Introduction.
Astro-navigation is based upon the concept of the “celestial sphere”. What is the “celestial sphere”? If one looked up into the night sky one would see what would appear to be an enormous black dome with all the heavenly bodies stuck to the inside like thousands of diamonds. This is what we refer to as the “celestial sphere”. It is infinite in size and the earth is the centre of the sphere. In order to be able to use it and the heavenly bodies, it is necessary to be able to situate the bodies in some form of reference system, similar to that we have on earth. We need a north/south axis and we need a base line to separate the hemispheres. To this end, the earth’s poles have been projected into the celestial sphere and we call them the “celestial poles”. In a like manner the earth’s equator has also been projected into the celestial sphere and we refer to it as the “celestial equator”.

Celestial sphere.

Celestial bodies.
When we speak about the celestial bodies we are referring to the stars, planets, sun and moon which are observed from the earth and are used for navigation.

Apparent motion of the celestial sphere.
To the observer on the earth’s surface it appears that the celestial sphere is rotating around us from east to west, but In reality, it is the earth that is rotating around it’s axis from west to east.

Angular distance between stars.
To anyone on the earth’s surface, the appearance of stars in the celestial sphere conveys no idea of their relative distances from the earth. It is impossible for the observer to determine the distances between him and the various stars or between the stars. He does have, however, the ability to measure the angular distance between them and with the assistance of spherical trigonometry, is able to solve all problems associated with the measurement of this distance. Because the stars are so far away from the earth, any movement they may have in space is not noticeable to the observer on earth.

The celestial meridians.
Celestial meridians are semi-great circles joining the celestial poles and they correspond exactly to the terrestrial meridians.

Position of heavenly bodies in the celestial sphere.
The position of heavenly bodies can be considered fixed in the celestial sphere in relation to the celestial equator and a particular celestial meridian, in the same way as the position of a place on earth is fixed in relation to the earth’s equator and the meridian running through Greenwich. The celestial meridian chosen is the one passing through the First Point of Aries. The latter point was chosen because it is the first point of intersection between the “ecliptic” and the “equinoctial”. The former is the plane of the earth’s orbit around the sun, whilst the latter is the plane of rotation of the earth around its own axis.

Position of heavenly bodies in the celestial sphere.

(Since it is not a requirement of this course to measure or use any celestial bodies other than the sun, it is not intended to pursue the stars, planets or moon any further).