Equatorial coordinate system

The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention.[1][2]

The equatorial coordinate system using spherical coordinates. The fundamental plane is formed by projection of Earth's equator onto the celestial sphere, forming the   celestial equator. The primary direction is established by projecting Earth's orbit onto the celestial sphere, forming the   ecliptic, and setting up the ascending node of the ecliptic on the celestial equator, forming the vernal equinox. Right ascension is measured eastward along the celestial equator from the equinox, and declination is measured positive northward from the celestial equator. (Two such coordinate pairs are shown here.) Projections of the Earth's north and south geographic poles form the north and south celestial poles, respectively.

The origin at the centre of Earth means the coordinates are geocentric, that is, as seen from the centre of Earth as if it were transparent.[3] The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.

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