![]() The celestial sphere seems to rotate around our motionless world once in about 24 hours.Ĭloseup of an equatorial mount. Of course Vega doesn't move it's the Earth that's turning. (This does mean that the one-to-one connection between right ascension and longitude is broken the moment after you imagine the lines ballooning out from Earth and printing themselves on the sky the two systems rotate with respect to each other.) Hours and Degrees That's why they can be permanently printed on star maps. Lines of both right ascension and declination stay fixed with respect to the stars. So once a day, Vega passes overhead as seen from the latitude of Kansas City. (By custom, declinations north and south of the equator are called + and – rather than N and S.) This is the declination of the bright star Vega. Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.Īt any other latitude - let's say Kansas City at 39° N - the corresponding declination line crosses your zenith: in this case declination +39°. If you stand on the Earth's equator, the celestial equator passes overhead. They are now called, respectively, declination and right ascension.ĭirectly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination. Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere, as shown at right. ![]() In the case of Earth, these are named latitude and longitude. ![]() Whenever you want to specify a point on the surface of a sphere, you'll probably use what geometers call spherical coordinates. The celestial sphere, with its infinitely large radius, appears to turn daily around our motionless Earth, from which we use telescopes to examine wonders painted on its inside surface. Perhaps for this reason, astronomers are quite comfortable living with both - as long as the two are kept in their proper relationship. ![]() In astronomy, appearances and reality are more different than in any other area of human experience. Never mind that we're on a moving dust mote orbiting a star in the fringe of a galaxy. At any time half of the celestial sphere is above the horizon, half below.Įven today this is how the cosmic setup actually looks. Part of the celestial sphere is always setting behind the western horizon, while part is always rising in the east. The celestial dome with its starry decorations had to be a complete celestial sphere, early skywatchers realized, because we never see a bottom rim as the dome tilts and rotates around the Earth once a day. When a telescope's right-ascension axis is lined up with the Earth's axis, as shown here, the telescope can turn on it to follow the rotating sky. On the celestial sphere, lines of right ascension and declination are similar to longitude and latitude lines on Earth. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".The Earth is at the center of the celestial sphere, an imaginary surface on which the planets, stars, and nebulae seem to be printed. You can find a full list of credits here. The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. The OAE Multilingual Glossary is a project of the IAU Office ofĪstronomy for Education (OAE) in collaboration with the IAU Office of Astronomy ![]() This term and its definition is still awaiting approval The longitude angle is typically stated as a time, with 24 hours corresponding to 360 degrees. This longitude is called right ascension. As the analog of the Greenwich meridian on Earth, choose the March equinox, where the Sun's apparent path crosses the celestial equator in spring. Define longitude and latitude on the resulting sphere so that each fixed star position has constant longitude and latitude values. The points directly above Earth's equator form the celestial equator, the point directly above Earth's geographic North Pole is the celestial North Pole (similarly for the South Pole). Description: Right ascension is a celestial coordinate in the equatorial coordinate system that is fixed relative to the fixed stars (the second coordinate is declination). ![]()
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