Calculating Local Sidereal Time from local time?

Question

I'm trying to write a Unix programming script that will calculate Local Sidereal Time (LST) for particular places and times. I know you need the longitude of the location, but I don't know how to actually calculate LST from Longitude and local time.

Thanks

Answer 1

Greenwich Sidereal Time (in degrees) is given by the formula:

GST=280.461+360.98564737D

where D = number of days (of Universal Time)  from Greenwich Mean Noon on January 1st 2000, equivalent to Julian Day 2451545.0

There are various online calculators for the Julian date such as http://www.nr.com/julian.html or you may be able to write your own script for this conversion as well. The formula ignores the difference between mean and apparent sidereal time (always less than 2 seconds) and the difference between UT and UTC (always less than 1 second).

Worked example:

Find the Sidereal Time at Los Angeles (118.25 degs W) at 11 pm PDT (10pm PST) on July 1st, 2008.

UT = PST + 8 hours, so the equivalent UT date is July 2nd at 6h. This converts to Julian Date 2454649.75 (the online calculator I referred to gives the JD as 2454650, which is the date starting at noon on July 2nd).

Days elapsed since Greenwich Mean Noon on January 1st, 2000 is 2454649.75 - 2451545.0 = 3104.75. Substituting into the formula:

GST = 280.461+360.98564737 x 3104.75 = 1121050.650 degrees. Eliminating multiples of 360 leaves GST = 10.650 degrees.

Subtracting the longitude of Los Angeles, we have 10.650 - 118.25 = -107.60. Addng 360 degrees gives 252.40 degrees (or 16h 49m 36s) as the local sidereal time at Los Angeles.

Answer 2

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Answer 3

isn't this backwards? first you get the algorithm, then you implement it.

like so many of these calculations,  try astronomical algorithms by meeus.