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Celestron Omni XLT 102ED Manual

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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.
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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.
Figure 4-1
The celestial sphere seen from the outside showing R.A. and DEC
… 23 …


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