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Greenwich Sidereal Time

This code is a MATLAB script that can be used to calculate Greenwich Sidereal Time, Julian Day

clc;
clear all;
% Input Date: April 11,2013.  UT time 20:11:30
year  = 2013;  month = 4; day   = 11;
hour  = 20; min   = 11; sec   = 30;
UT = hour + min/60 + sec/3600;
J0 = 367*year - floor(7/4*(year + floor((month+9)/12))) ...
    + floor(275*month/9) + day + 1721013.5;

JD = J0 + UT/24;              % Julian Day
fprintf('Julian day = %6.4f [days] \n',JD);
JC = (J0 - 2451545.0)/36525;
Julian day = 2456394.3413 [days]

JC is Julian centuries between the Julian day J0 and J2000(2,451,545.0) Greenwich sidereal time at 0 hr UT can be calculated by this equation [Seidelmann,1992]

GST0 = 100.4606184 + 36000.77004*JC + 0.000387933*JC^2 - 2.583e-8*JC^3; %[deg]
GST0 = mod(GST0, 360);  % GST0 range [0..360]
fprintf('Greenwich sidereal time at 0 hr UT %6.4f [deg]\n',GST0);
Greenwich sidereal time at 0 hr UT 199.3719 [deg]

Greenwich sidereal time at any other UT time

GST = GST0 + 360.98564724*UT/24;
GST = mod(GST, 360);  % GST0 range [0..360]
fprintf('Greenwich sidereal time at UT[hours] %6.4f [deg]\n',GST);
Greenwich sidereal time at UT[hours] 143.0761 [deg]

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