Astronomy: what Indians picked up from the ancients

One thing Indians quickly come to realise is how central astrology is to how many people choose to order their lives.

Astrology is of course for the most part bunkum. But the ancient Indians included among their number some decent world-class astronomers whose contributions were significant, and had a major impact on how modern mathematics and physics developed in the West.

This is sometimes taken much too far. Some ill-informed Indians occasionally speak as if ancient Indian knowledge has informed this native tradition all along. In their reading, the absence of any firm historical records means nothing and Indians independently discovered all the glories the ancient world had to offer.

The reality is different.

Indian astronomers must have been in contact with Babylonians (before 500 BC). We know this since Babylonian astronomers are known to have calculated various numerical time periods, e.g to do with the period of the moon's revolution round the earth, and the methods they used have survived. The Indians later developed their own mathematical models of moon behaviour and other phenomena but made use of these exact same figures.

Other data from Babylonian observations would prove to be crucial for the later development of Greek astronomy. The concept of the zodiac was theirs, the division of the circle into 360 degrees (for days of the year) too.

We also know that Indian astronomers must have been in contact with the Greeks.

Celestial bodies could, according to the Platonic dictates of Greek aesthetics, not move in anything other than circular paths. Yet there is no way the orbits of the planets as observed from earth can be made to fit circular paths with the earth at their centre. The particular problem lay in the need to explain the apparent retrograde motion of planets, where they appear to double back on themselves in the sky

Hipparchus (2nc entury BC) proposed an ingenious explanations for this, with more complex orbits composed of epicycles. The idea was that here you would have the planet in question travelling in a circlular orbit, still, except that the centre of this circle would itself travel along a circle centred at the earth.

Aryabhatta (5th century) is known to have used such a model, so it is fair to say he and Indian astronomers picked up this idea from the Greeks and were bound by such thinking, at least in this matter.

Ptolemy (2nd century) would go on to refine this model further, introducing epicycles of epicycles and so on. This complicated approach held sway across the Arab and Western worlds until Copernicus and Kepler came along.

However, Aryabhatta and his immediate successors had no inclination of this model, which would only reach India via Arab influences much later.

Interestingly, only in this century was it finally realised that Ptolemy might have been onto something. Apparently the circle and epicycle form the first and second approximations respectively, if you were to consider Fourier series expansiona of the mathematical expressions for the orbits.

It's not clear if Ptolemy had the mathematical nous or tools to take the calculations those extra steps further, though!

My source for all this is the fabulous From Eudoxus to Einstein: A History of Mathematical Astronomy by C M Linton, an outstanding volume. This fabulous book covers many of the gaps popular-science books have when it comes to explaining just how and why particular theories of the celestial skies were developed.