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solar_l(jd [, /full])
Returns the solar l angle (i.e., the heliographical longitude of the
center of the solar disk as would be seen from the center of the Earth)
om degrees, for Julian Ephemeris Date jd (i.e., Julian Date
in terrestrial dynamical time TDT), based on the Carrington rotation.
By default, the angle is reduced to the interval from 0 through 360
degrees. If keyword /full is specified, then the angle is not so
reduced. In that case, the number of full rotations that must be added
to obtain an angle between 0 (exclusive) and 360 degrees (inclusive)
indicates the number of the Carrington rotation.
For example, solar_l(calendar([2000,1,1]), /full) yields
-704511.9511 degrees (truncated). If 1957 full rotations (of 360
degrees each) are added to this number, then we obtain 8.0489 degrees
which is between 0 and 360, so on 0 TDT on 1 January 2000 we are in
Carrington rotation number 1957.