Equatorial Coordinates

The horizon coordinate system (altitude and azimuth) is not convenient for specifying the location of celestial objects because the horizon coordinates of stars and other objects are continuously changing with time (due to the rotation of the Earth). For example, at Sunrise, the Sun is near zero degrees altitude in the East, but a short 6 hours later it is high in the sky, with a completely different altitude and azimuth.

The exception occurs with objects near the North and South celestial poles. These are unique since they are close to the axis of rotation of the Earth and therefore move only in small circular paths. For example, Polaris, the North Star remains at a nearly constant altitude and azimuth. All celestial objects that are not near the poles change position from hour-to-hour.

In the equatorial coordinate system, the coordinates of all celestial objects remain fixed* from hour-to-hour, day-to-day and so on. An object's equatorial coordinates remain the same regardless of from where on Earth the object is viewed. This allows us to create star maps that apply to any place on Earth, or publish the anticipated position of an upcoming comet so that astronomers everywhere know where it is located among the stars.

* Equatorial coordinates change over long periods of time due to precession (wobbling of the Earth). TheSky computes this change in stars’ position for the input date and time. Precession, however, does not change the relative positions of objects with respect to one another.

The equatorial coordinate system used to specify the positions of celestial objects is directly analogous to the latitude-longitude coordinate system used on Earth. In fact, if you were to expand the latitude and longitude grid of the Earth so that it is out beyond all stars, you would have a sphere with identical geometry to the celestial sphere.

We suppose that all stars and deep-sky objects are located on a very large sphere (out beyond all stars). We call this the celestial sphere. For purposes of describing the positions of celestial objects, we consider all stars and deep sky objects to be on the celestial sphere, when actually they are all positioned at varying distances from the Earth.

The geometric relationship of Latitude-Longitude to Declination-Right Ascension.