It would be surprising for many Indians today to know that the concepts of atom (Anu, Parmanu) and relativity (Sapekshavada) were explicitly stated by an Indian philosopher. This was way back nearly 600 years before the birth of Christ. These ideas which were of fundamental import had been developed in India in a very abstract manner. This was because their exponents were not physicians in today's sense of the term.
They were philosophers. Their ideas about the physical reality were integrated with those of philosophy and theology. Apart from this, the ancient Indians classified the material world into five basic elements: earth, fire, air, water and ether/space.
From the 6th century BC onwards, they began formulating systematic atomic theories. This task commenced with Kanada and Pakudha Katyayana. Indian atomists believed that an atom could be one of up to 9 elements. Each of such elements had up to 24 properties.
They developed detailed theories of how atoms could combine, react, vibrate, move and perform other actions. Additionally, 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 were also laid down. This parallels with the structure of modern atomic theory. In this, pairs or triplets of supposedly fundamental quarks combine to create most typical forms of matter.
In the late Vedic arranging from the 9th to the 6th century BC, the astronomer Yajnavalkya referred to an early concept of heliocentrism. This concept formed a part of his work, Shatapatha Brahmana. According to this concept, the Earth was round while the Sun was 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. These 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. In this model, the planets rotated on their axes causing day & night and followed elliptical orbits around the Sun causing year. Additionally, the planets and the Moon do not have their own light but reflect the light of the Sun.
Aryabhatta also correctly explained the causes of the solar and lunar eclipses and predicted their times, gave the radii of planetary orbits around the Sun. Further, he also 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 around the 2400 B.C used shell objects as compasses to measure the angles of the 8-12 fold divisions of the horizon. Additonally, the sky was measured in multiples of 40-360 degrees, along with 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 was one of the five fundamental "subtle" elements. Out of these emerge the gross elements, which were taken to be continuous.