Physics in Ancient India

Indians in the Vedic era classified the material world into five basic elements: earth, fire, air, water and ether/space. From the 6th century BC, they formulated systematic atomic theories, beginning with Kanada and Pakudha Katyayana. Indian atomists believed that an atom could be one of up to 9 elements, with each element having up to 24 properties. They developed detailed theories of how atoms could combine, react, vibrate, move and perform other actions, as well as elaborate theories of how atoms can form binary molecules that combine further to form larger molecules, and how particles first combine in pairs, and then group into trios of pairs, which are the smallest visible units of matter. This parallels with the structure of modern atomic theory, in which pairs or triplets of supposedly fundamental quarks combine to create most typical forms of matter.

In the late Vedic era(9th–6th century BC), the astronomer Yajnavalkya, in his Shatapatha Brahmana, referred to an early concept of heliocentrism with the Earth being round and the Sun being the "centre of spheres". He measured the distances of the Moon and the Sun from the Earth as 108 times the diameters of these heavenly bodies, which were close to the modern values of 110.6 for the Moon and 107.6 for the Sun.

In 499 A.D., the mathematician-astronomer Aryabhata propounded a detailed model of the heliocentric solar system of gravitation, where the planets rotate on their axes causing day & night and follow elliptical orbits around the Sun causing year, and where the planets and the Moon do not have their own light but reflect the light of the Sun. Aryabhata also correctly explained the causes of the solar and lunar eclipses and predicted their times, gave the radii of planetary orbits around the Sun, and accurately measured the lengths of the day, year, and the Earth's diameter and circumference. Brahmagupta, in his Brahma Sputa Siddhanta in 628 A.D., recognized gravity as a force of attraction and understood the law of gravitation.

Harappan civilization (2400 B.C) used shell objects served as compasses to measure the angles of the 8–12 fold divisions of the horizon and sky in multiples of 40–360 degrees, and the positions of stars.

The Samkhya and Vaisheshika schools developed theories on light from the 6th–5th century BC. According to the Samkhya School, light is one of the five fundamental "subtle" elements out of which emerge the gross elements, which were taken to be continuous. The Vaisheshika School defined motion in terms of the non-instantaneous movement of the physical atoms. Light rays were taken to be a stream of high velocity fire atoms, which can exhibit different characteristics depending on the speed and the arrangements of these particles. The Buddhists Dignāga (5th century) and Dharmakirti (7th century) developed a theory of light being composed of energy particles, similar to the modern concept of photons.