Specifications

AUTOSTAR CCD PHOTOMETRY 53
Alternatively, the hour angle of a star can be determined directly
from the telescope's right ascension setting circle (if so equipped)
by noting the difference in right ascension between the star and the
meridian.
Because the celestial equator intersects the observer's horizon at
the due east and west points, a star with δ = 0° (on the celestial
equator) is above the horizon for exactly 12 hours; the star rises
with HA = –6 hour and sets with HA = +6 hours. For observers in
the Earth's northern hemisphere, a star with δ < 0° spends less than
12 hours above the horizon each day, while a star with δ > 0° is
visible more than 12 hours per day. As a final comment, note that
circumpolar stars never rise nor set, so that their hour angle can
range from –12 hours (12 hours East) to +12 hours (12 hours
West). The hour angle is zero for stars at upper culmination.
Determining the Air Mass
Now that we know the HA for the observation, the value of secZ
and thus the Air Mass, X, can be calculated:
secZ = (sinLat sinδ + cos Lat cosδ cosHA)
-1
Latitude for HPO (Phoenix, Arizona):
Lat = 33° 30m 06.0s North or +33.50166667 degrees
Declination and Right Ascension for α Aur:
δ = +45.998056 degrees
HA = –54.060105 degrees
Thus:
secZ = 1.4061942273781
To determine Air Mass, X:
X = secZ – 0.0018167 (secZ – 1) – 0.002875 (secZ – 1)
2
– 0.0008083 (secZ – 1)
3
X = 1.404928