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Understanding the Celestial Coordinate System
Components
The Celestial Coordinate System is very similar to the Terrestrial
Coordinate System. Use the following table and figure 6-2 to become
familiar with the components of the Celestial Coordinate System.
Term
Celestial
Equator
Declination
Description
The celestial equator is the point of reference for
measuring declination.
Declination can be thought of as the celestial
equivalent to latitude. Declination is expressed in the
same manner as latitude, measured north or south
from 0° through 90°.
First Point of
Aries
The first point of Aries can be thought of as the
reference point for measuring angles for stars and
planets (celestial bodies).
Hour Circles
The great circles that encircle the celestial sphere in
the same manner that meridians of longitude encircle
Earth. The major difference is that hour circles are
measured from 0° westward through 360°. As you
know, longitude on the other hand is measured east
or west from 0° through 180°. Hour circles move
with each celestial body. The 0° meridian is called
the Greenwich meridian.
Greenwich
Hour Angle
(GHA)
Local Hour
Angle (LHA)
GHA is the angular measurement of a celestial body
measured westward 0° through 360° from the
Greenwich meridian.
LHA of a celestial body is measured westward from
0° through 360° from the observers meridian to the
hour circle of the celestial body.
Sidereal Hour
Angle (SHA)
The SHA is the hour circle of a star or planet
measured westward from the first point of Aries from
0° through 360°.
6-3
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