Talk:Right ascension
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[edit] 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. —Preceding unsigned comment added by 165.158.9.93 (talk) 15:20, December 16, 2004
[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. —Preceding unsigned comment added by 131.107.0.73 (talk) 11:49, January 17, 2007
[edit] Rewrites in the second para .
I did an overhaul of the second paragraph, to make it flow better and to make it more accurate. RA is not a "time" as the previous version had all-but-implied; it's an angle that is customarily (but not always!) measured in units of time for the sake of convenience.
Also, I'd never heard the term Sidereal Hour Angle, which is apparently legitimate terminology in celestial navigation. If one were to confuse this with the definition of Hour Angle used in ordinary astronomy, serious confusion would result. Therefore, I added a sentence to clarify this. Jthorstensen 22:53, 29 April 2007 (UTC)
We're all new at this Prof. Thorstensen :) "Since an arc has 360 degrees, an hour of right ascension is equal to 15 degrees of arc, a single minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc." Uh, I'm about to flunk (anything less than an 'A' is flunking) my undergraduate for nonmajors astronomy course since I'd rather learn than follow dogma. Anyway, 'Sine there are 360 degrees in the arc of a full circle, an hour of right ascension is equal to 15 degrees of arc, an arcminute is equal to 15/60 degrees of an arc... 60 seconds in an minute and 60 minutes in a day and 24 hours in 360 degrees up, over, and around the sky? arcminute qv http://en.wikipedia.org/wiki/Arcminute 128.195.84.188 05:14, 10 June 2007 (UTC)David dlevine@uci.edu (soon to be no more...)
