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The celestial sphere was earlier described as ballooning outward of the longitude and latitude lines of the earth to a distance of billions of miles beyond it. We look at the celestial sphere as tiny creatures would at a transparent globe while suspended at its center. The lines of the celestial sphere have no more reality, or less, than the lines of longitude and latitude imagined around the planet Earth. Up until now we have been using the celestial sphere to help follow the movement of the stars and planets through the night sky without specifying exact points on the sphere.
Celestial sphere coordinates are frequently used to specify locations of the planets, the Moon and Sun, and comets. With a minimal knowledge of celestial sphere coordinates, you will be able to find objects in the sky relative to the fixed stars.
The celestial equivalents of latitude and longitude are declination and right ascension and will be found on any map of the celestial sphere. If you read that such and such planet is at declination –16 degrees 40 minutes, right ascensions 21 hours and 3 minutes, you can then go to a sky atlas and find its exact location among the fixed stars. The fixed stars also have a declination and right ascension except that theirs is almost constant over a period of years.
Declination is similar to latitude. Its number from +90 to –90 degrees, with each degree divided into 60 minutes and each minutes into 60 seconds. The North Celestial Pole has a declination of +90 degrees and the declination of any object along the celestial equator is zero degrees. A declination between the celestial equator and the Celestial South Pole has a negative declination. For instance, the brightest stars of the Southern Cross, Acrux, has a declination of –63 degrees.
Right ascension is the celestial equivalent of longitude. Instead of being specified in degrees, right ascension is traditionally measured from 0 to 24 hours, with each hour being divided into 60 minutes and each minute into 60 seconds.
Thanks to greater use of computers, many have abandoned the minutes and seconds and express both declination and right ascension as a number. So a declination of +45 degrees and 15 minutes would be equivalent to 45.25 while a right ascension of 12 hours would be equal to 180.0.
Every point on the celestial sphere has a unique declination and right ascension just as every point on Earth has a unique longitude and latitude. Declination, right ascension and apparent magnitude are the primary tools for describing objects in the sky. As explained in the chapter of the celestial sphere, the grid formed by the lines of the celestial coordinates is only an arbitrarily designed system to help us locate objects in the sky.
Learning to use a telescope is a bit like learning to drive. At first the controls of the care were difficult to use. Little by little, you gained proficiency and can now drive, listen to the radio and carry on a conversation at the same time. When you first use a telescope and its mount, everything will feel unfamiliar. After some practice, operating your telescope will become almost automatic and you will be able to concentrate on the beauty of the solar System, galaxy and beyond.