ich is available only in PostScript or PDF format. If you get excited when you see the type of math shown in Figure 35-1, RFC 1129 is for you. aj-1uj-1 Figure 35-1. Actual math from RFC 1129 If, like most people, you can't be bothered with the math, and just want to know what you need to do to make NTP work for you, read on. What Is Accurate Time? How do we measure time? How long is a second? Who defined how long a second is, and why does it matter? These are questions most people don't think about. Most of us take time for granted. Every electronic device you own, from your personal computer to your television, relies on time being accurate to a certain degree. The power in your home oscillates at 60 cycles per second (60 Hz). High-definition televisions update their screens at 60 frames per second. Modems, Tls, DS3s, and all other data services send information at a set number of bits per second. When two pieces of equipment communicate, they must agree on the length of a second. For a long time, a second was defined as 1/86,400th of a sidereal day. 24 hours * 60 minutes * 60 seconds = 86,400 seconds. A sidereal day is the time it takes for the Earth to spin once (360 degrees) on its axis. Astronomers will tell you that a sidereal day is measured as the amount of time it takes for a star in the sky to transit the meridian twice. A solar day is shorter than a sidereal day by four minutes, which is why we have leap years: the calendar has to reset itself due to the accumulation of these four-minute errors each day. Thus, 1 sidereal second = 1.00278 solar seconds. A solar second is defined with the same formula (1/86,400th of a solar day), but uses the sun as the reference point in the sky instead of a distant star. Because the Earth rotates around the sun, the position of the sun in the sky changes every day. As you can see, agreeing on the length of a second is not as straightforward as you might think. Currently, the definition of one second is accepted to be the duration of 9,192,631,770 periods of the radiati