When we look at the night sky, we can't tell how far away anything is.The Moon, planets, and distant stars and galaxies all look as if they're projected onto a sphere surrounding the Earth. Astronomers call this the celestial sphere.
From the diagram you can see that the celestial sphere has its own equator and poles. These are projections of the Earth's equator and poles. The “right ascension” and “declination” are the celestial equivalent of earthly lines of longitude (meridians) and latitude. This gives a coordinate system for the sky that is equivalent to the one we use for the Earth.
However there is one line marked on the diagram called the “ecliptic” which may need some explanation.
As the Earth orbits the Sun, the Sun's position changes against the background of fixed stars. The path the Sun seems to take in the sky is called the ecliptic. As it's the Earth that's moving, the ecliptic is actually the projection of Earth's path onto the celestial sphere.
Try to think of the Earth's orbit not just as a path, but as a path on a flat surface – a plane. Then imagine this plane stretching from the Sun outwards into the Solar System. The other seven planets also orbit close to this plane, which means that we also see them on the ecliptic. Pluto – now classified a dwarf planet – has an orbit tilted about 17 degrees to this plane.
The planets not only orbit in much the same plane, but also in the same direction. Astronomers think this because the planets were made from a disk of material that gathered around the Sun as it formed. They have found such disks around young stars elsewhere in the Galaxy.
If you're wondering why the Earth's orbit is called the ecliptic, it comes from the word eclipse. The orbit is tilted Moon's orbit is tilted by about 5 degrees to the Earth's orbit. This means that the Moon doesn't line up with the Earth and the Sun to cause eclipses every month. Eclipses can only occur when the Moon is near the ecliptic.
The ecliptic is also the center of the line of the Zodiac. The Zodiac is made up of twelve traditional constellations though which the Sun seems to pass during the year. The word zodiac comes from a Greek word meaning “circle of animals,” because most of the constellations represent animals.
You may recognize the names of the Zodiacal constellations because most are the same as the astrological star signs that you see in the newspaper. However in most western astrology the star signs don't correspond to the actual astronomical constellations.
The geographical coordinate system consists of lines of latitude parallel to the equator and meridians going from pole to pole crossing the equator at right angles. The equator is zero degrees of latitude and the Prime Meridian is zero degrees of longitude – it goes through Greenwich, England.
In the celestial coordinate system the celestial equator is zero degrees of declination and the starting point for right ascension is the March equinox. An equinox occurs when the Sun crosses the celestial equator. It does this twice a year, in March and in September.
The points at which the equinoxes occur are known traditionally as “the first point of Aries” (March) and the “first point of Libra” (September). The names came about because the Sun was in those constellations at the time. But not anymore. The Sun is in the constellation of Pisces at the March equinox and in Virgo at the first point of Libra. What's happening?
Precession is happening. The effect was discovered by Greek astronomer Hipparchus in about 127 BCE. There is a wobble in the Earth's axis that takes 26,000 years to complete. The angle the axis makes to the ecliptic doesn't change, but within a small circle – like a child's top – it changes the direction in which it points. Click to see an animation of a gyroscope precessing in the same way.
Because of precession the equinoxes move along the ecliptic. The March equinox is always the zero point of right ascension and declination, but its actual location against the stars changes. Therefore when giving the coordinates of celestial objects, astronomers must specify a time (called an epoch) at which these coordinates apply.
Precession also means that the way we see the Sun's place in the zodiac changes, so that the month represented by a star sign doesn't coincide with the month in which the Sun is in the constellation of that name.
Over long periods of time the pole star also changes. Currently, the north polar axis points to within 1 degree of Polaris, which is very useful for navigators. However Polaris hasn't always been the north polar star. In another 12,000 years Vega – a first magnitude star in Lyra – will be the pole star.