![]() ![]() ![]() RA and HA are both measured in hours (h) minutes (m) and seconds (s) of time. 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°). 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. The ancient astronomers measured this quantity referred to the rising point on the horizon and called it ascension. A celestial object with HA 2 hours east (HA=2h00mE) will cross the meridian in two hours. Due east on the eastern horizon (which contains the celestial equator) is six hours east, the western horizon six hours west. Rising objects are east and setting objects west. The projection of the local meridian on the celestial sphere is zero hours. Each hour corresponds to 15° of arc along the celestial equator. The meridian circles of earth localization system are replaced by great circles of hour angles (HA) measured east (or west) of the local meridian. At latitude f declination d=f is always on the zenith. 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. Now once a sky object is located it can be tracked with a single motion (the old azimuth motion).ĭeclination runs from d=+90° at the north celestial pole through zero at the celestial equator to -90° at the south celestial pole. This polar aligned axis is called the declination or dec axis which now points toward the celestial pole instead of the zenith. 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. It is very convenient to consider a system in which only a single coordinate depends on time. Alt-az coordinates of an object vary with time and are different for different places on earth. 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. To understand the origin of the expression right ascension One way to avoid confusion in visualizing either the Equatorial Coordinate System or the Ecliptic Coordinate System is to assume that the Earth does NOT spin, or rotate, around its own axis but only translates along its orbit around the Sun. ![]() And keep in mind that the angular speed of RA is not uniform in time. That's to say, it takes the RA roughly 3 months to advance 6 hours, or 1 year to advance 24 hours. In other words, it completes a 0 o - 360 o cycle in one year, NOT in one day. At the Vernal Equinox, the right ascension (RA) is 0 o at the Summer Solstice, the RA is 90 o at the Autumnal Equinox, the RA is 180 o at the Winter Solstice, the RA is 270 o and at yet another Vernal Equinox, the RA is 360 o, or back to 0 o. contribs) 11:16, 19 September 2010 (UTC) Reply No, it's not right.Preceding unsigned comment added by Slothman32 ( talk Looking up on the September equinox will make a star a little past 12 hours. Similarly, if you wait a month, 1 year/12 days, and do the same thing then it is at 2 hours. If you wait an hour and look up it is at 1 hour R.A. If you are on the Tropic of Cancer and looking directly straight up, at the zenith, on the March equinox, then the object is at 0 hours R.A. ![]()
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