Paramount GEM User Guide
17 | P a g e
Zero degrees longitude passes through Greenwich, England and is the designated reference line for
longitude. What, then, is the reference line for 0 hours right ascension? Astronomers use the vernal
equinox, the location where the Sun crosses the celestial equator during its apparent annual motion
against the background stars, as a "starting point" for right ascension.
The term "right ascension" comes from the fact that when viewed from the equator, all stars rise (or
ascend) at right angles to the horizon, so their times of rising are called their times of right ascension.
Figure 2: Right ascension and declination relative to the celestial sphere.
Declination
The declination (Dec) lines on the celestial sphere are like latitude lines on a globe of the Earth, ranging
from –90 degrees to +90 degrees. The “declination” of an object is the angle measured from the celestial
equator (0 degrees declination) along a meridian line through the object. Polaris has a declination of just
over 89 degrees, placing it near the north celestial pole.
Hour Angle
Hour angle (HA) is measured from the meridian westward, along the celestial equator and uses the same
units as right ascension (hours, minutes, and seconds). An object on the meridian is located at hour angle
0. Six hours of time later, the object is located at hour angle +6. When the object rises, its hour angle is
–6 h.
Hour angle can be used to describe a telescope’s mechanical orientation with respect to a horizon-based
hemisphere. For Paramount mounts with AutoHoming™, when the Find Home command is completed,
the resulting home position of the telescope is a fixed, mechanical mount orientation. In the northern
hemisphere, when homing is successful, the mount always points to hour angle 0 and declination 0 with
the telescope on the east side of the pier. The home position for pre-Series 6 mount is hour angle 2,
declination 0.
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