At the heart of symbolism

Description of the celestial sphere

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.

Apparent Sun orbit

As the annual motion orbit of the Earth around the Sun, called the ecliptic, is titled by an angle of 23.5° with respect to the Earth equator, the daily apparent Sun motion on the celestial sphere looks like a circle, also titled by an angle of 23.5° with respect to the celestial equator.

Moreover, during the annual motion of the Earth around the Sun, the inclination of the Sunbeams over the equator varies, producing seasonal changes. At our Winter solstice (around 21st December), the Sun beams titled an angle of 23.5° southern the equator where the days are the longest while they are the shortest northern the equator. At our Summer solstice (around 21st June), the Sunbeams titled an angle of 23.5° northern the equator where the days are the longest. At Spring and Autumn equinoxes (around 21st March and 21st September), Sunbeams are parallel to the equator, so that days and nights are equal.

As a result, the orbit of the apparent motion of the Sun on the celestial sphere is moving daily Northward between Winter and Summer solstice and Southward between Summer and Winter solstice. The ecliptic intersects the horizon at two points associated with Sunrise at east and Sunset at west. It meets the observer's meridian just between these two moments, at noon, and corresponds to the daily highest position in the sky.

According to the Northern and Southern move of the ecliptic over the year, the Sunrise and Sunset positions will also move. The Sun rises exactly at east and sets precisely at west only at equinox time. At Winter solstice, the Sun rises Southeast and sets Southwest whereas at Summer solstice, it rises Northeast and sets Northwest.