# Chapter 2 – Spheres Within Spheres (The Celestial Sphere and Coordinate Positions)

## The celestial sphere and coordinate positions

Just as your Spectrum uses a two-dimensional coordinate system to PRINT and PLOT on the screen, so astronomers use a virtually identical system to represent the skies above our heads. But, whereas the Spectrum TV display is effectively small and flat, the skies have a complete ‘wraparound’ through 360°, both horizontally and vertically. One hemisphere is above our heads and one below our feet making a complete sphere.

The computation of positions on this sphere is relatively complex and requires almost constant use of trigonometry. It is beyond the scope of this book to explain the mathematics involved: in fact, this is deliberately excluded, so that you can simply key in the programs and get results without any knowledge of mathematics or trigonometry.

There are a number of coordinate systems which are used by astronomers. One we can all relate to concerns the relationship between our local horizon and the stars as they pass across the sky. This involves the conversion of, say, a star position from one system to another. A couple of experimental programs follow (experimental in their means of presentation) to explain this relationship. If you would prefer a written explanation first, go to the section headed ‘The celestial sphere explained‘ and return to the programs afterwards.