Specifications

AUTOSTAR CCD PHOTOMETRY 49
Where: LST is the local sidereal time and α is the star's Right
Ascension. The LST is simply the Right Ascension of a star on the
observer's celestial meridian at the time of the observation.
Observer's Southern Horizon
3 hr
1.5 hr
0 hr
Observer's Meridian
(North -South Line)
+
+
+
West
East
Figure B-3. Illustration of Star's Hour Angle for Northern
Hemisphere Observers Facing The Southern Horizon.
Note: The star on the meridian may or may not be the star being
observed, and is referred to only to define LST.
Determining Local Sidereal Time (LST)
The LST can be calculated; however high accuracy is needed to
determine an accurate LST. Most computers and computer
programs have a difficulty in providing sufficient accuracy.
Another approach is to look up the Greenwich Mean Sidereal Time
(GMST) from an Astronomical Almanac for UT = 00:00:00
(GMS
0hUT
), subtract (if West of Greenwich) or add (if East of
Greenwich) the observation longitude (in hours) and then add the
UT of the observation times the factor 1.00274:
LST = (GMS
0hUT
– Long + 1.00274 * UT)