Celestial Sphere

CELESTIAL SPHERE:

                     If you look at the night sky, you will see that all stars always appear fixed relative to each other and the entire pattern of stars rigidly orbit the sun throughout the year in the same manner. We employ this artificial, earth-based view of the heavens to make celestial maps by pretending that the stars are attached to the inside of an enormous hollow shell which is basically the "Celestial Sphere" and the Earth is at the center of the sphere.

THE NIGHT SHY:

                  In day time we can't see the stars due to the sunlight. But at night we can watch the stars quite clearly. The pattern of stars is visible in the dark which is called "Constellation". But the same view is invisible in the big cities due to the light pollution. You can clearly see the difference of the night sky with and without the light pollution in the following picture. In this photograph you can clearly see the difference of the night sky with and without light in Goodwood, Ontario, Canada.[copyright Todd Carlson /skynews Magazine]

CELESTIAL SPHERE IN NAVIGATING THE SKY:

                In celestial sphere stars are seemed fixed because they are too far away. In reality they do move relative to each other. But we neither see their motion nor perceive their relative distances because they are so far from us. Astronomers use celestial sphere in finding stars, but the condition is the stars must be fixed on it, just as cities are fixed on maps of the Earth. They applied a coordinate system to it to locate this quickly.

• CELESTIAL EQUATOR: If we expand the Earth's equator onto the celestial sphere we obtain the "celestial equator",which divides the sky into northern and southern hemispheres same as the Earth's equator divides the Earth into two hemispheres. Now if we stretch the Earth's north and south poles out into space along the Earth's axis of rotation to the celestial sphere, it will give us "north celestial pole' and "south celestial pole"

• DECLINATION AND RIGHT ASCENSION: Using the same conditions as the celestial equator, astronomers divide up the surface of the celestial sphere in precisely the same way that latitude and longitude grid divides the Earth. The equivalent to latitude on Earth is called "declination" on the celestial sphere. It is measured from 0 degree to 90 degree north or south of the celestial equator. The  equivalent of longitude on Earth is called "right ascension" on the celestial sphere, measured from 0 h to 245 h around the celestial equator.

• VERNAL EQUINOX: The Sun also orbit around the our galaxy (Milky Way Galaxy) through out the year.So we can say the Sun also has an orbit in the celestial sphere.Just as the    location of Greenwich, England, defines the prime meridian, or zero of longitude on Earth, we need to establish a zero of right ascension. It is defined as one of the places where the Sun's annual path across the celestial sphere intersects the celestial equator. The celestial equator and the Sun's path intersect at two points. The equivalent on the celestial sphere of the Earth's prime meridian is where the Sun crosses the celestial equator moving northward. Angles of right ascension are measured from this point, called the "vernal equinox".