Node: Celestial coordinate system, Next: , Up: Basic astronomical terms

### 3.1 Celestial coordinate system

On a sheet of paper, every point has an x and a y coordinate. On planet Earth, every location has a longitude and a latitude.

With a very similar system astronomers give the coordinates of every point in the sky. The sky longitude is called rectascension and the sky latitude is called declination. There is a sky North Pole (near the Polar Star), a sky South Pole, and a sky equator. You can make globes of the sky like of the Earth, with the only difference that you must imagine being inside the globe.

So, in a way, the rectascension is the x coordinate. It is given in `hours' (h) from 0h to 24h. Since the sky is circular, both 0h and 24h are the same rectascension, and 12h is the opposite to it.

Usually the fraction of the rectascension that is smaller than 1h is given in minutes and seconds, however in PP3 the rectascension is one decimal fraction number.

The declination corresponds to the y coordinate. It is measured from -90^\circ (South Pole) over 0^\circ (equator) to +90^\circ (North Pole).