Astronomy Basics; The Celestial Coordinate System - Celestron CGEM 1100 Instruction Manual

Up to this point, this manual covered the assembly and basic operation of your telescope. However, to understand your
telescope more thoroughly, you need to know a little about the night sky. This section deals with observational astronomy
in general and includes information on the night sky and polar alignment.

The Celestial Coordinate System

To help find objects in the sky, astronomers use a celestial coordinate system that is similar to our geographical coordinate
system here on Earth. The celestial coordinate system has poles, lines of longitude and latitude, and an equator. For the
most part, these remain fixed against the background stars.
The celestial equator runs 360 degrees around the Earth and separates the northern celestial hemisphere from the southern.
Like the Earth's equator, it bears a reading of zero degrees. On Earth this would be latitude. However, in the sky this is
referred to as declination, or DEC for short. Lines of declination are named for their angular distance above and below the
celestial equator. The lines are broken down into degrees, minutes of arc, and seconds of arc. Declination readings south
of the equator carry a minus sign (-) in front of the coordinate and those north of the celestial equator are either blank (i.e.,
no designation) or preceded by a plus sign (+).
The celestial equivalent of longitude is called Right Ascension, or R.A. for short. Like the Earth's lines of longitude, they
run from pole to pole and are evenly spaced 15 degrees apart. Although the longitude lines are separated by an angular
distance, they are also a measure of time. Each line of longitude is one hour apart from the next. Since the Earth rotates
once every 24 hours, there are 24 lines total. As a result, the R.A. coordinates are marked off in units of time. It begins
with an arbitrary point in the constellation of Pisces designated as 0 hours, 0 minutes, 0 seconds. All other points are
designated by how far (i.e., how long) they lag behind this coordinate after it passes overhead moving toward the west.
Figure 5-1
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The celestial sphere seen from the outside showing R.A. and DEC
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