History

The Celestial Sphere was once believed to be real, a hollow ball, surrounding the Earth, with all the stars neatly fixed to its inside surface. It was first used as a description by Aristotle and it remained the standard scientific model for the universe outside the Solar System for millennia. When science became an observational and experimental discipline the Celestial Sphere was doomed to the fate of all obsolete theories.

Modelling space

Possibly because the Celestial Sphere was so ingrained in astronomical thought and literature, possibly because of the beautiful name, it has remained as a theoretical model for understanding the relationship of an observer on Earth and the astronomical phenomena being observed. Astronomers imagine that it still exists because it's the easiest way to describe some key components of astronomy.

The shape of the model

The Celestial Sphere is imagined as a perfect globe, with an equator, parallel to the Earth's equator, and poles, in direct line with the axis of the poles on Earth. The model is used for describing observations, so the Celestial Sphere is always perceived as rotating around the Earth even though it is actually the Earth that spins within the sphere.

From the point of view of an observer on the Earth's equator, the celestial equator would always be directly overhead, forming a line running due east to due west. Every star would move steadily from the eastern horizon to the western horizon overhead, while Polaris, the pole star, would remain on the northern horizon, unmoving, since its position on the Celestial Sphere is at the celestial North Pole. The Celestial Sphere appears to rotate once every 24 hours, fifteen degrees of angle per hour.

An observer standing at Earth's North Pole would see all the stars in the northern celestial hemisphere all the time, they would move in circles around Polaris. Such an observer would never be able to see any of the stars in the southern celestial hemisphere.

From any intermediate point, there would be some stars always visible above the horizon and others occasionally visible. Stars that are always visible are known as circumpolar Stars. At the Earth's North Pole, the entire northern celestial hemisphere is made of circumpolar stars, at Earth's equator there are no circumpolar stars at all.

The ecliptic

The Earth's axis is tilted relative to the Sun; this is why we have seasons. When the North Pole on Earth points closer to the Sun, we have summer in the Northern Hemisphere, winter in the southern. Because of this tilt, the Sun's path projected onto the Celestial Sphere is at an angle to the celestial equator.

For half the year the Sun appears to be in the northern celestial hemisphere, for the other half it appears to be in the southern. The points where the Sun crosses the celestial hemisphere are those two points of the year when the days and nights are equal in length. The technical term for this is an equinox.

The Sun's path on the Celestial Sphere is called the ecliptic.

The ecliptic is represented on maps of the Earth by the two tropics. These are imaginary lines, drawn on the surface of the globe parallel to the equator. The tropics are marked at the angle between the centre of the Earth and the most northerly and most southerly points of the ecliptic on the Celestial Sphere. This means that the area of Earth's surface between the two tropics can be defined as the area where it is possible that the Sun might appear directly overhead. The Sun never appears exactly 'up' to an observer outside the tropics.

Mapping astronomical bodies

A standard method is used for describing the location of any object in space. The angle between the object, for the sake of argument a star, and the celestial equator is measured, this angle is known as the declination. The angle around the equator is measured from a single point on the celestial equator called the First Point of Aries; this is the point where the ecliptic crosses the celestial equator from south to north. It is the apparent position of the Sun on the Celestial Sphere on the day of the Vernal Equinox.

The angle between the First Point of Aries and the position on the celestial equator directly north or south of the star is called the right ascension of the star. It can be measured in degrees, but classically it is measured in time, since the Celestial Sphere rotates at 15 degrees per hour it is very easy to convert between the two.