# What is meant by "revolution of Sun's apogee"?

While reading Translation of the Surya Siddhanta and of the Siddhanta Siromani(1861) - Bapu Deva Shastri, I stuck in the follwing.

Chapter I, verse 41 & 42

In a KALPA (4.32 billion years), there are 387 revolutions of the Sun's Apogee (about the earth), 204 of Mars' apogee, 368 of Mercury's apogee, 900 of Jupiter's apogee, 535 of Venus' apogee and 39 of Saturn's apogee.

Now my question is what is meant by 387 revolution of Sun's apogee? It will be helpful if someone can explain this.

This is the same question I have asked here, but as not getting proper explanation I am posting here too.

The apogee is the direction of the sun when it is at greatest distance from the Earth. We would normally call this the aphelion because we use a heliocentric model. The sanskrit doesn't have the precise terms for apogee and aphelion, it uses a word meaning "greatest height". The translator has converted the notion of "greatest height of the sun above the Earth" as "apogee"

This direction changes (relative to the stars) mostly as a result of perturbations of Venus and Jupiter (though the cause could not have been known to ancient authors without a model of gravity). Currently, aphelion (or apogee from the perspective of the sun) occurs on about July 4th, but this changes slowly.

It takes about 112000 years for the aphelion to revolve by 360 degrees. The calculation you quote gives about 10000000 years for a revolution. What are we to make of these numbers?

Measurement of the rotation of the aphelion is a hard thing to observe. It is harder than the observation of the precession of the equinoxes, which was well established by the time that the Surya Siddhanta was composed. And the value quoted here is incorrect by a factor of about 100. Apsidal precession could not have been observed at this level, and without a theory of gravity and calculus, it could not have been calculated. It is therefore reasonable to suppose that the value here is found by analogy with the apsidal revolutions of the moon (which were observed in India) although exactly how this analogy was constructed is not explicit in the text.

We should also note that the model of planetary motion was not base on ellipses, but on circles with the Earth placed off-centre and the sun moving around it. In reading ancient accounts of science, we need to put ourselves into an ancient mode of thought, even if the translation uses modern terminology.