Equatorial coordinate system

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 March equinox, and a right-handed convention.

A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.

Superimposed on this is a smaller motion of the ecliptic, and a small oscillation of the Earth's axis, nutation.

[4] In order to fix the exact primary direction, these motions necessitate the specification of the equinox of a particular date, known as an epoch, when giving a position.

Setting circles in conjunction with a star chart or ephemeris allow the telescope to be easily pointed at known objects on the celestial sphere.

The declination symbol δ, (lower case "delta", abbreviated DEC) measures the angular distance of an object perpendicular to the celestial equator, positive to the north, negative to the south.

Hour angle may be considered a means of measuring the time since upper culmination, the moment when an object contacts the meridian overhead.

When calculating topocentric phenomena, right ascension may be converted into hour angle as an intermediate step.

Model of the equatorial coordinate system. Declination (vertical arcs, degrees) and hour angle (horizontal arcs, hours) is shown. For hour angle, right ascension (horizontal arcs, degrees) can be used as an alternative.
Equatorial (red) and horizontal (blue) celestial coordinates.
As seen from above the Earth 's north pole , a star's local hour angle (LHA) for an observer near New York. Also depicted are the star's right ascension and Greenwich hour angle (GHA), the local mean sidereal time (LMST) and Greenwich mean sidereal time (GMST). The symbol ♈︎ identifies the March equinox direction.
Geocentric equatorial coordinates. The origin is the centre of the Earth . The fundamental plane is the plane of the Earth's equator. The primary direction (the x axis) is the March equinox . A right-handed convention specifies a y axis 90° to the east in the fundamental plane; the z axis is the north polar axis. The reference frame does not rotate with the Earth, rather, the Earth rotates around the z axis.