Nadi Vritta is the name used to refer to equator in ancient Hindu religion. It is an imaginary line which cuts the earth into two equal halves and which is perpendicular to Earth’s axis. All points on the equator are equidistant from the North and South Poles of the earth.
Aryabhata I (476 – 550 AD), a well known Indian astronomer, in his Aryabhatiyam, says that the Earth is round and that the Earth rotates on its own axis from west to east.
Due to its diurnal motion, the celestial bodies appear to rise in the east and set in the west. There are other related facts, when the axis of the earth’s rotation is extended, it meets the celestial sphere at two diametrically opposite points called the celestial poles, of which the one in the direction of the Earth’s North Pole is called the celestial North Pole, and the other, the celestial South pole.
In the figure, the great circle AB, whose plane is perpendicular to the line pp’ joining the celestial poles is called the Vishuvat Vritta (celestial equator).
The diurnal paths of the celestial bodies are circles of different radii, parallel to the celestial equator. The celestial equator AD divides the celestial sphere into two hemispheres. The one containing the North Pole, P (Uttara Dhruva), is called the northern hemisphere and the other, the souther (Dakshina Dhruva).
When an observer is positioned on the surface of the Earth, for him the sky appears to meet the Earth along a circle. This circle is called the celestial horizon (Kshitija). The point Z in the figure refers to the Zenith, and N is the Nadir. The apparent annual path of the Sun around the Earth with respect to the fixed stars is a great circle, called the ecliptic.
Bibliography
Aryabhatiya or Aryabhata (1976) Edited by Kripa Shankar Shankla – Indian National Service Academy New Delhi
Astronomy and Mathematical Astrology (1995) Deepak Kapoor – Ranjan Publications Delhi
Encyclopedia of Hinduism Volume VII page 300 - IHRF