C+
C NAME:
C	T_GST
C PURPOSE:
C	Calculate Greenwich Sidereal Time.
C	Good for years 1901 through 2099. Accuracy is 0.006 degree.
C CATEGORY:
C	Celestial mechanics
C CALLING SEQUENCE:
	function T_GST(iYr,iDoy,SECS)
C INPUTS:
C	iYr	integer
C	iDoy	integer
C	SECS	real		(iYr,iDoy,SECS) define the universal time (UT)
C OUTPUTS:
C	T_GST	real		Greenwich sidereal time (degrees)
C RESTRICTIONS:
C	Only valid for years 1901 through 2099
C PROCEDURE:
C	Local Sidereal Time	= Hour Angle of the vernal equinox
C				= Right Ascension of zenith
C	For star X : RA(X)+HA(X)=LST
C MODIFICATION HISTORY:
C	Extracted from:
C	Geophysical Coordinate Transformations, C.T. Russell, in:
C		Cosmic Electrodynamics 2 (1971) 184-196
C-
	    integer	iYr
	    integer	iDoy
	    real	SECS

	double precision	DJ

	if (iYr .lt. 1901 .or. iYr .gt. 2099) stop 'T_GST: Year must be in [1901,2099]'

	FDAY	= SECS/86400.0d0
	DJ	= 365*(iYr-1900)+(iYr-1901)/4+iDoy+FDAY-0.5d0
	T_GST	= dmod(279.690983d0+0.9856473354d0*DJ+360.0d0*FDAY+180.0d0,360.0d0)

	return
	end