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Old 03-01-2010
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Originally Posted by magnusmurphy View Post
What is the difference between GHA and RA (right ascension)? It sounds like the same thing to me. Is it?
The two coordinates are similar in that they measure east-west angular distances, but beyond that they are quite different.

The first difference is that GHA is an angular distance relative to the terrestrial prime meridian. The GHA of a star changes throughout the day, but the GHA of a geostationary satellite is constant.

RA, however, is measured relative to the celestial prime meridian or first point of Aries. The RA of a star barely changes at all during the day (it may change over a period of centuries because stars do move).

The second difference is the direction in which they are measured. GHA, like all hour angles, is measured westward from the Greenwich meridian to the body---think of an hour angle as representing the amount of time since a celestial body has passed a given point. RA is measured eastward (probably because that's the direction angles are measured in in righthanded coordinate systems).

The last distance is that mariners don't use RA; astronomers and maybe rocket scientists do

Also, is the celestial meridian (defined as the line extending from the North Celestial pole through the vernal equinox to the South Celestial Pole), the same thing as the Aries meridian?
There are many celestial meridians and one celestial prime meridian. Aries, a constellation, doesn't really have one meridian, but the first point of Aries does, and it is indeed the point where the celestial equator crosses the celestial prime meridian.
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