Celestron 4 - page 25

Up to this point, this manual coveredthe assembly and basic operation of your NexStar telescope. However, to

understand your telescopemore thoroughly , you need to know a little about the night sky. Thissection deals with

observationalastronomy in general and includesinformation on the night sky and polar alignment.

To help find objects in the sky, astronomersuse a celestial coordinate system that is similar to our geographical

coordinate system here on Earth. Thecelestial coordinate system has poles, lines of longitude and latitude, and an

equator. For the most part, these remain fixedagainst the background stars.

The celestial equator runs 360 degrees aroundthe Earth and separates the northern celestial hemisphere from the

southern. Like the Earth's equator, it bears a reading of zero degrees. OnEarththis would be latitude. However,in the

skythis is referred to as declination, or DEC for short. Linesof declination arenamed 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 carrya minussign (-) in front of the coordinate and those north of the

celestialeq uator are either blank (i.e., no designation) or preceded bya plu s sign (+).

The celestialequivalent 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. Si nce the

Earth rotatesonce every 24 hours, there are 24 lines total. Asa result, the R.A. coordinates are marked off in units of

time. It begins with an arbitrary point in the constellationof Pisces designated as 0 hours, 0 minutes, 0 seconds. All

otherpoints are designated by how far (i.e., how long) they lag behind this coordinate afterit passes overhead moving

towardthe west.

Figure6-1

Thecelestial s phere seen from the outside showing R.A. and DEC.