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Right ascension and declination are the components of a coordinate system that are used to identify the position of celestial objects in the sky. They are very similar in concept to longitude and latitude on the earth's surface. It is convenient to think of the earth as sitting at the center of a great hollow sphere, and the celestial objects are being pinned to the inside surface of that sphere. This is not reality of course- all the stars and planets, etc are at widely varying distances from earth. But when you look up at the night sky it definitely appears that way. Hence RA and declination tell you where an object is on the inside surface of that sphere, but do not tell you how far away from us that object is.
As the earth rotates is appears to us like we are standing still and the celestial sphere is rotating above us, as if it was spinning on an axis. The axis intersects the sphere at the north and south poles. These two points are directly above the earth's north and south poles. Midway in between is the celestial equator, which again is directly above the earth's equator. Declination is measured in degrees north or south of the equator, similar to latitude on the earth's surface. The north celestial pole is +90 degrees declination and the south pole is -90 degrees. Objects on the equator are at 0 degrees declination. Right ascension is the analog of longitude - it measures the angular distance of the object to the left or right of a defined point on the equator (just as longitude is measured from the Greenwich meridian on earth). One difference is that the angular distance along the equator is measured not in degrees but in hours, minutes, and seconds. One hour is 1/24 of the complete 360 degrees, or is equivalent to 15 degrees. One minute is 1/60 of an hour, and a second is 1/60 of a minute. The base point for measuring RA is taken to be one of two points where the equator intersects the ecliptic - which happens to be in the constellation Aries. That's it. Hope this helps. Here are two articles that go into a bit more detail: Declination - Wikipedia, the free encyclopedia Right ascension - Wikipedia, the free encyclopedia |
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