Introduction

The celestial sphere is an imaginary sphere of infinite radius centred on the Earth, on which all celestial bodies are assumed to be projected. This Earth-centred Universe is, of course, not an accurate model of the real Universe, but it turns out to be an extremely useful tool for understanding the relative positions of celestial objects, and their apparent motions through the sky. It is the most important concept in this course; so make sure you understand it well!

The celestial sphere is shown in the figure below. The celestial sphere is fixed. However, the Earth rotates inside the sphere, and so, from our point of view, the celestial sphere appears to rotate. The Earth rotates towards the east (counter-clockwise from a perspective looking down onto the north pole). So an observer on Earth appears to see the celestial sphere rotate towards the west (or clockwise, looking towards the north pole).

Celestial Co-ordinates

This apparent rotation of the celestial sphere presents us with an obvious co-ordinate system to locate objects upon it. The extensions of the north pole and south pole of the Earth intersect with the north celestial pole and the south celestial pole, respectively, and the projection of the Earth's equator on the celestial sphere defines the celestial equator. The celestial sphere can then be divided up into a grid in a similar manner to the way in which the Earth is divided up into a grid of latitude and longitude, as shown in the figure to the right (bottom of page on mobiles).

Declination

Declination
(DEC, or δ) is analogous to latitude on the Earth's surface. It is measured from the celestial equator, and extends from 0° at the celestial equator to +90° at the north celestial pole and from 0° at celestial equator to -90° at the south celestial pole. Declination is shown in the figure by the red line.

Right Ascension

Right Ascension (RA, or α) is analogous to longitude on the Earth's surface. It is measured around the celestial equator. RA is measured in hours, and increases to the east. Since the Earth rotates once in roughly 24 hours, RA extends from 0h to 24h. Much as we had to choice an (arbitrary) zero point for longitude, we also need a zero point for right ascension. This zero point is known as the first point of Aries. We'll learn more about why it was chosen when we look at how the Sun moves on the celestial sphere. RA is shown in the figure with the blue line.