Indian Astronomy III By Govind Kumar
When we look up at the heavens on a clear night, we find a multitude of celestial bodies, illuminating the sky with their radiance. It looks like a large hollow hemisphere, with the observer at the center. The planets and the luminaries seem to be scattered thorough the heavens at large distances.
This picture of the observer, the sky as a large crystalline hemisphere , is a very convenient model for the study of the Heavens. This hemispherical model of the heavens is called Khagola, the Celestial Sphere. Another geocentric model, the Khagola is an imaginary sphere of large radius.
Astronomy, Maths and Astrology ( Siddhanta, Samhita and Hora ) are considered to be Apaurusheya, divine in origin. They were revealed to the Seers in higher states of Consciousness and hence are revealed sciences. Descendit e Caelo, they cometh from Heaven ! Their date of compilation is believed to be 12th century BCE.
Rotation, & Diurnal Motion
The earth rotates about its own axis, from west to east in the course of a day. Due to this rotation, the observer, unaware of his motion in space, is carried eastward. To the observer, the Celestial Sphere with all the heavenly bodies, is seen as rotating from east to west. This apparent westward motion of the heavenly objects is known as the diurnal motion.
The Celestial Poles
All celestial bodies appear to rise in the east, travel westward and set in the west, because of this relative diurnal motion. When the axis of the earth's rotation is extended, it meets the Celestial Sphere on two diametrically opposite points called Celestial Poles. The one in the direction of the Earth's north pole is called the North Celestial Pole and the one in the opposite direction is called the South Celestial Pole.
The Celestial Horizon, the Kshitija
To the observer standing at a place on the surface of the earth and looking around describing a full circle, the earth appears to meet the Heavens along a circle. From the observers' frame of reference, this circle is called the Celestial Horizon, the Khshitija.
The Celestial Meridian, the Nadi Vritta
Almost 90 degrees to the Celestial Horizon, you will find another circle ( the line perpendicular to the plane of the horizon ). This Circle is the Celestial Meridian. The highest point on this Celestial Meridian is called Zenith, the MC and the lowest, the Nadir or IC.
Aryabhata gives a classic example of diurnal motion. For an observer moving in a boat on a river, all the trees etc on the bank of that river appear to move inthe opposite direction. Astronomical Knowledge is known in Sanskrit as Siddhanta. Transcendental Knowledge is Vedanta. There are 18 astronomical treatises compiled by 18 Seers. Each Siddhanta is named after its author. They deal exhaustively with Astronomy.
These are the main astronomical treatises. There are others like Arya Siddhanta by Aryabhata II, Brahma Sphuta Siddanta by Brahmagupta and Maha Bhaskareeya by Bhaskara.