A coordinate system designed to give the stars fixed coordinates. It is split into right ascension (ra) and declination (dec). Declination measures from the north celestial pole to the south celestial pole in units of degrees, +90 to -90, modelled on latitude.

Ra is measured in hours, the sphere is split into 24 hours each hour into minutes and seconds. A star (apart from the sun) of a given ra will appear in the same place in the sky at the same time of the day. The time of year will dictate whether or not the sun will also be in the sky rendering the star invisible.

The principal axis of the celestial sphere is aligned with the axis of rotation of the earth. The pole star, Polaris, is so called because it is the most resaonably bright star closest to this axis on the sky. There is no southern equivalent.

The zero point of ra is set so that it corresponds to 0 latitude on the summer solstice.