The Equatorial Coordinate System is used to locate the position of star on Celestial Sphere. The Equatorial Coordinate System is similar to the coordinates of longitude and latitude that we use to locate the position of any place on the surface of Earth. Let us first learn how to locate an observer on the surface of Earth.
We know that Earth moves from West to East along an axis which passes through North and South Poles.
We know that there is an equator which is the great circle that can be obtained by cutting the sphere through a plane passing through Center of Earth and perpendicular to axis of rotation.
Now we have Earth on which poles and equator are marked.
To locate an observer on the surface of Earth, we can measure its angular distance from the equator. On doing so we get a circle centered on the axis of rotation and perpendicular to axis but note that this may not be a great circle means it may not be passing through Center of Earth.
This angular distance is called latitude. Latitude may vary from 0 degree to 90 degree toward North Pole and from 0 degree to 90 degree toward South Pole. In this way we will get a large number of concentric circles with different Latitudes.
Note that we need one more thing to locate an observer because observer may be present on any position on the same circle with particular latitude. As Earth rotates the observer also rotates along his latitude.
So we draw vertical great circles which pass through poles and Center of Earth. These circles will cut the circles of latitudes perpendicularly at large number of points.
These vertical circles are called Longitudes. The Longitudes are measured in degrees. The reference longitude is called the prime meridian which passes through the Greenwich England and corresponds to 0 degree.
The longitudes towards East are measured in +ve degrees. So the longitudes vary from 0 to 360 degrees.
Now we can locate an observer by knowing his latitude and longitude.
We have learned the coordinates of longitudes and latitudes. Now we can move towards equatorial celestial coordinate system which is similar to it.
The Earth and the Celestial sphere are concentric spheres where Earth looks like a points and the celestial sphere looks very large.
As Earth moves from west to east, this large celestial sphere moves from east to west along the same axis which is followed by Earth. We can obtain Celestial North and South Poles by projecting out the axis of rotation of Earth. The celestial equator corresponds to the equator of Earth. We can also consider celestial equator as the shadow of equator if a bulb is lighted up inside Earth.
Similar to latitudes equatorial coordinates also has angular distances measured from celestial equator but they are known as declination. Declination is measured in degrees which vary from 0 to 90 degrees towards North Pole and 0 to -90 degrees toward South Pole.
Similar to longitude, here we have Right Ascension which are vertical circles passing through poles and center and cut the declination perpendicularly. Right Ascension is measured in hours which are different from longitudes. Right Ascension is measured along east direction and 1 h is equal to 15 degree.
360° = 24h
1h = 15°
We can choose any vertical circle as 0 h Right Ascension but as a convention a circles through Pisces is chosen as 0 h.
So now we can locate a star by knowing its declination and Right Ascension. For e.g. a star with 6 h RA and 30 degree declination.
Note that these hours of RA are similar to hours of time means after 1 h the celestial sphere will move by RA of 1 h.