GV-8 PHOTOGRAMMETRIC ENGINEERING 
chart. The preset azimuth (Az), vertical angle (A), and radio times converted 
to sidereal time are employed in these preliminary reductions: 
1. Right Ascension (RA). The right ascension of a star is the angle measured 
eastward in the plane of the celestial equator from the celestial meridian passing 
through the vernal equinox to the celestial meridian passing through the star. 
RA = LST — t (time) 
where 
LST is the right ascension of the observer's meridian and is measured in the 
same manner. 
t is the hour angle of the star. 
The hour angle of the star is the angle measured westward in the plane of 
the celestial equator from the observer's meridian to the meridian passing 
  
FiG. 3. Astronomic notation. 
through the star. The hour angle, like the right ascension, may vary from 0° 
to 360°, or 0^ to 24" sidereal time. The hour angle is computed with the cosine 
formula: 
sin # — sin ó sin $ 
cos | = — —— eters 
cos Ó COS $ 
where 
h — altitude of star, 
6 =declination of star, and 
¢ =latitude of observer. 
The declination of a star is the angle measured in the plane of the star’s 
meridian above or below the celestial equator. The latitude of the observer is 
the angle measured in the plane of the observer's meridian above or below the 
equator. These various notations and the equation for cos ¢ are illustrated in 
Figure 3.