Tim TrottDon't look back - you're not going that way

# The Celestial Sphere

By , Friday 4th April 2008 in Astronomy Basics

The most fundamental property that any astronomer needs to know is our place in the heavens and the position of astronomical objects that lie on the surface of the celestial sphere. We will look at the ecliptic and how to locate objects using Right Ascension and Declination.

The celestial sphere is a projected sphere centred on the Earth that we can imagine all the stars are painted on. As the Earth rotates on its axis, this gives the impression of the sphere continually rotating overhead and creates the effect of stars rising in the East and setting in the West.

The Celestial Sphere

In order to identify the position of an object on the celestial sphere we need a coordinate system that can cope with an objects position on the sphere at a given time from a given location. Imagine that you are stood on the North Pole and when you look directly up you will see the Moon. Now imagine another observer stood on the Equator, where would they see the Moon? It certainly won't be overhead; instead it will be on the horizon. In the same manner an objects position will appear to change if one observer was in Europe and another in America, however over time the object will move into the position observed as the Earth rotates.

For these reasons the celestial coordinate system is based on the latitude and longitude system used on the surface of Earth. The coordinate system consists of two figures, Right Ascension (RA) and Declination (Dec). Right Ascension is a measure of an objects position from the First Point of Aries and can be thought of as the celestial sphere equivalent of longitude, while Declination is a measure of the position relative to the celestial equator and is similar to latitude. The celestial equator is a projection of the Earth's equator onto the celestial sphere.

In the diagram above, declination is marked on the blue lines and is analogous to latitude as it measures the angular distance in degrees from the equator, from 0° at the equator to +90° at the North Pole and -90° at the South Pole.

## First Point of Aries and Right Ascension

Right Ascension is analogous longitude in that it measures angular distance around the equator, however this is where the comparison ends. The First Point of Aries acts as the zero point for the Right Ascension scale instead of starting from the Greenwich meridian like longitude does.

As the Earth orbits the Sun, we see that the Suns position changes with respect to the background stars over the period of a year. If you were able to the position of the Sun compared with fixed background stars, the Sun would appear to move in a large ellipse on the surface of the celestial sphere. This ellipse is imaginatively called the elliptic. As the Sun moves along the elliptic, it crosses the celestial equator twice per year - in March we know it as the Vernal Equinox and in September the Autumnal Equinox. The First Point of Aries is the point in the sky where the Celestial Meridian, the Celestial Equator and the Ecliptic all meet. It is presently in the southwest of Pisces, moving slowly towards Aquarius.

First Point of Ares

When this point was first conceived the point was located within the constellation Aries, hence its name, however over time this position has moved westwards due to the precession of the equinoxes.

Right Ascension is measured in hours, minutes and seconds from the local sidereal time.