phones.

Linux has a reputation of supporting old hardware very well by maintaining standardized drivers for a long time. There are several industry associations and hardware conferences devoted to maintaining and improving support for diverse hardware under Linux, such as FreedomHEC. Over time, support for different hardware has improved in Linux, resulting in any off-the-shelf purchase having a "good chance" of being compatible.

In 2014, a new initiative was launched to automatically collect a database of all tested hardware configurations.

Uses

Market share and uptake

Many quantitative studies of free/open-source software focus on topics including market share and reliability, with numerous studies specifically examining Linux. The Linux market is growing, and the Linux operating system market size is expected to see a growth of 19.2% by 2027, reaching $15.64 billion, compared to $3.89 billion in 2019. Analysts and proponents attribute the relative success of Linux to its security, reliability, low cost, and freedom from vendor lock-in.

 Desktops and laptops
 According to web server statistics (that is, based on the numbers recorded from visits to websites by client devices), , the estimated market share of Linux on desktop computers is around 2.5%. In comparison, Microsoft Windows has a market share of around 75.5%, while macOS covers around 14.9%.

 Web servers

 W3Cook publishes stats that use the top 1,000,000 Alexa domains, which  estimate that 96.55% of web servers run Linux, 1.73% run Windows, and 1.72% run FreeBSD.

W3Techs publishes stats that use the top 10,000,000 Alexa domains and the top 1,000,000 Tranco domains, updated monthly and  estimate that Linux is used by 39% of the web servers, versus 21.9% being used by Microsoft Windows. 40.1% used other types of Unix.

IDC's Q1 2007 report indicated that Linux held 12.7% of the overall server market at that time; this estimate was based on the number of Linux servers sold by various companies, and did not include server hardware purchased separately that had Linux installed on it later.

 Mobile devices
 Android, which is based on the Linux kernel, has become the dominant operating system for smartphones. In April 2023, 68.61% of mobile devices accessing websites using StatCounter were from Android. Android is also a popular operating system for tablets, being responsible for more than 60% of tablet sales . According to web server statistics,  Android has a market share of about 71%, with iOS holding 28%, and the remaining 1% attributed to various niche platforms.

 Film production
 For years, Linux has been the platform of choice in the film industry. The first major film produced on Linux servers was 1997's Titanic. Since then major studios including DreamWorks Animation, Pixar, Weta Digital, and Industrial Light & Magic have migrated to Linux. According to the Linux Movies Group, more than 95% of the servers and desktops at large animation and visual effects companies use Linux.

 Use in government
 Linux distributions have also gained popularity with various local and national governments. News of the Russian military creating its own Linux distribution has also surfaced, and has come to fruition as the G.H.ost Project. The Indian state of Kerala has gone to the extent of mandating that all state high schools run Linux on their computers. China uses Linux exclusively as the operating system for its Loongson processor family to achieve technology independence. In Spain, some regions have developed their own Linux distributions, which are widely used in education and official institutions, like gnuLinEx in Extremadura and Guadalinex in Andalusia. France and Germany have also taken steps toward the adoption of Linux. North Korea's Red Star OS, developed , is based on a version of Fedora Linux.

Copyright, trademark, and naming

The Linux kernel is licensed under the GNU General Public License (GPL), version 2. The GPL requires that anyone who distributes software based on source code under this license must make the originating source code (and any modifications) available to the recipient under the same terms. Other key components of a typical Linux distribution are also mainly licensed under the GPL, but they may use other licenses; many libraries use the GNU Lesser General Public License (LGPL), a more permissive variant of the GPL, and the X.Org implementation of the X Window System uses the MIT License.

Torvalds states that the Linux kernel will not move from version 2 of the GPL to version 3. He specifically dislikes some provisions in the new license which prohibit the use of the software in digital rights management. It would also be impractical to obtain permission from all the copyright holders, who number in the thousands.

A 2001 study of Red Hat Linux 7.1 found that this distribution contained 30 million source lines of code. Using the Constructive Cost Model, the study estimated that this distribution required about eight thousand person-years of development time. According to the study, if all this software had been developed by conventional proprietary means, it would have cost about  to develop in  in the United States. Most of the source code (71%) was written in the C programming language, but many other languages were used, including C++, Lisp, assembly language, Perl, Python, Fortran, and various shell scripting languages. Slightly over half of all lines of code were licensed under the GPL. The Linux kernel itself was 2.4 million lines of code, or 8% of the total.

In a later study, the same analysis was performed for Debian version 4.0 (etch, which was released in 2007). This distribution contained close to 283 million source lines of code, and the study estimated that it would have required about seventy three thousand man-years and cost  (in  dollars) to develop by conventional means.

In the United States, the name Linux is a trademark registered to Linus Torvalds. Initially, nobody registered it. However, on August 15, 1994, William R. Della Croce, Jr. filed for the trademark Linux, and then demanded royalties from Linux distributors. In 1996, Torvalds and some affected organizations sued him to have the trademark assigned to Torvalds, and, in 1997, the case was settled. The licensing of the trademark has since been handled by the Linux Mark Institute (LMI). Torvalds has stated that he trademarked the name only to prevent someone else from using it. LMI originally charged a nominal sublicensing fee for use of the Linux name as part of trademarks, but later changed this in favor of offering a free, perpetual worldwide sublicense.

The Free Software Foundation (FSF) prefers GNU/Linux as the name when referring to the operating system as a whole, because it considers Linux distributions to be variants of the GNU operating system initiated in 1983 by Richard Stallman, president of the FSF. They explicitly take no issue over the name Android for the Android OS, which is also an operating system based on the Linux kernel, as GNU is not a part of it.

A minority of public figures and software projects other than Stallman and the FSF, notably Debian (which had been sponsored by the FSF up to 1996), also use GNU/Linux when referring to the operating system as a whole. Most media and common usage, however, refers to this family of operating systems simply as Linux, as do many large Linux distributions (for example, SUSE Linux and Red Hat Enterprise Linux). By contrast, Linux distributions containing only free software use "GNU/Linux" or simply "GNU", such as Trisquel GNU/Linux, Parabola GNU/Linux-libre, BLAG Linux and GNU, and gNewSense.

, about 8% to 13% of the lines of code of the Linux distribution Ubuntu (version "Natty") is made of GNU components (the range depending on whether GNOME is considered part of GNU); meanwhile, 6% is taken by the Linux kernel, increased to 9% when including its direct dependencies.

