Keywords: astro

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## Setting the time

Atfter having set the observatory, it is necessary to define the time at which ephemeris must be calculated, using the command:

`TIME hour date`

Formats are ``hh:mm:ss.ss'' and ``jj-mmm-yyyy''. Entering only `TIME` will use the current system time and date. By default, time and date are supposed to be in UTC (Universal Time Coordinated). However, time may also be entered in:

- local time, using the option `/ZONE z` where `z` is the number of hours to be substracted to the local time to convert it to UTC (example: `/ZONE 2` corresponds to Summer Time in Western Europe).

- UT1, using the option `/UT1`. The difference between UTC and UT1 can be modifed using the `SET DUT1 dUT1_value` command. As of January 2008, dUT1 is 0.3 sec (Bulletin D 98 of IERS). By construction of UTC, the value of dUT1 is always lower than 1 sec. The difference is therefore very small for all usual ephemeris computations.

- TDT (Terrestrial Dynamical Time), using the option `/TDT`. The difference between UTC and TDT can be modifed using the `SET DTDT dTDT_value` command. Since January 2006, dTDT1 is 65.184 sec (last introduction of a leap second, Bulletin C 30 of IERS).

Definitions - The definition of the various times used in astronomy is given in a number of excellent textbooks. To make a quite complex story short: time standard is the TAI (International Atomic Time), which is not defined by an astronomical measurement. The TDT (Terrestrial Dynamic Time) is defined as TDT=TAI, the offset being introduced to allow for a continuity with older definitions. UT1 is defined by astronomical measurements of the Earth rotation, corrected for various effects. UT1 differs from TAI, because of the irregularity of the Earth rotation period at the millisecond level, and drifts slowly with time, because of the overall slowing down of the Earth rotation. UTC is defined as TAI + a certain offset, such that UTCUT1 sec, i.e. UTC is an atomic-based time that is tracking the astronomical-based time within one second. Since UT1 drifts compared to TAI (and thus UTC), a leap second in UTC may be introduced on the 1st of January or 1st of June. Hence, the difference UTCUT1 is kept sec.

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