Celestial Equatorial Coordinate System
The Celestial Equatorial Coordinate System is based on the concept of the celestial sphere. The celestial sphere is an imaginary sphere of infinite radius surrounding the earth. Locations of objects in the sky are given by projecting their location onto this infinite sphere. While technically impossible to depict the perspective of looking down on the celestial sphere (being infinite in size), it is often convenient to depict a celestial sphere of finite radius – such as was done with the figure below.
The celestial sphere is fixed with respect to the universe. Its orientation does not change. However, because the earth rotates from west to east (counterclockwise from the perspective of looking down at the north pole), an observer standing on the earth will see the celestial sphere rotate from east to west (or clockwise when looking up at sky).
Like terrestrial coordinates (ie longitude and latitude), two coordinates define a point on the celestial sphere. The rotation of the earth defines a direction in the universe and it is convenient to base a coordinate off that rotation/direction. The celestial equator is the line coplanar with the earth's equator (and 90° to the axis of rotation). The north celestial pole is directly above the earth's north pole and likewise for the south celestial pole. The coordinate indicating where an object is between those poles is declination.
Declination is measured from the celestial equator. It extends from 0° at the celestial equator to +90° at the north celestial pole and from 0° at celestial equator to -90° at the south celestial pole. While declination does not use the designation N and S, latitude and declination are nonetheless closely related coordinates. Declination is depicted by the red line in the figure to the right.
The second coordinate in the celestial equatorial system is right ascension. It is analogous to (but not the same as) longitude. Much has Greenwich is the arbitrary zero point for longitude, right ascension also has a zero reference point. That point is the Vernal Equinox Point. Why that point is a convenient choice is discussed in the next section of the module (Seasons and the Ecliptic).
Because the earth rotates, from the perspective of the earth, the celestial sphere rotates once about every 24 (sidereal) hours. Right ascension, consequently, is measured in (sidereal) hours, 0h to 24h east from the Vernal Equinox Point. That is, east is the direction of increasing right ascension. The half-circle with right ascension 0h is called the 0 hour circle. As a circle is 360°, 1 hour right ascension = 15°