Developers guide

101
E.2. Astronomical times
E.2.1. Universal time
Universal time (UT) is the precise measurement of time used as the basis for civil time keeping [ 5
]. By definition, it is the solar time at the Greenwich meridian, also know as Greenwich Mean Time
(GMT)
i
. Such that the sun passes directly over the Greenwich meridian at noon, UT.
Universal time is calculated in several ways:
• UT0 is the universal time calculated from astronomical observations.
• UT1 is UT0 adjusted to compensate for long term, uniform errors in the earths rotation
(known as the Chandler Wobble).
• UT2 is UT1 adjusted to compensate for annual and semi-annual changes in the earths
rotation rate.
• UTC, Coordinated Universal Time is calculated from TAI. UTC is identical to TAI except for
the addition of leap seconds which are added to keep UTC accurate to within 0.9
seconds of UT1. Astronomers will typically quote UTC when recording observation data.
E.2.2. Atomic time
International Atomic Time (Temps Atomique International – TAI) is a time scale based on the SI
unit for a second, defined as the duration of 9,192,631,770 cycles of radiation corresponding to
the transition between two hyperfine levels of the ground state of caesium 133. TAI is a fixed
scale and consequently does not compensate for irregularities in the earths rotation. It is
maintained by approximately 200 atomic clocks worldwide.
E.2.3. Sidereal time
Sidereal time is used with the equatorial coordinate system. If a certain hour circle is on the
celestial meridian, one sidereal hour later the next hour circle will be on the celestial meridian.
The celestial sphere completes one rotation in relation to the earth every sidereal day [ 5 ]. Thus,
Local Sidereal Time (LST) is defined as the Right Ascension of the local meridian, or hour circle, as
shown in Figure E-1.
E.2.4. Ephemeris time
Ephemeris Time (ET) was introduced in 1960 by astronomers to account for errors caused by
variations in the earths rotation
i
. ET is currently one minute ahead of UT and gradually diverging.
i
Now obseleted by UTC