The book that moved Mercury, the gist

The book that moved Mercury

In 1500 a Kerala astronomer called Nilakantha found that the standard Indian method for locating Mercury and Venus was built on the wrong centre, rebuilt it, wrote a separate book arguing that old rules must be checked against the sky and corrected, and then almost nobody outside his own district ever heard about it.

This is the short way in. The essay has the working, the sources and the sky.

The correction was right, it was published, it was explained, and it travelled about as far as a person could walk in a day. Being right is not a means of transport.

Indian astronomy books worked out where a planet was in stages. Start with a pretend planet that moves at a perfectly steady speed. That is easy arithmetic and it is wrong, because real planets speed up and slow down. So you apply a first correction for that unevenness, and a second correction that links the planet to the sun.

For Mars, Jupiter and Saturn this worked as described. For Mercury and Venus, the books had wired it up differently. The thing they called the planet's steady position was actually the sun's steady position. So the first correction, the one meant to fix a planet's own uneven speed, was being applied to the sun instead of to the planet.

You can live with that for a while. Where it broke down was height. A planet is not just left or right along the sun's yearly track through the stars; it also sits a little above or below it. That up-and-down amount depends on the tilt of the planet's own path, so you have to measure it from the point that path actually goes around. The old rule measured from the wrong point. It also needed one recipe for the outer planets and a different one for the inner planets, which is usually a sign that something underneath is broken.

Nilakantha Somayaji, born 1444, fixed it. He gave Mercury and Venus their own positions back, which made one single rule for height work for every planet. In a commentary he spelled out what that implies: the five planets go round the sun, and the sun goes round the earth carrying them. He did not move the earth. The earth stays the centre for the sun and the moon.

He also wrote a short book arguing that this kind of correction is allowed: that astronomy is not settled by old authority, that predictions must be checked against what you actually see, and that the numbers get revised whenever the two disagree.

Tycho Brahe published a similar arrangement in Europe 88 years later, with no known connection between them.

Nearly four centuries later, in Odisha, a man named Samanta Chandra Sekhar observed with bamboo sighting tubes and wooden instruments he built himself, with no lens at any point, and arrived at the same arrangement in a book he finished in 1869. There is no evidence that a Kerala manuscript ever reached him.

Words decoded

Tantrasangraha (तन्त्रसङ्ग्रह): the astronomy book Nilakantha finished in 1500, in Sanskrit verse
siddhanta (सिद्धान्त): a standard astronomy treatise, the kind of book Indian astronomers inherited and computed from
manda correction (मन्द), also called the equation of centre: the adjustment for a body moving faster in one part of its circuit and slower in another
shighra correction (शीघ्र): the second adjustment, which relates the planet to the sun and is what makes a planet appear to go backwards for a few weeks
longitude: how far along the sun's yearly track a planet stands, measured round the circle
latitude: how far above or below that track it stands
ecliptic: the technical name for that yearly track of the sun through the stars
geo-heliocentric: an arrangement with the planets going round the sun and the sun going round a fixed earth; Nilakantha's scheme and Tycho Brahe's are both of this kind
Aryabhatiyabhasya (आर्यभटीयभाष्य): Nilakantha's long commentary on Aryabhata's book of 499, where he sets out the geometry
Jyotirmimamsa (ज्योतिर्मीमांसा): his short book, around 1504, arguing that astronomical constants must be checked against observation and revised
Yuktibhasa (युक्तिभाषा): a book of about 1530, written in Malayalam prose by his fellow pupil Jyeshthadeva, that demonstrates the revised scheme
Siddhanta Darpana (सिद्धान्त दर्पण): the book finished in 1869 by Samanta Chandra Sekhar of Odisha, which arrives at a similar arrangement

Why anyone should carePeople argue about whether a tradition can change its mind, and usually the argument runs on assertion. Here is one documented case where it did, in public, with the fault named and the repair defended in writing. It is also a case where the correct answer lost, not to a better answer but to the postal system: knowledge that moves only from teacher to pupil, on palm leaves, in one river valley, stays there. If you want to know why a tradition can hold a right answer and still not benefit from it, this is the cleanest example available.

Read it properly, 6 min The book that moved Mercury