The object itself has not moved – just the coordinate system. In J2000.0 coordinates, this object is at RA = 22h 37m, Dec = +03 o 21’. (Two such coordinate pairs are shown here.) Projections of the Earth's north and south geographic poles form the north and south celestial poles, respectively. This epoch is recorded in the designations of many celestial objects.įor example, the Einstein Cross (2237+0305) was located at RA = 22h 37m, Dec = +03 o05’ using epoch B1950.0. Right ascension is measured eastward along the celestial equator from the equinox, and declination is measured positive northward from the celestial equator. The previous epoch used for many astronomical surveys was B1950.0 – celestial coordinates as they were in 1950. The current epoch is referred to as J2000.0, so that locations of celestial objects are given relative to the coordinates as they were in the year 2000. The result, known as the precession of the equinox, is a gradual change in the location on the sky where the Sun crosses the celestial equator. I understand that Right Ascension is a longitude-like celestial coordinate that varies from 0-24hrs, taken from a reference point of the vernal equinox.
Over a period of 26,000 years, the axis traces out a circle on the sky, resulting in a change in the zero line of RA westwards by 50 arcseconds every year. The Earth’s rotation axis does not point towards a fixed point on the celestial sphere, as the gravitational pull of the Moon and Sun cause the axis to ‘wobble’. The apparent motion of the Sun across the sky is really due to the Earth’s daily rotation on its axis (tilted at 23.5 degrees to the plane of the ecliptic) and yearly orbit around the Sun. The zero line of RA is the point where the Sun crosses the celestial equator, moving from southern declinations to northern declinations. This point is located on the celestial equator. The zero point for celestial longitude (that is, for right ascension) is the Vernal Equinox, which is that intersection of the ecliptic and the celestial. Instead, the RA and Dec of an object vary slightly every year, so we need to specify the epoch or time period which we are using as a reference for the celestial coordinate system. These are called hour circles of right ascension and they are analogous to meridians of longitude on the Earth Remember that a point called the vernal equinox (VE) is the origin of our celestial coordinate system. Although this coordinate system is very similar to latitude and longitude used to locate objects on the surface of the Earth, it is not ‘fixed’ on the sky (the celestial sphere). It is alternatively defined as the ascension (point on the celestial equator which rises with a specified celestial object) as seen from any point on the Earth's equator, where the celestial equator intersects the horizon at a right angle.Celestial coordinates of right ascension ( RA) and declination ( Dec) are used to help locate objects on the sky. H = LST − α, where LST is the local sidereal time.Īlong with declination, right ascension determines the direction of a point on the celestial sphere in the equatorial coordinate system. (Alternatively, the local sidereal time is RA of the observers meridian. The RA of the point transiting zenith at any given time is the equivalent of the local sidereal time.
The hour-angle, H, and the right ascension, α, are related by the simple formula 1 Answer Sorted by: 2 The declination of the point overhead (zenith) is the same as the observers latitude.
The sun's right ascension in time is useful to the practical astronomer in regular observatories, who adjusts his clock by sideral time. 1819, William Nicholson, Ascension, entry in American Edition of the British Encyclopedia, Volume 2, unnumbered page,.( astronomy ) The angular distance east of the vernal point (the solar zenith at the march equinox) the celestial equivalent of longitude.Right ascension ( plural right ascensions ) Diagram of a star's right ascension and declination as seen from outside the celestial sphere Noun
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