Astronomical Applications Department, U.S. Naval Observatory - NOVASC (Page 17)

SIDEREAL_TIME

void sidereal_time (double julianhi, double julianlo, double ee,

double *gst)

PURPOSE:
Computes the Greenwich apparent sidereal time, at Julian date
'julianhi' + 'julianlo'.

INPUT
ARGUMENTS:
julianhi (double)
Julian date, integral part.
julianlo (double)
Julian date, fractional part.
ee (double)
Equation of the equinoxes (seconds of time)

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,

julianhi

should be set to be equal to the integral part of the Julian date, and

julianlo

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

julianhi

+

julianlo

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

julianhi

contains the entire Julian date and

julianlo

is set to 0.0.

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.