CT2LST
Name
CT2LST
Purpose
To convert from Local Civil Time to Local Mean Sidereal Time.
Calling Sequence
CT2LST, Lst, Lng, Tz, Time, [Day, Mon, Year]
or
CT2LST, Lst, Lng, dummy, JD
Inputs
Lng - The longitude in degrees (east of Greenwich) of the place for
which the local sidereal time is desired, scalar. The Greenwich
mean sidereal time (GMST) can be found by setting Lng = 0.
Tz - The time zone of the site in hours, positive East of the Greenwich
meridian (ahead of GMT). Use this parameter to easily account
for Daylight Savings time (e.g. -4=EDT, -5 = EST/CDT), scalar
This parameter is not needed (and ignored) if Julian date is
supplied. ***Note that the sign of TZ was changed in July 2008
to match the standard definition.***
Time or JD - If more than four parameters are specified, then this is
the time of day of the specified date in decimal hours. If
exactly four parameters are specified, then this is the
Julian date of time in question, scalar or vector
Optional Inputs
Day - The day of the month (1-31),integer scalar or vector
Mon - The month, in numerical format (1-12), integer scalar or vector
Year - The 4 digit year (e.g. 2008), integer scalar or vector
Outputs
Lst The Local Sidereal Time for the date/time specified in hours.
Restrictions
If specified, the date should be in numerical form. The year should
appear as yyyy.
Procedure
The Julian date of the day and time is question is used to determine
the number of days to have passed since 0 Jan 2000. This is used
in conjunction with the GST of that date to extrapolate to the current
GST; this is then used to get the LST. See Astronomical Algorithms
by Jean Meeus, p. 84 (Eq. 11-4) for the constants used.
Example
Find the Greenwich mean sidereal time (GMST) on 2008 Jul 30 at 15:53 pm
in Baltimore, Maryland (longitude=-76.72 degrees). The timezone is
EDT or tz=-4
IDL> CT2LST, lst, -76.72, -4,ten(15,53), 30, 07, 2008
==> lst = 11.356505 hours (= 11h 21m 23.418s)
The Web site http://tycho.usno.navy.mil/sidereal.html contains more
info on sidereal time, as well as an interactive calculator.
Procedures Used
jdcnv - Convert from year, month, day, hour to julian date
Modification History
Adapted from the FORTRAN program GETSD by Michael R. Greason, STX,
27 October 1988.
Use IAU 1984 constants Wayne Landsman, HSTX, April 1995, results
differ by about 0.1 seconds
Longitudes measured *east* of Greenwich W. Landsman December 1998
Time zone now measure positive East of Greenwich W. Landsman July 2008
Remove debugging print statement W. Landsman April 2009