Graha-Gati: Planetary Motion
Discover how Indian astronomers built sophisticated models of planetary motion, with epicycle corrections that reproduced retrograde motion, Āryabhaṭa's bold argument that the Earth turns on its axis, and Nīlakaṇṭha's model in which the planets circle the Sun.
Lessons in this chapter
- Graha: What 'Planet' Means in Indian Astronomy — The seven seizers, the nine grahas, and a category drawn by motion, not matter
- Madhyama-Gati: Mean Motion and Orbital Periods — How a whole number of revolutions turned wandering lights into calculable clocks
- Spaṣṭa-Graha: True Position Calculations — The two corrections that turn a smooth average planet into the one you actually see
- Manda-Paridhi: The Epicycle of Apsis — The little circle that captured a planet's changing speed, and a first glimpse of a mathematics of motion
- Śīghra-Paridhi: The Epicycle of Conjunction — The correction that explained retrograde motion, and quietly hid the Sun at the centre
- Āryabhaṭa-Darśana: Āryabhaṭa's Rotating Earth — The claim that the sky only seems to turn, argued with a boat on a river
- Nīlakaṇṭha-Siddhānta: A Sun-Centred Planetary System — The Kerala astronomer who fixed the inner planets and set the five planets circling the Sun