ConvertRADecToThetaPhi - function to convert right ascension and 
        declination of an astronomical source to theta and phi in the geocentric 
        coordinate system. 
            
        RA - Right ascension of source. 
        Dec - Declination of source. 
        GPSTime - GPS time that the GW signal from the source reached the
		  geocenter. 
                
        Returns Theta - Polar angle of source's sky position in geocentric 
                        coordinates. 
                Phi - Azimuthal angle of source's sky position in geocentric 
                      coordinates. 
            
        Sarah Gossan 2012.

def ConvertRADecToThetaPhi(RA, Dec, GPSTime): 
    """ ConvertRADecToThetaPhi - function to convert right ascension and 
        declination of an astronomical source to theta and phi in the geocentric 
        coordinate system. 
            
        RA - Right ascension of source. 
        Dec - Declination of source. 
        GPSTime - GPS time that the GW signal from the source reached the
		  geocenter. 
                
        Returns Theta - Polar angle of source's sky position in geocentric 
                        coordinates. 
                Phi - Azimuthal angle of source's sky position in geocentric 
                      coordinates. 
            
        Sarah Gossan 2012. """ 
                
    # Get Greenwich mean sidereal time corresponding to GPSTime 
    GMST = ComputeGMST(GPSTime) 
            
    # RA -> Phi
    Phi = RA - GMST
            
    # Dec -> Theta
    Theta = np.pi/2. - Dec
            
    return Theta,Phi