See also

 Comparison of Linux distributions
 Comparison of open-source and closed-source software
 Comparison of operating systems
 Comparison of X Window System desktop environments
 Criticism of Linux
 Linux kernel version history
 Linux Documentation Project
 Linux From Scratch
 Linux Software Map
 List of Linux distributions
 List of games released on Linux
 List of operating systems
 Loadable kernel module
 Usage share of operating systems
 Timeline of operating systems

Notes

References

External links

 Graphical map of Linux Internals (archived)
 Linux kernel website and archives
 The History of Linux in GIT Repository Format 1992–2010 (archived)

1991 software
Computing platforms
Cross-platform software
Finnish inventions
Free software programmed in C
Linus Torvalds
Operating systems
Unix variants

Linux


Fernand Gustave Halphen (18 February 1872 – 16 May 1917) was a French Jewish composer.

Life and career
Fernand Halphen was the son of Georges Halphen, a diamond merchant, and of Henriette Antonia Stern (1836–1905), who was from the Stern banking family. From the age of ten, he studied under the direction of Gabriel Fauré before entering the Paris Conservatory where he took a composition course taught by Ernest Guiraud, who also taught Paul Dukas, Claude Debussy and Erik Satie. After Guiraud's death in 1892, Halphen studied with Jules Massenet, who also taught Henri Rabaud, Florent Schmitt, Charles Koechlin and Reynaldo Hahn. He won first prize for his fugue in 1895, and the next year won second place for the second Grand Prix de Rome with his cantata Mélusine, behind Jules Mouquet and Richard d'Ivry.

Fernand Halphen is known principally as a composer. Among his notable works are the one-act opera Le Cor Fleuri (libretto by Ephraïm Mikhael and André-Ferdinand Hérold), which debuted in the national theatre Opéra-Comique, 10 May 1904, several symphonies, one of which was performed in Paris and in Monte Carlo, a suite for orchestra, the pantomime Hagoseida, the ballet Le Réveil du faune, some chamber music such as a sonata for violin and piano, works for organ as well as songs.

Captain of the thirteenth territorial infantry regiment during World War I, Halphen was killed in action on 16 May 1917.

Personal life
On 15 February 1899, Fernand Halphen married  (1878–1963). She also assembled an important collection of paintings including the works of Monet, Pissarro and of Henri Rousseau, as well as the portrait of Fernand Halphen which was painted by Renoir in 1880.

The couple had one daughter, Henriette, born on 26 February 1911, and one son, Georges, born 9 March 1913. In 1995, Georges Halphen offered the portrait of his father, painted by Renoir, to the Musée d'Orsay.  It was to offer his wife a view which enchanted her, he said, that Fernand Halphen bought the house at la Chapelle-en-Serval, near Chantilly (Oise) and decided in 1908 to erect a country house in a wooded valley there: the château Mont-Royal.

His niece, Germaine (1884-1975) married Baron Édouard de Rothschild of the prominent Rothschild banking family of France.

Halphen Foundation
Alice Halphen created the Halphen Foundation (Fondation Halphen) whose purpose was to help students of composition in the Conservatory to publish and perform their works.

The Foundation also created social housing on the Ile St. Louis in Paris, taking advantage of a controversial scheme to demolish one side of an original, seventeenth-century street. 10-12 rue des Deux-Ponts housed around 50 rent-controlled apartments in two blocks dating from 1926 and 1930 and aimed at large families. In the round-ups of Jews at the end of September 1942, all 112 tenants, among them 40 young children, were deported to Auschwitz-Birkenau.

By 2003, the apartment block had deteriorated but still housed low-income tenants, some of whom had lived there for decades, in what is France's single most expensive district for real estate. By 2004, the tenants had all left and developers began a major overhaul, marketing "luxurious, prestigious" one- to four-bedroomed air-conditioned apartments. The first act of the builders renovating the building was to chisel off the coloured mosaic plaque above the main entrance, bearing the words "Fondation Fernand Halphen 1926". No trace of the Foundation's presence remains. In August 2006, a one-bedroom apartment in the building was on sale for 600,000 euros, 39.7 times the annual gross French minimum wage.

Works, editions, recordings
His chief works are:
 a Sicilian, a suite for orchestra, 1896
 a symphony, Monte Carlo, 1897
 a sonata for piano and violin, 1899
 "Le Cor Fleuri", lyric opera in one act, based on the play by the late Ephraim Micaël
He also composed several songs, and pieces for the piano, violin, horn, etc

 Mélodies, pieces for piano & chamber music (2CD):  Jeff Cohen (piano), Alexis Galpérine (violin), François Le Roux (baritone), Jean McManama (horn), Clara Novakova (flute), Sonia Wieder-Atherton (cello). Patrimoines musicaux des juifs de France, vol. 5 (2006). Orphée d'or 2007 of the Académie du Disque Lyrique, and Prix Gabriel Fauré 2007.

See also
 Halphen

References

External links

1872 births
1917 deaths
19th-century classical composers
19th-century French musicians
19th-century French male musicians
20th-century classical composers
Conservatoire de Paris alumni
19th-century French Jews
French male classical composers
French military personnel killed in World War I
French opera composers
Male opera composers
Musicians from Paris
Prix de Rome for composition
Pupils of Ernest Guiraud
Pupils of Jules Massenet
Pupils of Gabriel Fauré
Stern family (banking)

Fernand Halphen


The McAllen–Hidalgo–Reynosa International Bridge is a road bridge completed in 1926, crossing the Rio Grande between the state of Tamaulipas in northeastern Mexico and the state of Texas in the southwestern United States.

Route
The Hidalgo Texas Port of Entry is associated with the bridge that connects the cities of Hidalgo and McAllen in Texas to the city of Reynosa in Tamaulipas.

The road bridge has two spans, north and south bound, of four lanes each. This bridge has a SENTRI Lane.

Jurisdictions
The American portion of the bridge is within Hidalgo, but is operated-maintained by the larger city of McAllen to the north. It is one of the largest operations undertaken by McAllen to date.

Crossing the bridge north into Texas can take up to 1-2 hours at the Hidalgo Texas Port of Entry, as the U.S. border agents stop all vehicles and the lines can be very long. Recently a Secure Electronic Network for Travelers Rapid Inspection−SENTRI lane was designated, which for an extra yearly fee shortens the U.S. entry time from Mexico.

Crossing the bridge south into Tamaulipas is generally quicker, as the Mexican border agents usually check few incoming vehicles.

Since 1996, all northbound trucks are required to use the Pharr–Reynosa International Bridge to enter the U.S.

Border crossing

The Hidalgo Texas Port of Entry is located at the northern end of the McAllen–Hidalgo–Reynosa International Bridge, in the city of Hidalgo. The northbound traffic is coming from the city of Reynosa.  Since 1996, all northbound trucks are required to use the Pharr–Reynosa International Bridge to enter the U.S. from Reynosa.

See also
 Anzalduas International Bridge — next Reynosa + McAllen road bridge upriver-west.
 Pharr-Reynosa International Bridge — 2nd downriver between the cities, for trucks.

