The clocks displayed on the main page are based on a combination of the clock setting on your computer and the cookie values set when you first registered with AstronomyDaily.com.

If the time, date, or time zone setting on your computer are wrong, then the clocks displayed on AstronomyDaily.com will be wrong as well.

 

Local Time
Local time is read directly from the time setting on your computer. It should always be the same as your local computer clock, as long as the time zone setting on your computer is correct.
Coordinated Universal Time (UTC)
Universal Time is obtained by using javascript functions to read the time from your computer clock and convert it to UTC based on your time zone setting. Again, if your clock and time zone are correct, then this time will be correct. UTC is like Greenwich Mean Time (GMT), but not quite. The term GMT is technically obsolete; as a standard, it was replaced by UTC in 1972. However, the abbreviation is still used in some places (e.g., the BBC, for obvious patriotic reasons), but whenever the term "GMT" is used it now means UTC. While most people believe that UTC is a hyper-accurate time standard obtained from an atomic clock; the actual details are bit more complicated... TAI (International Atomic Time) is defined in terms of a particular transition of the electrons of the cesium atom. The frequency or rate of TAI is computed by the International Bureau of Weights and Measures (BIPM) located near Paris, France, using a weighted average from about 250 atomic clocks located in about 50 national laboratories. However, this standard is more precise than the actual rotation of the earth, so the Universal Time family of time scales was created. UTC is based on TAI, but "leap seconds" are inserted (or possibly deleted) on a semi-regular basis (roughly every 18 months) to keep it within ± 0.9 seconds of UT1 (see below). As of January 1, 1999, TAI was ahead of UTC by 32 seconds. UT (Universal Time) is the modern equivalent of "solar time". It is used by astronomers and others who need to keep their equipment synchronized with the rotation of the earth. UT is actually a family of time scales, with UT0 providing the basis as the precise solar time at the zero meridian. A network of radio telescopes maintains UT0 by observing distant quasars. UT1 is UT0 corrected for a periodic effect known as Chandler wobble or "polar wandering", i.e., small changes in the longitude/latitude of all places on the Earth due to the fact that the rotational axis of the Earth "wanders" in semi-regular patterns. It is available from most time services in the form of an offset (± 0.9 seconds) from UTC, and as of July 2000 it was hovering near +200 ms. UT2 is an even better-corrected version of UT0 that accounts for seasonal variations in the Earth's rotation rate and is sometimes used in astronomy. Another frequently seen time standard, GPS (Global Positioning System) system time, is a virtual time standard maintained as a composite of the atomic clocks in the GPS, both on the satellites and on the ground. It is similar to TAI in that it has no leap seconds, but it has a fixed offset of 19 seconds based on the fact that it was started on January 6, 1980, when this was the offset between TAI and UTC. However, most GPS receivers actually display UTC, since they can get the current offset from GPS system time to UTC from the GPS navigation message.
Greenwich Sidereal Time (GST)
Greenwich Sidereal Time is computed from the time and the date setting on your computer. If your time is set correctly, but the date is off, then this clock will be wrong even if the previous two clocks are correct. GST is local sidereal time for Greenwich, England. See the detailed discussion of sidereal time below.
Local Sidereal Time (LST)
Local Sidereal Time is computed from GST based on your longitude. Your longitude is stored in the cookie sent to you when you first registered with AstronomyDaily.com and was initially set based on your selections on the user registration maps. The UT family of time standards discussed above all have one thing in common: they measure the rotation of the Earth relative to the position of the Sun. Since the Earth orbits the Sun, it's location in the sky appears to shift slightly less than 1° each day (since one orbit is 360°, and it covers this space in about 365 days, the daily shift is about 360/365 = 0.98°). Instead, if we measure the Earth's rotation relative to the stars, we get sidereal time. Sidereal time is defined as the length of time since the vernal equinox has crossed the local meridian. An equivalent definition of sidereal time is the right ascension of any star presently located on the local celestial meridian. For example, the star Sirius is located at R.A. 6hr 45m and the LST when Sirius is on the meridian is 6:45. Looked at another way, the time indicated on the clock above is the right ascension of the point directly overhead in the sky right now. Because our everyday clocks are based on solar time, sidereal time generally does not coincide with the clock time. The exception is on the date of autumnal equinox (around September 21) when the sidereal time agrees with the solar time. Six months later at the date of vernal equinox (around March 21) they are exactly 12 hours apart.