Using Equation 28 in Section 3.8.1 of Solar Position Algorithm (Reda, I. and A. Andreas, Jan. 2008), I computed the mean sidereal time at Greenwich for 2003 October 17 for four different times (UT), using a Casio fx-991MS. The computed values were cross-schecked online using the URL Computation of GAST, GMST, and ERA
Equation 28 used to compute GMST is:
v0=280.46061837 + 360.98564736629*(JD - 2451545) + 0.000387933*JC^2 - JC^3/38710000
JC is computed as:
JC = (JD - 2451545)/36525
The computed values of JD and JC are as follows:
1. 00:00:00 UT: 2452929.5 (JD) & 0.037905544 (JC) 2. 05:30:00 UT: 2452929.729 (JD) & 0.037911818 (JC) 3. 11:00:00 UT: 2452929.958 (JD) & 0.037918083 (JC) 4. 19:21:00 UT: 2452930.306 (JD) & 0.037927611 (JC)
However, there is a clear offset between the computed values and the values provided by the aforementioned URL. For example:
1. 00:00:00 UT: 22.32738 hour (Computed) & 1.6726265 hour (URL) 2. 05:30:00 UT: 16.7231 hour (Computed) & 7.1960077 hour (URL) 3. 11:00:00 UT: 11.30528667 hour (Computed) & 12.702743 hour (URL) 4. 19:21:00 UT: 2.93042 hour (Computed) & 21.075605 hour (URL)
Moreover, if I subtract 22.32738 hour from 24.0, I will get 1.67262, which is the value provided by the URL.
At the same time, using the same set of equation , if I compute GMST for 1987 April 10 at 00:00:00, the computed and URL values are the same.
April 10 at 00:00:00:
JD=2446895.5 JC=-0.127296372 GMST = 13.18 hour (Computed) & 13.17954 hour (URL)
Kindly help me to understand where exactly lies the problem. If the problem lies with the concept regarding the interpretation, please help for a better understanding.
Note: Knew gmst is now deprecated , but since i am a beginner thought of understanding gmst foremost and then will dwell into current scenario.