Imagine that the Earth is at the centre of a huge sphere in space. Then imagine that all the objects we see in the night sky are held at points on the sphere. This imaginary sphere is known as the celestial sphere. We can measure locations on the sphere using coordinates called right ascension (RA) and declination (Dec).

This system is an Equatorial system. This is because the coordinates are relative to the centre of the Earth. They do not depend on the location of the observer or on the time of night of the observation.

However, the Earth's orbit does change over long periods of time. This means that an object's RA and Dec change slightly each year. When using Equatorial coordinates, you must refer to the position of an

on a particular date. The current date used is the J2000.0 epoch. This means the RA and Dec are given relative to their positions on 1st January 2000.

How does it work? 

Just as the Earth has poles and equator, so does the celestial sphere. All points on the celestial equator lie in the same plane as the Earth’s equator. 

The declination (Dec) measures how far an object is above or below the celestial equator. It is an angle, measured in degrees and ranges from -90° to 90°. A negative value means south of the equator. Smaller amounts of declination can be expressed in arcminutes and arcseconds. 1 hour of arc = 60 arcminutes and 1 arcminute = 60 arcseconds.

The right ascension (RA) measures the angle around the celestial equator. The angle is measured from a point on the sphere called the vernal equinox. The measurement stops at the point on the celestial equator closest to the object. The RA angle is measured in hours instead of degrees and ranges from 0 to 24 hours. 1 hour of RA = 15°.

Here's an example: how do you write the RA and Dec of the bright star Rigel?

RA = 05 14 32 or 05 hours 14 minutes 32 seconds
Dec = -08 12 06 or -08° 12' (arcminutes) 06" (arcseconds)

What is the vernal equinox? 

This is the point on the celestial sphere where the Sun will be at the equinox. It is also the point where the celestial equator intersects the ecliptic. At the equinox, both the northern and southern hemispheres get roughly equal amounts of daytime and nighttime. The spring vernal equinox is sometimes called the first point of Aries.