This Sidereal Clock uses your computer clock's definition of Coordinated Universal Time (UTC) to compute the corresponding Mean Sidereal Time at the Greenwich meridian of 0°: Longitude.
The program operates automatically. The display is only as accurate as your computer's clock and it assumes that your clock is setup to maintain UTC. Otherwise, the display will be incorrect.
The upper display shows the current day of the year at Greenwich in the leftmost column. The leftmost column of the lower display shows the hour angle of the Vernal Equinox in units of degrees. The sidereal time is the same hour angle expressed in time units.
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The Java Script source code for this program can be viewed by using the View|Source command of your web browser.
You may use or modify this source code in any way you find useful, provided that you agree that the author has no warranty, obligations or liability. You must determine the suitablility of this source code for your use.
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The mean sidereal time at zero longitude is often called Greenwich Mean Sidereal Time or GMST. The formula below is based on Meeus[1] formula 11.4 with terms in the square and the cube of the time left out. GMST is in degrees!
GMST = 280.46061837 + 360.98564736629·d
where
d = UT in days since J2000.0, including parts of a day
Multiples of 360 are subtracted to bring the answer into the range 0 to 360 degrees.
To get the sidereal time at your longitude, known as Local Mean Sidereal Time, just add your longitude in degrees, taking East as Positive, so
LMST = 280.46061837 + 360.98564736629·d + Longitude
and again, subtract multiples of 360 to bring the answer into the range 0 to 360. To convert into time units divide the time in degrees by 15.0 to get the time in hours. Then, multiply the fractional part of hours by 60 to get minutes. Finally, multiply the fractional part of minutes by 60 to get seconds.
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