Dates and time scales
The date types that can be handled in CelestLab are:
JD (Astronomical Julian date): Number of (decimal) days elapsed since January 1st, -4712 12h (julian calendar)
MJD (Modified Julian date): Number of (decimal) days elapsed since November 17th, 1858 0h. (MJD = JD - 2400000.5)
CJD (another modified Julian date): Number of (decimal) days elapsed since January 1st, 1950 0h (CJD = JD - 2433282.5)
CAL (Calendar date): [year;month;day;hour;minute;second]
- Years are counted astronomically. The year 1-BC is 0, 2-BC is -1, ...
- The dates are relative to the Julian calendar before 4th october 1582 24h (Julian calendar), and to the Gregorian calendar after 5th October 1582 0h (Julian calendar), that is, after 15th october 1582 0h (Gregorian calendar).
- Julian day 0 is 1st January -4712 12h (Julian calendar).
References
Jean Meeus - Astronomical Algorithms - 1991
Most time arguments in CelestLab functions are defined as CJD.
Date types can be converted by the function CL_dat_convert.
The time scales most often seen are the following:
TAI (Temps Atomique International or International Atomic Time):
This uniform time scale is produced by the BIPM (Bureau International des poids et mesures) in Paris. It is based on the readings of approximately 150 atomic clocks across the world.
UT1 (Universal Time):
UT1 is a measure of the orientation of the Earth. It is related to the Greenwich mean sidereal time (GMST) by a conventional relationship.
UTC (Coordinated Universal Time):
This is the international standard on which civil time is based.
It is a non-continuous time scale, 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. Leap seconds are introduced whenever UTC deviates by more than 0.9 second from UT1. (typically every few years)
The quantity UT1-UTC is available in IERS bulletin A and B and the quantity UTC-TAI is available in IERS bulletin C. IERS bulletins can be retrieved at: http://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html
TT (Terrestrial Time):
This time scale is a modern astronomical time standard defined by the International Astronomical Union primarily for time-measurements of astronomical observations made from the surface of the Earth.
It is defined by: TT = TAI + 32.184 seconds.
This time scale was previously known as TDT (Terrestrial Dynamical Time) and ET (Ephemeris Time).
TCB (Barycentric Coordinate Time):
This is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the Solar system. It is equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the barycenter of the Solar system: that is, a clock that performs exactly the same movements as the Solar system but is outside the system's gravity well. It is therefore not influenced by the gravitational time dilation caused by the Sun and the rest of the system.
Note: TCB is now the coordinate time scale recommanded for use in the Barycentric Celestial Reference System. However, TDB is still used most of the time (e.g. for ephemerides in the Solar system).
TDB (Barycentric Dynamical Time):
This is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar System. TDB is now (since 2006) defined as a linear scaling of TCB, and a feature that distinguishes TDB from TCB is that TDB, when observed from the Earth's surface, has a difference from TT that is about as small as can be practically arranged with consistent definition: the differences are mainly periodic, and overall will remain at less than 2 milliseconds for several millennia.
Note: TDB is not explictly defined in CelestLab. It is considered (generally) acceptable to use TT instead.
Time scales can be converted by the function CL_dat_scaleConvert.
When time scale matters, time arguments are given relative to a specific time scale called TREF. TREF is defined implicitly in relation to UT1 and TT.
The default setting is such that:
TREF is the same as UTC at the time of CelestLab release, and varies as TAI,
UT1 is the same as UTC at the time of CelestLab release, and UT1-TREF is constant.
It can be changed if necessary:
by modifying (or initializing) the constants %CL_TT_TREF (TT minus TREF) and %CL_UT1_TREF (UT1 minus TREF). This will impact all subsequent calls to CelestLab functions.
by using the optional arguments ut1_tref and tt_tref whenever these arguments are available.