References

International bridges in Texas
International bridges in Tamaulipas
Road bridges in Texas
Toll bridges in Mexico
Toll bridges in Texas
Bridges completed in 1926
Reynosa
Buildings and structures in Hidalgo County, Texas
Transportation in McAllen, Texas
Transportation in Hidalgo County, Texas
1926 establishments in Texas
1926 establishments in Mexico
20th-century architecture in Mexico
1920s architecture in the United States


A solar tracker is a device that orients a payload toward the Sun. Payloads are usually solar panels, parabolic troughs, Fresnel reflectors, lenses, or the mirrors of a heliostat.

For flat-panel photovoltaic systems, trackers are used to minimize the angle of incidence between the incoming sunlight and a photovoltaic panel, sometimes known as the cosine error. Reducing this angle increases the amount of energy produced from a fixed amount of installed power-generating capacity. In standard photovoltaic applications, it was predicted in 2008–2009 that trackers could be used in at least 85% of commercial installations greater than one megawatt from 2009 to 2012.

As the pricing, reliability, and performance of single-axis trackers have improved, the systems have been installed in an increasing percentage of utility-scale projects. According to data from WoodMackenzie/GTM Research, global solar tracker shipments hit a record 14.5 gigawatts in 2017. This represents growth of 32 percent year-over-year, with similar or greater growth projected as large-scale solar deployment accelerates.

In concentrator photovoltaics (CPV) and concentrated solar power (CSP) applications, trackers are used to enable the optical components in the CPV and CSP systems. The optics in concentrated solar applications accept the direct component of sunlight light and therefore must be oriented appropriately to collect energy. Tracking systems are found in all concentrator applications because such systems collect the sun's energy with maximum efficiency when the optical axis is aligned with incident solar radiation.

Basic concept

Sunlight has two components: the "direct beam" that carries about 90% of the solar energy and the "diffuse sunlight" that carries the remainder – the diffuse portion is the blue sky on a clear day, and is a larger proportion of the total on cloudy days. As the majority of the energy is in the direct beam, maximizing collection requires the Sun to be visible to the panels for as long as possible. However, on cloudier days the ratio of direct vs. diffuse light can be as low as 60:40 or even lower.

The energy contributed by the direct beam drops off with the cosine of the angle between the incoming light and the panel.  In addition, the reflectance (averaged across all polarizations) is approximately constant for angles of incidence up to around 50°, beyond which reflectance increases rapidly.

Notes

For example, trackers that have accuracies of ± 5° can capture more than 99.6% of the energy delivered by the direct beam plus 100% of the diffuse light. As a result, high-accuracy tracking is not typically used in non-concentrating PV applications.

The purpose of a tracking mechanism is to follow the Sun as it moves across the sky. In the following sections, in which each of the main factors are described in a little more detail, the complex path of the Sun is simplified by considering its daily east-west motion separately from its yearly north-south variation with the seasons of the year.

Solar energy intercepted
The amount of solar energy available for collection from the direct beam is the amount of light intercepted by the panel. This is given by the area of the panel multiplied by the cosine of the angle of incidence of the direct beam (see illustration above). Put another way, the energy intercepted is equivalent to the area of the shadow cast by the panel onto a surface perpendicular to the direct beam.

This cosine relationship is very closely related to the observation formalized in 1760 by Lambert's cosine law. This describes that the observed brightness of an object is proportional to the cosine of the angle of incidence of the light illuminating it.

Reflective losses

Not all of the intercepted light is transmitted into the panel; some is reflected at its surface.  The amount reflected depends on both the refractive index of the surface material and the angle of incidence of the incoming light. The amount reflected also differs depending on the polarization of the incoming light. Incoming sunlight is a mixture of all polarizations, with equal amounts in direct sunlight. Averaged over all polarizations, the reflective losses are approximately constant at angles of incidence up to around 50°, beyond which they increase rapidly. See for example the accompanying graph, appropriate for glass.

Solar panels are often coated with an anti-reflective coating, which is one or more thin layers of substances with refractive indices intermediate between those of silicon and air. This causes destructive interference in the reflected light, diminishing the reflected amount. Photovoltaic manufacturers have been working to decrease reflectance with improved anti-reflective coatings and with textured glass.

Daily east-west motion of the Sun
The Sun travels through 360° east to west per day, but from the perspective of any fixed location, the visible portion is 180° degrees during an average half-day period (more in summer, slightly less in spring and fall, and significantly less in winter). Local horizon effects reduce this somewhat, making the effective motion about 150°. A solar panel in a fixed orientation between the dawn and sunset extremes will see a motion of 75° to either side, and thus, according to the table above, will lose over 75% of the energy in the morning and evening. Rotating the panels to the east and west can help recapture those losses. A tracker that only attempts to compensate for the east-west movement of the Sun is known as a single-axis tracker.

Seasonal north-south motion of the Sun
Due to the tilt of the Earth's axis, the Sun also moves through 46° north and south during a year. The same set of panels set at the midpoint between the two local extremes will thus see the Sun move 23° on either side. Thus according to the above table, an optimally aligned single-axis tracker (see polar aligned tracker below) will only lose 8.3% at the summer and winter seasonal extremes, or around 5% averaged over a year. Conversely a vertically- or horizontally-aligned single-axis tracker will lose considerably more as a result of these seasonal variations in the Sun's path. For example, a vertical tracker at a site at 60° latitude will lose up to 40% of the available energy in summer, while a horizontal tracker located at 25° latitude will lose up to 33% in winter.

A tracker that accounts for both the daily and seasonal motions is known as a dual-axis tracker. Generally speaking, the losses due to seasonal angle changes are complicated by changes in the length of the day, increasing collection in the summer in northern or southern latitudes. This biases collection toward the summer, so if the panels are tilted closer to the average summer angles, the total yearly losses are reduced compared to a system tilted at the spring/fall equinox angle (which is the same as the site's latitude).

There is considerable argument within the industry about whether the small difference in yearly collection between single- and dual-axis trackers makes the added complexity of a two-axis tracker worthwhile. A recent review of actual production statistics from southern Ontario suggested the difference was about 4% in total, which was far less than the added costs of the dual-axis systems. This compares unfavorably with the 24–32% improvement between a fixed-array and single-axis tracker.

Other factors

Clouds
The above models assume uniform likelihood of cloud cover at different times of day or year. In different climate zones cloud cover can vary with seasons, affecting the averaged performance figures described above. Alternatively, for example in an area where cloud cover on average builds up during the day, there can be particular benefits in collecting morning sun.

Atmosphere
The distance that sunlight travels through the atmosphere increases as the sun approaches the horizon, as the sunlight travels diagonally through the atmosphere. As the path length through the atmosphere increases, the solar intensity reaching the collector decreases. This increasing path length is referred to as the air mass (AM) or air mass coefficient, where AM0 is at the top of the atmosphere, AM1 refers to the direct vertical path down to sea-level with Sun overhead, and AM greater than 1 refers to diagonal paths as the Sun approaches the horizon.

