Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v1.c6.13

v1.c6.13

One complete revolution of a planet in the zodiac is called *bhagaṇa*. It is obvious that the time for one *bhagaṇa* must have been determined after observing the times taken by the planet in making several revolutions. The astro-mathematical works give the number of *bhagaṇas* which each planet completes in the period of one *Kalpa* or one *Mahāyuga*. The motion of a planet as calculated from these periods using the *bhagaṇas* mentioned by the Pañca-Siddhāntikā must be the same for every day. This is called the ‘mean motion’. But the motion of a planet as actually seen in the sky is not always the same. For instance, Jupiter takes about twelve years for one revolution and hence its mean daily motion comes to be about 5′; but the actual observation of the planet shows that Jupiter sometimes moves faster than that and sometimes slower; its daily motion is sometimes found to be as much as 15′ and sometimes it is slower than even 1′; not only this but sometimes the planet also appears even to move in a reverse direction *i. e.* from east to west (this is called retrograde motion). The planet’s daily motion as is actually seen is termed ‘true or apparent’ motion; similary the planet is actualy found to be somewhat ahead of or behind the position calculated from its mean motion. The real position of a planet is called its ‘true place’ and that found by adopting the mean motion is called its ‘mean place’.