). A sidereal day is the time it takes for Earth to spin once (360 degrees) on its axis. Astronomers will tell you that a sidereal day is 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: 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 (1/86,400 of a solar day) but uses the sun as the reference point in the sky instead of a distant star. Because 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 radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. That clears things up, doesn't it? This definition refers to the reference used by atomic clocks. Atomic clocks are the most accurate clocks available to the public. They are considered to be the reference by which all other clocks should be set. If we consider atomic clocks to be the gold standard by which other clocks should be measured, we can also categorize other clocks by how accurate they are compared to the atomic clock standard. In NTP, each clock exists inside a stratum. An actual atomic clock is a stratum-zero time source. A device that serves NTP requests and is directly connected to the atomic clock is considered to be at stratum one. If you set up an NTP server that learns time from a stratum-one NTP server, your server will become a stratum-two server, and SO on down the line. Strata in NTP reference how many levels of clocks the device is removed from the actual time source. A clock operating in stratum 16 is considered to be unsynchronized. Strata are defined differently in NTP than they are in teleco