Astronomy Basics; The Celestial Coordinate System - Celestron AstroMaster LT Instruction Manual

ASTRONOMY BASICS

Up to this point, this manual covered the assembly and basic operation of your telescope. However, to un-
derstand 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.
For telescopes with equatorial mounts, the users have setting circles and polar alignment methods to help
them find objects in the sky. With your altazimuth mount, you can use a method called "star hopping"
which is described in the "Celestial Observing Section" later in this manual. Good star maps are essential in
helping you locate deep sky objects and current monthly astronomy magazines will help you locate where
the planets are.

THE CELESTIAL COORDINATE SYSTEM

To help find objects in the sky, astronomers use a
celestial coordinate system that is similar to our
geographical co-ordinate 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 back-
ground 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.
Figure 4-1
Lines of declination are named for their angular
The celestial sphere seen from the outside
distance above and below the celestial equator.
showing R.A. and DEC.
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 lon-
gitude, 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.
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