Astronomy Basics

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Astronomy BAsics
ENGLISH
The Celestial Coordinate System and Motion of the Stars
sections below are applicable to all types of telescopes, and if
you have a non-computerized altazimuth telescope, these are
the only sections that apply to your telescope.
The sections about Latitude Scale/Pointing at Polaris/Finding
the North & South Celestial Poles/Polar Alignment in the
Northern & Southern Hemisphere, etc. are basic information for
non-computerized equatorially mounted telescopes — actual
telescope models may differ somewhat from the descriptions
about basic functions.
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 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.
The CelesTial CoordinaTe sysTem
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1
0
3
20
19
18
17
21
22
23
60
-60
-40
40
20
0
The celesTial sphere seen from The ouTside showing r.a. and dec