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Glossary of Terms

Glossary of Terms

Right ascension

- Right ascension, measures the easterly distance of an object from the vernal point, along the plane of the celestial equator. In this, it is the celestial equivalent of terrestrial longitude (measuring positions east or west of the equator), in the same way that declination is the celestial equivalent of terrestrial latitude (measuring positions north or south of the equator).

Right ascension makes it easy to use the apparent diurnal rotation of the celestial sphere as a means to telling time. In 24 hours a fixed point in the celestial sphere will move 360° in right ascension; in one hour it will move 15° in right ascension.

Right ascension can therefore be measured in degrees or hours. Whereas terrestrial longitude uses meridians of longitude, right ascension uses hour circles which run between the north and south celestial poles. Any object on the same hour circle will have the same right ascension, just as any place on earth on the same meridian of longitude has the same longitude.

Right ascension and hour circles

A table to convert degrees of zodiacal longitude to their equivalent right ascension is available on this link.

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