;+ ; NAME: ; GST ; ; PURPOSE: ; Calcualtes Greenwich Sidereal Time ; ; CATEGORY: ; Utility ; ; CALLING SEQUENCE: ; result = GST(year,doy,ut) ; ; INPUTS: ; year Date in yyyy, integer or longword integer ; doy Day of year in ddd, integer or longword integer ; ut Time of day in seconds, UTC, floating point or double precision ; ; OUTPUTS: ; gst Greenwich sidereal time, radians, double precision ; ; COMMON BLOCKS: ; None. ; ; PROCEDURE: ; Greenwich Sidereal Time (the angle between the Greenwich meridian ; and the vernal equinox) is calculated using formula from C.T. Russell. ; Array input is OK. ; Will not work properly before 1900 or after 2100 due to lack of leap year. ; ; REFERENCE: ; C.T. Russell, Geophysical Coordinate Transforms. ; ; MODIFICATION HISTORY: ; 3/15 Adapted from SUNCOR by Stan Solomon ; ;+ function gst, year, doy, ut fday=ut/86400.d iyr=long(year)-1900 iday=long(doy) dj=365*iyr+(iyr-1)/4+iday+fday-0.5 gst=(279.696678+.9856473354*dj+360.*fday+180.) mod 360. * !dpi/180. return, gst end