## Celestial sphere

The only purpose of the present article is to recall the description of the celestial sphere commonly used in astronomy to represent astral object movements. This description provides an illustration of the relationships between terrestrial and celestial views as well related symbols.

To get an overview of the apparent motion of stars and other astral objects in the sky, it is useful to imagine them as projections on a fictitious celestial sphere surrounding the Earth and centred at the observer position. While the Earth is undertaking its 24 hours revolution around its polar axis, the whole sky appears to the observer to turn the other way, around an axis parallel to the polar axis. This axis intersects the celestial sphere at the North and South Celestial Poles. It is perpendicular to the celestial equator also parallel to the Earth equator. The observer looking at the sky “sees” only half of the celestial sphere, the part over the horizon containing either the North or South Celestial Pole (designed as NP and SP in the remaining text).

The imaginary point of the celestial sphere just above the observer's head at a given time is called the Zenith; the point right under his feet, the Nadir. The angle between NP-SP and Zenith-Nadir axis will depend on the latitude of the observer's position (see diagram below). The imaginary circle, centred at the observer and passing through the Zenith (or Nadir) and North (or South) Celestial Pole, is termed the meridian of the observer. The observer's meridian intersects the horizon alongside the north-south compass points. The celestial equator cuts the horizon alongside the east-west compass points, obviously perpendicular to the north-south axis.