Definition:Equatorial Coordinate System

Definition

The equatorial coordinate system is an astronomical coordinate system in which measurements are based on the celestial equator.

Hence a point on the celestial sphere is located by means of $2$ angular measurements:

the right ascension $\alpha$, which is the angular distance measured eastwards along the celestial equator from the vernal equinox

the declination $\delta$, which is the angular distance measured north or south of the terrestrial equator.

Also defined as

The equatorial coordinate system can also be defined such that a point on the celestial sphere is located by means of:

the hour angle $t$, which is the angular distance measured westwards along the celestial equator from the vernal equinox, instead of the right ascension

the north polar distance, which is the complement $90 \degrees - \delta$ of the declination $\delta$.

Also see

• Results about the equatorial coordinate system can be found here.

Historical Note

The equatorial coordinate system is the most widely used of the astronomical coordinate systems.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): equatorial coordinate system
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): equatorial coordinate system