Nīlakaṇṭha-Siddhānta: A Sun-Centred Planetary System
The Kerala astronomer who fixed the inner planets and set the five planets circling the Sun
Mercury and Venus had always been computed by an odd special rule. In 1501, the Kerala astronomer Nīlakaṇṭha Somayājī traced the error to its root, and the fix led him to a startling new model: the five planets orbit the Sun, which in turn orbits the Earth. Learn how he completed the chapter's long hint of a Sun-centred sky.
The Error That Would Not Go Away
On the banks of the Nila river in Kerala, around the year 1500, an astronomer named Nīlakaṇṭha Somayājī was wrestling with a stubborn, centuries-old flaw. Mercury and Venus, the two planets that never wander far from the Sun, were being computed the wrong way. The inherited rules handled them by a different procedure from every other planet, most visibly in their latitudes, how far they stray north or south of the Sun's path. Generations of astronomers had noticed the oddity, and the only defence offered was that it matched the sky: a special rule for the two inner planets that worked, roughly, without anyone explaining why it was needed.
Nīlakaṇṭha would not leave it alone. He was heir to one of the greatest schools of mathematics India ever produced, the Kerala school, and its habit was to demand the reason behind every rule. He asked the question others had left hanging: why do Mercury and Venus need a different treatment from every other planet? Following that question to its end would lead him, in his masterwork of 1501, the Tantrasaṅgraha, and in his later commentary on the Āryabhaṭīya, to redraw the heavens, and to set the planets circling the Sun.
The Clue in the Old Rule
The flaw was hiding in plain sight, written into the classical procedure itself. Āryabhaṭa's own text had singled out the two inner planets with a special instruction.
शीघ्रोच्चादर्धोनं कर्तव्यमृणं धनं स्वमन्दोच्चे। स्फुटमध्यौ तु भृगुबुधौ सिद्धान्मन्दात्स्फुटौ भवतः॥
śīghroccād ardhonaṃ kartavyam ṛṇaṃ dhanaṃ svamandocce sphuṭamadhyau tu bhṛgubudhau siddhān mandāt sphuṭau bhavataḥ
Take half the śīghra correction and apply it, with its sign reversed, to the planet's own manda apex. From the apex so fixed, Venus and Mercury become true-mean, and then true.
Āryabhaṭīya, Kālakriyāpāda 24
Look at what the verse admits. For every other body the corrections follow one clean pattern, but Venus and Mercury (bhṛgu-budhau) require a special handling: the half-correction goes onto their apex rather than onto the planet. The tradition had preserved this exception faithfully for a thousand years without resolving it. To Nīlakaṇṭha, the exception was not a nuisance to be memorised. It was a clue, a sign that the model was being forced to fit a truth it had not admitted.