Equatorial coordinates of a direction

Equatorial coordinates of a direction are the coordinates of the direction referred to the celestial equator and the origin direction of this plane defined by the equinox. These coordinates are called true when they are referred to the true equator and the true equinox of the date, mean equinox of date and mean of a reference date when they are referred to the mean equator and mean equinox of that reference date (see True coordinates and Mean coordinates). Two kinds of equatorial coordinates are used: Cartesian equatorial coordinates and polar equatorial coordinates with right ascension and declination.

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