Calculate the sidereal time (in degrees).
UT Julian day number as double (day)
swe_sidtime returns the sidereal time as double (deg)
Determine the sidereal time.
Section 15 in http://www.astro.com/swisseph/swephprg.htm. Remember that array indices start in R at 1, while in C they start at 0!
swe_sidtime(2451545)#>  18.69714