$LastChangedDate: 27-Oct-2009 $

# Dates and Time

Dates and Time

## Dates and time scales used in CelestLab

- TAI (Temps Atomique International or International Atomic Time) is a continuous time scale produced by the "Bureau International de l’Heure" in Paris. This time scale is based on the readings of approximately 150 atomic clocks across the world.

- UTC (Universal time Coordinated) is deduced from TAI so as not to lose the relation between time and Earth‘s orientation (defined by UT1). UTC and TAI differ by a whole number of seconds, and adjustments are made whenever UTC deviates by more than 0.9 second from UT1. The quantity UT1-UTC is available on the IERS web site.

- ET (Ephemeris Time) is a time scale originally based on Earth motion around the Sun. It is defined by: ET = TAI + 32.184 seconds

The time scale used in CelestLab is generally UTC. For accurate computation (in relation to reference frame conversions), an additional argument (UT1-UTC) sometimes exists. Functions that define (Earth) sidereal time have time arguments relative to UT1.

Most functions in CelestLab have time arguments defined as "modified julian days from 1950.0", that is, number of (fractional or decimal) days from 1st january 1950 0h (generally in the UTC time scale).

Conversions to calendar dates (gregorian calendar) are possible using dedicated functions.

Other types of Julian days defined in CelestLab are :

- JD (astronomical Julian days) : days are counted from January 1st, -4712 12h (julian calendar)

- MJD (modified) Julian days) : MJD = JD - 2400000.5.