By imagining Earth at the center of a celestial sphere, he used a coordinate system similar to latitude and longitude, which had recently been devised, to measure the precise positions of the stars.

Today, I want to talk about the equatorial coordinate system ... If we project Earth’s equator and poles onto the celestial sphere, we get the celestial equator and the north and the south ...

View image: http://sdw.webhop.net/tmp/problem.jpg <BR><BR>I want to step through the 2D coordinates (x,y) in the above picture. Say for example that x=0...1 and y=0 ...

case 1<BR>In 2D coordinate system the sum of the angles of all quadrants=360 degree= 2pi radian<BR>in (x,y) plane<BR>Case 2<BR>In 3D coordinate system (x,y,z) I took 3 possible planes<BR><BR>i.e ...