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Right ascension is measured eastward up to 24 h along the celestial equator from the primary direction. Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth. [1]
Model of the equatorial coordinate system. Declination (vertical arcs, degrees) and hour angle (horizontal arcs, hours) is shown. For hour angle, right ascension (horizontal arcs, degrees) can be used as an alternative. The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects.
For geocentric orbits, Earth's equatorial plane as the reference plane, and the First Point of Aries (FPA) as the origin of longitude. In this case, the longitude is also called the right ascension of the ascending node (RAAN). The angle is measured eastwards (or, as seen from the north, counterclockwise) from the FPA to the node.
German equatorial mount. In the German equatorial mount, [4] (sometimes called a "GEM" for short) the primary structure is a T-shape, where the lower bar is the right ascension axis (lower diagonal axis in image), and the upper bar is the declination axis (upper diagonal axis in image).
The equatorial describes the sky as seen from the Solar System, and modern star maps almost exclusively use equatorial coordinates. The equatorial system is the normal coordinate system for most professional and many amateur astronomers having an equatorial mount that follows the movement of the sky during the night. Celestial objects are found ...
The term Right Ascension took its name from early northern hemisphere observers for whom "ascending stars" were on the east or right hand side. In the southern hemisphere the east is on the left when an equatorial mount is aligned on the south pole. Many Right Ascension setting circles therefore carry two sets of numbers, one showing the value ...
Equations derived from spherical trigonometry allow for the conversion from equatorial coordinates to ecliptic coordinates. As points in the ecliptic have no latitude (β =0º) and the East point of the horizon has a right ascension 6 h higher than that of the meridian (or 90º more in hour angle), the equation that determines East Point's longitude can be written as:
Right ascension (blue) and declination (green) as seen from outside the celestial sphere. The orbiting body direction cosine vector can be determined from the right ascension and declination (from Topocentric Equatorial Coordinate System) of the orbiting body from the observation points via: