THE CELESTIAL SPHERE

As shown in the diagram, the Celestial Sphere is an imaginary sphere centred on the Earth on which the stars and other celestial bodies are projected. Half of this sphere is seen by an observer  and is what is called the sky. The Earth’s axis projected to meet the sphere marks the positions of the celestial poles and the plane of the Earth’s equator projected in a similar way marks the celestial equator

                              Fig. 1     The Celestial Sphere

As shown on the World Star Chart, the celestial sphere is divided by coordinates of latitude and longitude. Celestial latitude is called declination. It is measured in degrees north and south of the celestial equator and parallels of declination are extensions of the Earth’s parallels of latitude extended to touch the celestial sphere. The celestial equator is overhead at the equator and stars of declination 10½º pass through the zenith, or overhead point,  in Trinidad.  Declination is + in the northern hemisphere and – in the southern hemisphere.

Celestial longitude is called right ascension (R.A)  and it is measured in hours, minutes and seconds. There are 24 hours of right ascension and one hour of right ascension is equal to 15º. It is measured eastward from the vernal equinox which is also called The First Point of Aries. This is the point where the Sun crosses the celestial equator going from south to north.

                         Fig.2 The Position of the Vernal Equinox

           Fig.3  The Position of the Meridian