Even though the sun may not feel particularly hot in the early mornings or during the winter months, the diagonal path through the atmosphere has a less than expected impact on the solar intensity. Even when the sun is only 15° above the horizon the solar intensity can be around 60% of its maximum value, around 50% at 10° and 25% at only 5° above the horizon. Therefore, if trackers can follow the Sun from horizon to horizon, then their solar panels can collect a significant amount of energy.

Solar cell efficiency

The underlying power conversion efficiency of a photovoltaic cell has a major influence on the end result, regardless of whether tracking is employed.

Temperature
Photovoltaic solar cell efficiency decreases with increasing temperature, at the rate of about 0.4%/°C. For example, there is about 20% higher efficiency at 10 °C in early morning or winter than at 60 °C in the heat of the day or summer. Therefore, trackers can deliver additional benefit by collecting early morning and winter energy when the cells are operating at their highest efficiency.

Summary
Trackers for concentrating collectors must employ high-accuracy tracking so as to keep the collector at the focus point.

Trackers for non-concentrating flat-panel do not need high accuracy tracking:
 low power loss: under 10% loss even at 25° misalignment
 reflectance consistent even to around 50° misalignment
 diffuse sunlight contributes 10% independent of orientation, and a larger proportion on cloudy days.

The benefits of tracking non-concentrating flat-panel collectors flow from the following:
 power loss rises rapidly beyond about 30° misalignment
 significant power is available even when the Sun is very close to the horizon, e.g. around 60% of full power at 15° above the horizon, around 50% at 10°, and even 25% at only 5° above the horizon – of particular relevance at high latitudes and/or during the winter months
 photovoltaic panels are around 20% more efficient in the cool of the early mornings as compared with during the heat of the day; similarly, they are more efficient in winter than summer – and effectively capturing early morning and winter sun requires tracking.

Types of solar collector
Solar collectors may be non-concentrating flat-panels, usually photovoltaic or hot-water, or concentrating systems, of a variety of types.

Solar collector mounting systems may be fixed (manually aligned) or tracking. Different types of solar collector and their location (latitude) require different types of tracking mechanism. Tracking systems may be configured as a fixed collector / moving mirror – a Heliostat – or as a moving collector

Non-tracking fixed mount
Residential and small-capacity commercial or industrial rooftop solar panels and solar water heater panels are usually fixed, often flush-mounted on an appropriately-facing pitched roof. Advantages of fixed mounts over trackers include the following:
 Mechanical Advantages: Simple to manufacture, lower installation and maintenance costs.
 Wind-loading: it is easier and cheaper to provision a sturdy mount; all mounts other than fixed flush-mounted panels must be carefully designed having regard to wind loading due to greater exposure.
 Indirect light: approximately 10% of the incident solar radiation is diffuse light, available at any angle of misalignment with the Sun.
 Tolerance to misalignment: effective collection area for a flat panel is relatively insensitive to quite high levels of misalignment with the Sun – see the table and diagram at Basic concept section above – for example even a 25° misalignment reduces the direct solar energy collected by less than 10%.

Fixed mounts are usually used in conjunction with non-concentrating systems; however, an important class of non-tracking concentrating collectors, of particular value in the third world, are portable solar cookers. These use relatively low levels of concentration, typically around 2 to 8 Suns and are manually aligned.

Trackers
Even though a fixed flat panel can be set to collect a high proportion of available noon-time energy, significant power is also available in the early mornings and late afternoons when the misalignment with a fixed panel becomes too excessive to collect a reasonable proportion of the available energy. For example, even when the Sun is only 10° above the horizon, the available energy can be around half the noon-time energy levels (or even greater depending on latitude, season, and atmospheric conditions).

Thus the primary benefit of a tracking system is to collect solar energy for the longest period of the day, and with the most accurate alignment as the Sun's position shifts with the seasons.

In addition, the greater the level of concentration employed, the more important accurate tracking becomes, because the proportion of energy derived from direct radiation is higher, and the region where that concentrated energy is focused becomes smaller.

Fixed collector / moving mirror

Many collectors cannot be moved, such as high-temperature collectors where the energy is recovered as hot liquid or gas (e.g. steam). Other examples include direct heating and lighting of buildings and fixed in-built solar cookers, such as Scheffler reflectors. In such cases it is necessary to employ a moving mirror so that, regardless of where the Sun is positioned in the sky, the Sun's rays are redirected onto the collector.

Due to the complicated motion of the Sun across the sky, and the level of precision required to correctly aim the Sun's rays onto the target, a heliostat mirror generally employs a dual axis tracking system, with at least one axis mechanized. In different applications, mirrors may be flat or concave.

Moving collector
Trackers can be grouped into classes by the number and orientation of the tracker's axes. Compared to a fixed mount, a single-axis tracker increases annual output by approximately 30%, and a dual axis tracker an additional 10–20%.

Photovoltaic trackers can be classified into two types: standard photovoltaic (PV) trackers and concentrated photovoltaic (CPV) trackers. Each of these tracker types can be further categorized by the number and orientation of their axes, their actuation architecture and drive type, their intended applications, their vertical supports, and foundation.

Floating mount
Floating islands of solar panels are being installed on reservoirs and lakes in the Netherlands, China, the UK, and Japan. The sun-tracking system controlling the direction of the panels operates automatically according to the time of year, changing position by means of ropes attached to buoys.

Floating ground mount
Solar trackers can be built using a "floating" foundation, which sits on the ground without the need for invasive concrete foundations. Instead of placing the tracker on concrete foundations, the tracker is placed on a gravel pan that can be filled with a variety of materials, such as sand or gravel, to secure the tracker to the ground. These "floating" trackers can sustain the same wind load as a traditional fixed mounted tracker. The use of floating trackers increases the number of potential sites for commercial solar projects since they can be placed on top of capped landfills or in areas where excavated foundations are not feasible.

Motion-Free Optical Tracking
Solar trackers can be built without the need for mechanical tracking equipment. These are called motion-free optical tracking. Renkube pioneered a glass based design to redirect light using motion-free optical tracking technology.

Non-concentrating photovoltaic (PV) trackers
Photovoltaic panels accept both direct and diffuse light from the sky. The panels on standard photovoltaic trackers gather both the available direct and diffuse light. The tracking functionality in standard photovoltaic trackers is used to minimize the angle of incidence between incoming light and the photovoltaic panel. This increases the amount of energy gathered from the direct component of the incoming sunlight.

The physics behind standard photovoltaic trackers works with all standard photovoltaic module technologies. These include all types of crystalline silicon panels (either mono-Si, or multi-Si) and all types of thin film panels (amorphous silicon, CdTe, CIGS, microcrystalline).

Concentrator photovoltaic (CPV) trackers

The optics in CPV modules accept the direct component of the incoming light and therefore must be oriented appropriately to maximize the energy collected. In low-concentration applications, a portion of the diffuse light from the sky can also be captured. The tracking functionality in CPV modules is used to orient the optics such that the incoming light is focused to a photovoltaic collector.

