Celestron CR-150 HD - Manual (Page 28)

28 • Astronomy Basics
A S T R A S T R A S T R A S T R A S T R O N O M Y B O N O M Y B O N O M Y B O N O M Y B O N O M Y B A S I C S A S I C S A S I C S A S I C S A S I C S
This section deals with observational astronomy in general. It includes infor-
mation on the night sky, polar alignment, and using your telescope for astro-
nomical observations.
In order to help find objects in the sky, astronomers use a celestial coordinate
system which 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 now referred to as declination, or DEC for short. Lines of
declination above and below the celestial equator are labeled for their angular
distance from the equator. The lines are broken down into degrees, minutes,
and seconds of arc. Declination readings south of the equator carry a minus
sign (-) in front of the number and those north are often 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. 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.
Your Celestron telescope comes equipped with setting circles that translate
the celestial coordinates into a precise location for the telescope to point. The
setting circles will not work properly until you have polar aligned the telescope
and set the R.A. setting circle. Note that the process of polar alignment sets
the declination setting circle.
The Celestial Coordi- The Celestial Coordi- The Celestial Coordi- The Celestial Coordi- The Celestial Coordi-
nate System
nate System nate System nate System nate System
Figure 5-1 Figure 5-1 Figure 5-1 Figure 5-1 Figure 5-1