Since we are gifted with two eyes, we can get a sense of depth and determine if an object is close to us or not in everyday situations. When we look at something, both our eyes don't see the same image. Because of the distance of about 7cm between our eyes, we notice that closer objects seem to 'move' relative to the background if we alternately look through one eye and then the other.
The Parallax View
This phenomenon is known as parallax, and the distance between the two points of observation (in this case our eyes) is called the base. The farther an object is from us, the smaller the parallax it has. The stars, however, are located at such a great distances that a base of a mere 7cm isn't sufficient to notice any differences in stellar distance.
Some astronomical objects, such as distant stars and galaxies, appear to be fixed in position relative to each other. This means that the patterns they make are relatively static, and can be used to act as a background to identify the positions of objects, such as planets1, that move relative to them. This is important because the Earth itself is moving, and also spinning, making it difficult to use any terrestrial reference point.
Those astronomical bodies that do appear to be fixed are scattered throughout near space, some a few light-years from Earth, others much further away. The model used to map their positions as seen from Earth is called the celestial sphere.
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.
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.
At any given time, from any point on the Earth's surface, one half of the celestial sphere is visible, the part that is above the horizon. The other half of the sphere is hidden by the mass of the Earth itself. The point directly over the head of an observer is called the zenith, while the point exactly opposite on the celestial sphere is called the nadir, the point on the sphere which would be seen if the observer could look directly through the centre of the Earth.
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 (alpha Ursae Minoris), 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 roughly once every 24 hours, 15° of angle per hour2.
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 which would remain fixed at the Zenith. 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.
While it is convenient to assume that the stars are in fixed positions on the celestial sphere, it is actually somewhat misleading. Stars do move their relative positions; however, this motion is extremely slow from the point of view of an observer on Earth. It is sufficient to require maps of the stars around Earth to have a date associated with them, identifying the year in which the stars occupied the positions marked on the map. This date is known as the epoch of a star map. Most modern star maps are based on the year 2000. Even the most rapidly moving stars on the map would take many centuries to move far enough for that change to be visible to the naked eye. This movement of stars, the actual motion rather than the conventional heuristic rule that they can be assumed to be fixed, is called the star's proper motion.
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 at the zenith. 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. Declination is measured in degrees, from +90° to -90°. A declination of +0° would refer to an object on the celestial equator. This only provides one of the two co-ordinates needed to locate the star, however. 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. A line drawn through this point and extended to the celestial poles would effectively have the same value as the Greenwich Meridian has on the surface of the Earth. While there is no actual necessity for the meridian to be in that location, it is always fixed there for reference purposes.
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° per hour, it is very easy to convert between the two.