CPV modules that concentrate in one dimension must be tracked normal to the Sun in one axis. CPV modules that concentrate in two dimensions must be tracked normal to the Sun in two axes.

Accuracy requirements

The physics behind CPV optics requires that tracking accuracy increases as the system's concentration ratio increases. However, for a given concentration, nonimaging optics provide the widest possible acceptance angles, which may be used to reduce tracking accuracy.

In typical high-concentration systems, tracking accuracy must be in the ± 0.1° range to deliver approximately 90% of the rated power output. In low concentration systems, tracking accuracy must be in the ± 2.0° range to deliver 90% of the rated power output. As a result, high-accuracy tracking systems are typical.

Technologies supported

Concentrated photovoltaic trackers are used with refractive and reflective concentrator systems. There are a range of emerging photovoltaic cell technologies used in these systems. These range from conventional, crystalline-silicon-based photovoltaic receivers to germanium-based triple junction receivers.

Single-axis trackers
Single-axis trackers have one degree of freedom that acts as an axis of rotation. The axis of rotation of single-axis trackers is typically aligned along a true North meridian. It is possible to align them in any cardinal direction with advanced tracking algorithms. There are several common implementations of single-axis trackers. These include horizontal single-axis trackers (HSAT), horizontal single-axis tracker with tilted modules (HTSAT), vertical single-axis trackers (VSAT), tilted single-axis trackers (TSAT), and polar-aligned single-axis trackers (PSAT). The orientation of the module with respect to the tracker axis is important when modeling performance.

Horizontal

Horizontal single axis tracker (HSAT)

The axis of rotation for a horizontal single-axis tracker is horizontal with respect to the ground. The posts at either end of the axis of rotation of a horizontal single-axis tracker can be shared between trackers to lower the installation cost. This type of solar tracker is most appropriate for low-latitude regions. Field layouts with horizontal single-axis trackers are very flexible. The simple geometry means that keeping all of the axes of rotation parallel to one another is all that is required for appropriately positioning the trackers with respect to one another. Appropriate spacing can maximize the ratio of energy production to cost, with this being dependent upon local terrain and shading conditions and the time-of-day value of the energy produced. Backtracking is one means of computing the disposition of panels. Horizontal trackers typically have the face of the module oriented parallel to the axis of rotation. As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation. In single-axis horizontal trackers, a long horizontal tube is supported on bearings mounted upon pylons or frames. The axis of the tube is on a north-south line. Panels are mounted upon the tube, and the tube will rotate on its axis to track the apparent motion of the Sun through the day.

Horizontal single-axis tracker with tilted modules (HTSAT)

In HSATs, the modules are mounted flat at 0°, while in HTSATs, the modules are installed at a certain tilt. It works on the same principle as HSAT, keeping the axis of tube horizontal in north-south line and rotates the solar modules from east to west throughout the day. These trackers are usually suitable in high-latitude locations but do not take as much land space as vertical single-axis trackers (VSATs). Therefore, it brings the advantages of VSATs in a horizontal tracker and minimizes the overall cost of solar project.

Vertical

Vertical single-axis tracker (VSAT)

The axis of rotation for vertical single-axis trackers is vertical with respect to the ground. These trackers rotate from east to west over the course of the day. Such trackers are more effective at high latitudes than horizontal single-axis trackers are. Field layouts must consider shading to avoid unnecessary energy losses and to optimize land use. Als, optimization for dense packing is limited due to the nature of the shading over the course of a year. Vertical single-axis trackers typically have the face of the module oriented at an angle with respect to the axis of rotation. As a module tracks, it sweeps a cone that is rotationally symmetric around the axis of rotation.

Tilted
Tilted single-axis tracker (TSAT)

All trackers with axes of rotation between horizontal and vertical are considered tilted single-axis trackers. Tracker tilt angles are often limited to reduce the wind profile and decrease the elevated end height. With backtracking, they can be packed without shading perpendicular to their axes of rotation at any density. However, the packing parallel to their axes of rotation is limited by the tilt angle and the latitude. Tilted single-axis trackers typically have the face of the module oriented parallel to the axis of rotation. As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation.

Dual-axis trackers
Dual-axis trackers have two degrees of freedom that act as axes of rotation. These axes are typically normal to one another. The axis that is fixed with respect to the ground can be considered a primary axis. The axis that is referenced to the primary axis can be considered a secondary axis. There are several common implementations of dual-axis trackers. They are classified by the orientation of their primary axes with respect to the ground. Two common implementations are tip-tilt dual-axis trackers (TTDAT) and azimuth-altitude dual-axis trackers (AADAT). The orientation of the module with respect to the tracker axis is important when modeling performance. Dual-axis trackers typically have modules oriented parallel to the secondary axis of rotation. Dual-axis trackers allow for optimum solar energy levels due to their ability to follow the Sun vertically and horizontally. No matter where the Sun is in the sky, dual-axis trackers are able to angle themselves to point directly at the Sun.

Tip-tilt

A tip-tilt dual-axis tracker (TTDAT) is so named because the panel array is mounted on the top of a pole. Normally the east-west movement is driven by rotating the array around the top of the pole. On top of the rotating bearing is a T- or H-shaped mechanism that provides vertical rotation of the panels and provides the main mounting points for the array. The posts at either end of the primary axis of rotation of a tip-tilt dual axis tracker can be shared between trackers to lower installation costs.

Other such TTDAT trackers have a horizontal primary axis and a dependent orthogonal axis. The vertical azimuthal axis is fixed. This allows for great flexibility of the payload connection to the ground mounted equipment because there is no twisting of the cabling around the pole.

Field layouts with tip-tilt dual-axis trackers are very flexible. The simple geometry means that keeping the axes of rotation parallel to one another is all that is required for appropriately positioning the trackers with respect to one another. Normally the trackers would have to be positioned at fairly low density to avoid one tracker casting a shadow on others when the Sun is low in the sky. Tip-tilt trackers can make up for this by tilting closer to horizontal to minimize up-Sun shading and therefore maximize the total power being collected.

The axes of rotation of many tip-tilt dual-axis trackers are typically aligned either along a true north meridian or an east-west line of latitude.

Given the unique capabilities of the tip-tilt configuration and the appropriate controller, totally-automatic tracking is possible for use on portable platforms. The orientation of the tracker is of no importance and can be placed as needed.

