29
4.14
SIDEREAL_TIME
void sidereal_time (double jd_high, double jd_low, double ee,
double *gst)
PURPOSE:
Computes the Greenwich apparent sidereal time, at Julian date
`jd_high' + `jd_low'.
INPUT
ARGUMENTS:
jd_high (double)
Julian date, integral part.
Jd_low (double)
Julian date, fractional part.
Ee (double)
Equation of the equinoxes (seconds of time). [Note: this
quantity is computed by function `earthtilt'.]
OUTPUT
ARGUMENTS:
*gst (double)
Greenwich apparent sidereal time, in hours.
RETURNED
VALUE:
None.
Discussion:
This function computes Greenwich sidereal time. To obtain the Greenwich mean
sidereal time, set input argument
ee
= 0.0. To obtain Greenwich apparent sidereal time,
supply the correct value for the equation of the equinoxes (
ee
) which can be computed
by calling function earthtilt.
The input Julian date may be split into two parts to ensure maximum precision in the
computation. For maximum precision,
jd_high
should be set to be equal to the integral
part of the Julian date, and
jd_low
should be set to be equal to the fractional part. For
most applications the position of the split is not critical as long as the sum
jd_high
+
jd_low
is correct: for example, when used with computers providing 16 decimal digits
of precision in double variables, this function will yield values of
gst
precise to better
than 1 millisecond even if
jd_high
contains the entire Julian date and
jd_low
is set to
0.0. For ICRS/IERS compatibility when computing apparent sidereal time at millisecond
precision or better, you should also use function cel_pole and supply the published
celestial pole offsets.
For most uses, the input Julian date should be in the UT1 time scale. If the input
Julian date is in the TDB time scale, the output must be considered to be `dynamical'
sidereal time.
