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CT2LST

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