Right ascension

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Right ascension and declination on the celestial sphere (inside the globe)

The right ascension α or a ( Latin ascensio recta , straight ascent) is one of the two polar coordinates in the rotating equatorial coordinate system of spherical astronomy . Its second coordinate is the declination .

The right ascension is the angle measured on the celestial equator between the vernal equinox and the point of intersection of the hour circle that goes through the celestial body to be measured . It is the analogue of the geographical longitude of a place on the globe (second coordinate: geographical latitude ).

Right ascension is more often given in terms of time than degrees or radians . The indication of time refers to the fact that the apparent rotation of the stars around the earth is proportional to the time and time differences can be determined more easily than angles. If a star z. For example, when the equator rises one star hour after the vernal equinox, it has the right ascension 1h, corresponding to 15 ° (in a star day with 24 star hours the stars seem to move on a full circle with 360 °).

Word meaning

The word right ascension is the abbreviation of recta ascension , the reverse of the Latin ascensio recta . In English, the term right ascension (common abbreviation RA ) is used as a literal translation of the old Latin term. Also the literal translation from the Latin was used in the older German literature, namely straight rise or Geradeaufsteigung .

Ascensio recta refers to how the ascent ( ascensio, ascension, or ascent ) of a star occurs above the horizon at the Earth's equator , namely at right angles ( recta, right, or straight ) to the horizon. At the moment of the star's rise, its longitude coincides with the horizon circle. The point of intersection of this longitude with the celestial equator running vertically there is the east point of the equatorial horizon and thus the momentary point up to which the right ascension of the star from the vernal point is to be measured.

Equatorial coordinate system

Celestial sphere with equator and ecliptic circle

The spring equinox serves as the zero point of the right ascension. Like longitude, it increases towards the east.

In the celestial navigation and the widening to the west will take the right ascension star angle (Engl. Sidereal hour angle (SHA) ) is used.

In the stationary equatorial coordinate system, the hour angle is used instead of the right ascension .

The right ascension α and the declination δ correspond in the ecliptical coordinate system to the ecliptical length  λ and the ecliptical latitude  β. The ecliptic  inclination ε is used for the conversion , see the small triangle on the right of the sketch. This coordinate transformation is essential for calculating the orbits of planets and other bodies in the solar system .

Individual evidence

  1. For the observer on earth, the point of reference of the vernal point rotates on the celestial equator .
  2. ^ Friedrich Wilhelm Bessel: Astronomical investigations. Volume 2, Königsberg 1842, p. 107 ( online )
  3. ^ John Lathrop: A Compendious Treatise on the Use of Globes and Maps . Wells and Lilly and JW Burditt, Boston. P. 39 (187.). 1821. Retrieved June 11, 2012.

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