Azimuth-altitude
An azimuth-altitude (or alt-azimuth) dual axis tracker (AADAT) has its primary axis (the azimuth axis) vertical to the ground. The secondary axis, often called elevation axis, is then typically normal to the primary axis. They are similar to tip-tilt systems in operation, but they differ in the way the array is rotated for daily tracking. Instead of rotating the array around the top of the pole, AADAT systems can use a large ring mounted on the ground with the array mounted on a series of rollers. The main advantage of this arrangement is the weight of the array is distributed over a portion of the ring, as opposed to the single loading point of the pole in the TTDAT. This allows AADAT to support much larger arrays. Unlike the TTDAT, however, the AADAT system cannot be placed closer together than the diameter of the ring, which may reduce the system density, especially considering inter-tracker shading.

Construction and (Self-)Build

As described later, the economic balance between the costs of panels and trackers. The steep drop in cost for solar panels in the early 2010s made it more challenging to find a sensible solution. As can be seen in the attached media files, most constructions use industrial and/or heavy materials unsuitable for small or craft workshops. Even commercial offers may have rather unsuitable solutions (a big rock) for stabilization. For a small (amateur/enthusiast) construction, the criteria that must be met include economy, stability of end product against elemental hazards, ease of handling materials, and joinery.

Tracker type selection
The selection of tracker type is dependent on many factors including installation size, electric rates, government incentives, land constraints, latitude, and local weather.

Horizontal single-axis trackers are typically used for large distributed generation projects and utility scale projects. The combination of energy improvement, lower product cost, and lower installation complexity results in compelling economics in large deployments. In addition, the strong afternoon performance is particularly desirable for large grid-tied photovoltaic systems so that production will match the peak demand time. Horizontal single-axis trackers also add a substantial amount of productivity during the spring and summer seasons when the Sun is high in the sky. The inherent robustness of their supporting structure and the simplicity of the mechanism also result in high reliability which keeps maintenance costs low. Since the panels are horizontal, they can be compactly placed on the axle tube without danger of self-shading and are also readily accessible for cleaning.

A vertical-axis tracker pivots only about a vertical axle, with the panels at a fixed, adjustable, or tracked elevation angle. Such trackers with fixed or (seasonally) adjustable angles are suitable for high latitudes, where the apparent solar path is not especially high, but which leads to long days in summer, with the Sun traveling through a long arc.

Dual-axis trackers are typically used in smaller residential installations and locations with very high government feed in tariffs.

Multi-mirror concentrating PV

This device uses multiple mirrors in a horizontal plane to reflect sunlight upward to a high-temperature system requiring concentrated solar power. Structural problems and expense are greatly reduced since the mirrors are not significantly exposed to wind loads. Through the employment of a patented mechanism, only two drive systems are required for each device. Because of the configuration of the device, it is especially suited for use on flat roofs and at lower latitudes. The units illustrated each produce approximately 200 peak DC watts.

A multiple-mirror reflective system combined with a central power tower was employed at the Sierra SunTower, located in Lancaster, California. This generation plant, operated by eSolar, operated from 2009 to 2014. This system, which used multiple heliostats in a north-south alignment, used pre-fabricated parts and construction as a way of decreasing startup and operating costs.

Drive types

Active tracker
Active trackers use motors and gear trains to perform solar tracking. They can use microprocessors and sensors, date-and-time-based algorithms, or a combination of both to detect the position of the sun. To control and manage the movement of these massive structures, special slewing drives are designed and rigorously tested. The technologies used to direct the tracker are constantly evolving and recent developments at Google and Eternegy have included the use of wire-ropes and winches to replace some of the more costly and more fragile components.

Counter-rotating slewing drives sandwiching a fixed-angle support can be applied to create a "multi-axis" tracking method which eliminates rotation relative to longitudinal alignment. This method, if placed on a column or pillar, will generate more electricity than fixed PV, and its PV array will never rotate into a parking lot drive lane. It will also allow for maximum solar generation in virtually any parking lot lane/row orientation, including circular or curvilinear.

Active two-axis trackers are also used to orient heliostats – movable mirrors that reflect sunlight toward the absorber of a central power station. As each mirror in a large field will have an individual orientation, these are controlled programmatically through a central computer system, which also allows the system to be shut down when necessary.

Light-sensing trackers typically have two or more photosensors, such as photodiodes, configured differentially so that they output a null when receiving the same light flux. Mechanically, they should be omnidirectional (i.e. flat) and are aimed 90 degrees apart. This will cause the steepest part of their cosine transfer functions to balance at the steepest part, which translates into maximum sensitivity. For more information about controllers, see active daylighting.

Since the motors consume energy, one wants to use them only as necessary. So instead of a continuous motion, the heliostat is moved in discrete steps. Also, if the light is below some threshold, there would not be enough power generated to warrant reorientation. This is also true when there is not enough difference in light level from one direction to another, such as when clouds are passing overhead. Consideration must be made to keep the tracker from wasting energy during cloudy periods.

Passive tracker

The most common Passive trackers use a low-boiling-point compressed gas that is driven to one side or the other (by solar heat creating gas pressure) to cause the tracker to move in response to an imbalance. As this is an imprecise orientation, it is unsuitable for certain types of concentrating photovoltaic collectors but works fine for common PV panel types. These will have viscous dampers to prevent excessive motion in response to wind gusts. Shader/reflectors are used to reflect early morning sunlight to "wake up" the panel and tilt it toward the Sun, which can take some hours, depending on shading conditions. The time to do this can be greatly reduced by adding a self-releasing tiedown that positions the panel slightly past the zenith (so that the fluid does not have to overcome gravity) and using the tiedown in the evening. (A slack-pulling spring will prevent release in windy overnight conditions.)

A newly emerging type of passive tracker for photovoltaic solar panels uses a hologram behind stripes of photovoltaic cells so that sunlight passes through the transparent part of the module and reflects on the hologram. This allows sunlight to hit the cell from behind, thereby increasing the module's efficiency. Also, the panel does not have to move since the hologram always reflects sunlight from the correct angle towards the cells.

Manual tracking
In some developing nations, drives have been replaced by operators who adjust the trackers. This has the benefits of robustness, having staff available for maintenance, and creating employment for the population in the vicinity of the site.

Rotating buildings
In Freiburg im Breisgau, Germany, Rolf Disch built the Heliotrop in 1996, a residential building that is rotating with the sun and has an additional dual-axis photovoltaic sail on the roof. It produces four times the amount of energy the building consumes.

The Gemini house is a unique example of a vertical axis tracker. This cylindrical house in Austria (latitude above 45 degrees north) rotates in its entirety to track the Sun, with vertical solar panels mounted on one side of the building, rotating independently, allowing control of the natural heating from the Sun.

ReVolt House is a rotating, floating house designed by TU Delft students for the Solar Decathlon Europe competition in Madrid. The house was completed in September 2012. An opaque façade turns itself towards the Sun in summer to prevent the interior from heating up. In winter, a glass façade faces the Sun for passive solar heating of the house. Since the house is floating frictionlessly on water, rotating it does not require much energy.

