Specifications

48 AUTOSTAR CCD PHOTOMETRY
Similarly, a star situated at the South Celestial Pole (SCP) would
have δ = –90°, while those between that pole and the celestial
equator have declinations of –90° ≤ δ ≤ 0°.
NCP
Earth
Equ
a
t
o
r
+90
-90
0
0
+30
-30
+
+
Celes
t
rial
SCP
o
o
o
o
o
o
Figure B-2. Illustration of a Star's Declination.
Determining a Star's Hour Angle (HA)
As illustrated in Figure B-3, The observer's celestial meridian is
the north-south line passing directly overhead (i.e., through the
Zenith). The hour angle, HA, of a star is the amount of time since
the star crossed the celestial meridian or until the crossing will
occur. Stars East of the meridian are designated either as a
negative value or with the symbol E, while stars to the West have
positive values or a symbol W. The HA of a star increases with
time as the celestial sphere rotates.
Mathematically, the Hour Angle is defined as:
HA = (LST – α) hours
Note: HA is defined in hours, but must be converted to degrees
(angle). To produce this, multiply HA in hours by 15 (the sky
appears to rotate 15 degrees per hour).
HA = (LST – α) * 15 degrees