Paramount User Guide
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Right Ascension
The geometry of the right ascension (RA) lines on the celestial sphere is the same as the longitude lines
on Earth. Longitude lines divide the Earth into 360 degrees. Right ascension lines divide the celestial
sphere into 24 hours, based on one revolution of Earth. Therefore one hour of right ascension equals 15
degrees (360 divided by 24). See the definition of Local Sidereal Time for additional information on why
24 hours are used for right ascension instead of 360 degrees.
Zero degrees longitude passes through Greenwich, England and is the designated reference line for
longitude. What, then, is the reference line for zero hour’s 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 similar to latitude lines on 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, the North Star has a declination
of just over 89 degrees so it is very close to 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). A celestial object on the meridian is located at
hour angle 0. Six hours of time later, the object is located at hour angle +6. Twelve hours later, when the
object rises, its hour angle is –6 h.