Disadvantages
Trackers add cost and maintenance to the system – if they add 25% to the cost, and improve the output by 25%, then the same performance can be obtained by making the system 25% larger, eliminating the additional maintenance. Tracking was very cost effective in the past when photovoltaic modules were expensive compared to today. Because they were expensive, it was important to use tracking to minimize the number of panels used in a system with a given power output. But as panels get cheaper, the cost effectiveness of tracking vs using a greater number of panels decreases. However, in off-grid installations where batteries store power for overnight use, a tracking system reduces the hours that stored energy is used, thus requiring less battery capacity. As the batteries themselves are expensive (either traditional lead acid stationary cells or newer lithium ion batteries), their cost needs to be included in the cost analysis.

Tracking is also not suitable for typical residential rooftop photovoltaic installations. Since tracking requires that panels tilt or otherwise move, provisions must be made to allow this. This requires that panels be offset a significant distance from the roof, which requires expensive racking and increases wind load. Also, such a setup would not make for an aesthetically pleasing install on residential rooftops. Because of this (and the high cost of such a system), tracking is not used on residential rooftop installations, and is unlikely to ever be used in such installations. This is especially true as the cost of photovoltaic modules continues to decrease, which makes increasing the number of modules for more power the more cost-effective option. Tracking can (and sometimes is) used for residential ground mount installations, where greater freedom of movement is possible.

Tracking can also cause shading problems. As the panels move during the course of the day, it is possible that, if the panels are located too close to one another, they may shade one another due to profile angle effects. As an example, if one has several panels in a row from east to west, there will be no shading during solar noon, but in the afternoon, panels could be shaded by their west neighboring panel if they are sufficiently close. This means that panels must be spaced sufficiently far to prevent shading in systems with tracking, which can reduce the available power from a given area during the peak Sun hours. This is not a big problem if there is sufficient land area to widely space the panels. But it will reduce output during certain hours of the day (i.e. around solar noon) compared to a fixed array. Optimizing this problem with math is called backtracking.

Further, single-axis tracking systems are prone to becoming unstable at relatively modest wind speeds (galloping). This is due to the torsional instability of single-axis solar tracking systems.  Anti-galloping measures such as automatic stowing and external dampers must be implemented.

See also
 Air mass coefficient
 Heliostat
 Solar energy
 Sun path

References

Solar energy
Tracking
Photovoltaics

Solar tracker


A tongue-twister is a phrase that is hard to say. They often use alliteration and homophones. They are commonly employed to help warm up singers and actors

Examples
 How much wood would a woodchuck chuck if a woodchuck could chuck wood?  He would chuck as many wood as a woodchuck chucks if a woodchuck could chuck wood.
 She sells sea-shells on the sea-shore. The sea shells she sells are sea shells for sure.
 Six sick hicks nick six slick bricks with picks and sticks.
 The lips, the teeth, the tip of the tongue.

References

Linguistics

Tongue-twister


Howard Joseph Ruff (December 27, 1930 – November 12, 2016) was a financial adviser and writer of the pro-hard money investing newsletter The Ruff Times.  Ruff was the author of Famine and Survival in America (1974), How to Prosper During the Coming Bad Years (1979), Survive and Win in the Inflationary Eighties (1981), Making Money (1984), and other books. He updated and re-released his most successful book, re-titling it How to Prosper During the Coming Bad Years in the 21st Century (2008).

Career
Ruff advised investors to avoid stocks and bonds and instead to put their portfolios into gold, silver, platinum group metals, and collectibles such as art and numismatic coins.  He also advised his readers to store a year's supply of food in preparation for hard times.  Ruff believed (as of his 1979–1981 writings) that the United States was headed for a hyperinflationary economic depression and that there was a danger that both government and private pension plans were about to collapse.

In the late 1970s and early 1980s, Ruff hosted a syndicated television show called Ruff House.

During the financial crisis of 2007–08, Ruff began appearing on CNBC  making economic predictions that were similar to those he had made in the 1970s and 1980s.

Ruff gained a sizable mainstream audience for a while during the late 1970s until about 1981, because those who had been taking his investment advice and buying precious metals saw large capital gains during that period.  The New York Times labeled Howard Ruff "The Prophet of Doom" after his book How to Prosper During the Coming Bad Years reached the number #1 seller in 1979.  His popularity fell off after the peak in the gold and silver speculative bubble in 1980.

Ruff raised money to oppose the election of Hillary Clinton to the U.S. Senate in 2000.

Ruff was a member of the Church of Jesus Christ of Latter-day Saints.  In How to Prosper During the Coming Bad Years (1979), he relates that his recommendation of food storage is in accord with a policy of the Church of Jesus Christ of Latter-day Saints.

Ruff has also been involved in other endeavors such as a vinyl LP record album of his singing.

Ruff has also been cited on various occasions by Kiplinger's Personal Finance magazine.

Ruff died from complications of age-related diseases in Lehi, Utah, at the age of 85.

Bibliography

See also
Survivalism
Retreat (survivalism)

References

1930 births
2016 deaths
American economics writers
American male non-fiction writers
American Latter Day Saints
American finance and investment writers
Survivalists
People from Berkeley, California

Howard Ruff


Johnny Doughboy is a 1942 American black-and-white musical comedy film directed by John H. Auer for Republic Pictures. It stars Jane Withers in a dual role as a 16-year-old actress who is sick of playing juvenile roles, and her lookalike fan who is persuaded by a group of "has-been" child stars to perform with them in a U.S. troop show. The film features cameos by ex-child stars Bobby Breen, Carl "Alfalfa" Switzer, George "Spanky" McFarland, Baby Sandy, and others. It received an Academy Award nomination for Best Musical Score.

Plot
Sixteen-year-old film star Ann Winters is sick of playing juvenile roles and decides to run away for a vacation, to the consternation of her manager, Harry Fabian, who has just arranged for her to play a 12-year-old in Ann of Honeysuckle Farm. As soon as Ann drives off, Penelope Ryan, president of the Ann Winters fan club back in Oriole, Nebraska, arrives for a two-week stay as the winner of a lookalike contest that Harry dreamed up. Though she looks just like Ann, Penelope is sweet and agreeable, unlike Ann's brash and impetuous personality. Meanwhile, Ann runs out of gas in the middle of the mountains at night and seeks help at a private home. The housekeeper, Mammy, lets her in and gives her a bed for the night. Later that night, Ann is awakened by the entrance of the owner of the house, Oliver Lawrence, a playwright. Oliver goes along with Ann's desire not to reveal her true identity, and in the morning Mammy arranges for them to spend Ann's vacation together, hiking, boating, and dancing. Though she is only 16 and Oliver is old enough to be her father, Ann develops a crush on him.

