In astronomy, straight ascension (often referred to as AR, or with the first Greek letter α, or even RA from the beginnings of its English right ascension) is a term associated with the equatorial coordinate system. Right Ascension is similar to longitude, but projected on the celestial sphere rather than on the Earth's surface. It is defined as the angular distance between the fundamental meridian (equinocial colu) and the meridian passing through the object chosen, measured along the parallel to the celestial object. The zero corresponds to the first point of Aries (spring equinox point). The right ascension is measured in hours, minutes and seconds, corresponding to the Earth's rotation: 24 hours of straight ascension are a full turn. An hour equals 15 degrees (note that straight ascension uses sidereal and not civilian time).
Right ascension can be used to find the position of a star and to calculate how long it will take to find itself at some point in the sky. For example, if a star with AR = 01 30 00 is in zenith, a star with AR = 10 00 00 will be on the vertical in 8 hours and 30 minutes sideways. Due to the slow movements of the Earth (first of all the equinox precession), the coordinate system on which the right ascension is based changes slowly over time, and it is necessary to specify the astronomical epoch that we are referring to. Notemodify wikitesto Voices correlateemodify wikitesto
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