Surya Siddhanta is an ancient Indian astronomical text that provides detailed knowledge about the measurement and calculation of various celestial phenomena. It is considered one of the earliest and most important treatises on astronomy in Hinduism. The exact date of its composition is uncertain, but it is believed to have been written during the Vedic age.

The Surya Siddhanta is attributed to the Surya, as it is believed to have been revealed to the Mayasura to aid in the understanding of astronomy and timekeeping. The text consists of 14 chapters (adhikaranas) that cover a wide range of topics related to astronomy, including planetary motions, eclipses, celestial measurements, and the calculation of time.

Some key concepts and topics discussed in the Surya Siddhanta include:

Solar System: The text identify the solar system with minute details. But the planetary positions are calculated with respect to earth, but this doesn’t meant that earth is stationary and all other celestial objects including sun is revolving around it.

Planetary Motion: The Surya Siddhanta describes the motion of planets, their orbits, and the calculation of their positions in the sky at any given time. It introduces mathematical formulas and algorithms for predicting the planetary positions.

Eclipses: The text provides methods for predicting solar and lunar eclipses, including the calculation of their timings, durations, and locations.

Timekeeping: The Surya Siddhanta explains different units of time, such as the day, month, year, and yuga (cosmic cycle). It discusses the measurement of time using celestial bodies and introduces the concept of the sidereal year.

Trigonometry: The Surya Siddhanta contains early trigonometric methods and tables that were used for astronomical calculations.

As per Surya Siddhanta, the axis of rotation of Earth is inclined at 24 degrees from the perpendicular to the orbit of rotation of Earth around the Sun (The actual figure is 23.5 degrees)

The Surya Siddhanta had a significant influence on Indian astronomy and its subsequent developments. It served as a foundational text for later astronomers and was widely studied and commented upon. Its calculations and principles contributed to the refinement of Indian calendrical systems and the development of later astronomical treatises.

Till some centuries earlier, while most of the world considered the earth as a flat disc, the text remains a wonderful historical and cultural document that reflects the intellectual achievements of ancient India in the field of astronomy.