Back at Ann's home, Penelope is accosted by members of the "20 Minus Club", a group of ex-child stars who want to put on a show to entertain the troops before they go off to war. The club members explain to Penelope that the only way they can get funding is if they have a name star on their bill. Believing she is Ann, they convince her to say yes. They perform their song and dance act for Penelope, but when she is asked to sing in a duet with Johnny, Ann's former sweetheart, Penelope realizes that she can't do the show and reneges on her promise. She feels awful about it, and as she packs to go back to Nebraska, she confides in Ann's secretary, "Biggy" Biggsworth, what she's done. Biggy hints to her where Ann is vacationing and Penelope gets the "20 Minus Club" to drive her there to see Ann.

When Ann sees Penelope for the first time, she is taken aback by their resemblance. Then she confides in Penelope that she has given up film for the theater, and has also taken up with Oliver on a personal level. Oliver hears their conversation from the next room and confronts Ann, convincing her there is nothing between them by having his daughter Jennifer pose as his girlfriend. Ann runs outside in tears and Johnny, who has been waiting all this time for Penelope to return, finds her. Ann realizes she still is fond of Johnny though the studio broke up their friendship, and she agrees to perform in the show. The final act is the show itself, with Ann and Johnny singing and dancing in the lead.

Cast
Jane Withers as Ann Winters and Penelope Ryan
Henry Wilcoxon as Oliver Lawrence
Patrick Brook as Johnny Kelly
William Demarest as Harry Fabian
Ruth Donnelly as "Biggy" Biggsworth
Etta McDaniel as Mammy
Joline Westbrook as Jennifer Lawrence
Bobby Breen as himself
Baby Sandy as herself
Carl "Alfalfa" Switzer as himself
George "Spanky" McFarland as himself
Butch (Kenneth Brown) as himself
Buddy (Billy Lenhart) as himself
Cora Sue Collins as herself
Robert Coogan as himself
Grace Costello as herself
Karl Kiffe as himself
The Falkner Orchestra as Themselves

Production

Development

Johnny Doughboy is based on an original story by Frederick Kohner; Lawrence Kimble adapted the screenplay.

Casting
This was Jane Withers' first film after completing her seven-year contract with 20th Century Fox.  It was also Withers's third screen pairing with Etta McDaniel, sister of actress Hattie McDaniel and a former vaudevillian. According to The Hollywood Reporter, actor Henry Wilcoxon announced he would donate his fee for the film to the United States Coast Guard Relief.

The fictional "20 Minus Club" is depicted as an organization for once famous child stars. The "members" (all of whom portray themselves) really were Hollywood "has-beens", and this was the last time some of them appeared on screen. Members of the club include actor/singer Bobby Breen, Baby Sandy, former Our Gang stars Carl "Alfalfa" Switzer and George "Spanky" McFarland (reunited on film for the final time), musicians "Butch" and "Buddy" (Kenneth Brown and Billy Lenhart), actress Cora Sue Collins, actor Robert Coogan, dancer Grace Costello, drummer Karl Kiffe, and the Faulkner Orchestra.

Music
Breen performs "Ave Maria" by Franz Schubert in one musical segment. Songs performed in the film's grand finale "victory chorus" include:

"It Takes a Guy Like I", "You Do My Eyes a Favor", "Baby's a Big Girl Now", "All Done, All Through", "Give the Kids a Chance", "The Victory Caravan" — music and lyrics by Jule Styne and Sammy Cahn
"Better Not Roll Those Blue, Blue Eyes" — music and lyrics by Al Goodhart and Kay Twomey
"Johnny Doughboy Found a Rose in Ireland" — music by Al Goodhart and Allan Roberts, lyrics by Kay Twomey

"Johnny Doughboy Found a Rose in Ireland" was one of the top 10 best-selling songs, and songs with the most radio plays, in the United States in 1942 according to Billboard.

Filming
Filming took place from mid-August to September 11, 1942. Filmmakers had originally planned to use the exterior of Withers's own home as the home of the child star in the first scene, but decided it didn't look the part. The home of an electrician working on the set was filmed instead.

Release
The film was released on December 31, 1942.

Critical reception
Reviews in The New York Times and The Boston Globe observed that the film did not have much of a plot beyond Jane Withers playing a dual role, but the film was entertaining nevertheless. The New York Times review complimented the scriptwriters for cleverly helping the now-teenaged Withers transition from child stardom to adult roles by having her "be both her old and new self" in the film. "It gets a little confusing at times to be seeing two Jane Witherses, but at least it makes for contrast", this review concluded. The Boston Globe called Withers "a slim, personable young lady" who "sings, dances and even carries on a long conversation with herself". This review also praised Patrick Brook's acting and singing, and called Ruth Donnelly's performance "amusing".

The Brooklyn Citizen noted the tie-in between Withers's real-life participation in war bond drives and her fictional musical performance at a soldiers' camp in this film. During World War II, Withers participated in more than 100 U.S. war bond drives and camp tours.

In a 2019 review, James L. Neibaur calls the film "breezy" and "a pleasant diversion", adding that it is "a good example of the type of second-feature that was popular on double feature programs during the war years".

Accolades
Johnny Doughboy was nominated for Best Musical Score at the 15th Academy Awards, but lost to Ray Heindorf and Heinz Roemheld's score for Yankee Doodle Dandy.

References

External links

1942 films
1940s English-language films
American black-and-white films
Republic Pictures films
Films directed by John H. Auer
Films scored by Walter Scharf
American musical comedy films
1942 musical comedy films
Films about actors
1940s American films
English-language musical comedy films

Johnny Doughboy


The Great Fire of New York was a large fire in New York City. It burned through the night of September 21, 1776 on the west side of the former boundaries of New York City at the southern end of the island of Manhattan.  It started when the British Army took control of the city during the American Revolutionary War. Many think that Washington's men, who had recently lost the Battle of Long Island, started the fire to hamper British shipping of materials.  Others believe that the British Army started the fire in an attempt to burn Whitehall.

The fire destroyed 10 to 25% of the city. Some of the unburned parts of the city were ransacked.  The lack of housing made difficulties for war refugees who went to the city to escape the Rebels elsewhere.

1776 in New York City
American Revolutionary War
Fires in New York (state)
History of Manhattan

Great Fire of New York (1776)


A variable-width encoding is a type of character encoding scheme in which codes of different lengths are used to encode a character set for representation in a computer. All of the common Unicode encodings are variable-width encodings, e.g. UTF-8 and UTF-16. It's a common mistake to think that UTF-16 isn't, so that's not a good reason to prefer UTF-16 (only its obsolete predecessor UCS-2 is fixed-width).

ASCII is a fixed-width encoding. So are many other legacy encodings, but no modern text encoding. Note, ASCII is legal UTF-8 text, but it's only fixed-width in that sense only when that subset is used. As soon as text uses even one letter outside of the ASCII subset, or even if software expects UTF-8 text, and can't rely on only the ASCII subset used, then the (UTF-8) encoding used is variable-length.

Computer science
Encodings

Variable-width encoding