Talk:Right ascension
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To understand the origin of the expression right ascension
The altitude-azimuth (alt-az) coordinate system is appropriate for localization of terrestrial objects but the earth's rotation causes celestial objects to move with time both in altitude and azimuth at a variable rate making it really difficult to "track" objects. Alt-az coordinates of an object vary with time and are different for different places on earth.
It is very convenient to consider a system in which only a single coordinate depends on time. One way is to tilt the alt-az mounting of the observing instrument by an angle equal in order that it is aligned parallel to the earth's axis. This polar aligned axis is called the declination or dec axis which now points toward the celestial pole instead of the zenith. Now once a sky object is located it can be tracked with a single motion (the old azimuth motion).
Declination runs from d=+90° at the north celestial pole through zero at the celestial equator to -90° at the south celestial pole. A given declination is represented by a circle of declination on the celestial sphere except for zero declination, the greatest circle called the celestial equator. At latitude f declination d=f is always on the zenith.
The meridian circles of earth localization system are replaced by great circles of hour angles measured east (or west) of the local meridian. Each hour corresponds to 15° of arc along the celestial equator. The projection of the local meridian on the celestial sphere is zero hours. Rising objects are east and setting objects west. Due east on the eastern horizon (which contains the celestial equator) is six hours east, the western horizon six hours west. A celestial object with HA 2 hours east (HA=2h00mE) will cross the meridian in two hours. The ancient astronomers measured this quantity referred to the rising point on the horizon and called it ascension.
In the RA-DEC system, now used, the great circles of HA are fixed on the celestial sphere with the zero point defined as one of the intersections of the celestial equator and the ecliptic, the vernal equinox. This coordinate is called right ascension and increases to the east from zero hours at the vernal equinox around the celestial sphere through 24 hours (360°). RA and HA are both measured in hours (h) minutes (m) and seconds (s) of time. The sky is divided into 24 hours so each hour corresponds exactly to 15° of arc (angle), each minute 15' of arc and each second 15" of arc. One degree of arc corresponds to 4 miutes of time.
[edit] What A Terrible Diagram
The illustrative diagram used for both RA and Dec is horrible! It falls into the category of "crummy diagram for somebody who already knows the subject, and for someone who does not: Just about useless". I mean this in the nicest possible way: It would be great if it could be replaced by one or better yet about three diagrams that serve to illustrate the point better.
First of all the declination angle looks like a vector sticking up from the equator. Second, the ecliptic plane as a heavy yellow band is visually emphasized over the equatorial plane which is the entire point of the coordinate system.
