We begin this journey covering the works of ancient astronomers with Aryabhata.

      Aryabhata was one of the revolutionaries in science whose work, the Aryabhatiya was almost forgotten. Aryabhata is regarded as the greatest mathematician-astronomer of India. It was with this honour that India's first satellite was named after him. 

     Aryabhatta was born in 476 A.D. He wrote his first work, Aryabhatiya in 499 A.D. at the age of 23. The Aryabhatiya deals with both mathematics and astronomy and is divided into four parts: Gitikapada (preliminaries), Ganitapada (mathematics), Kalakriyapada (reckoning of time) and Golapada (astronomy).

     Aryabhata (476 - 550 A.D.) believed that the earth rotated on its axis and the stars were fixed in space.  He goes on to say that the apparent rotation of the heavens was due to the fact that the earth revolved around its axis.  According to him the period of one rotation of the earth is 23 hours 56 mn 4.1s while the modern value is 23 hours 56 mn 4.091s.  His accuracy regarding this is amazing. To justify this point, he stated:

        "Just as a man in a boat moving forward sees the stationery objects (on either
          side of the river) as moving backward, just so are the stationery stars seen
             by people at Lanka (on the equator), as moving exactly towards the west."

     Aryabhata was among the first astronomers to make an attempt at measuring the Earth's circumference. Aryabhata accurately calculated the Earth's circumference as 24,835 miles, which was only 0.2% smaller than the actual value of 24,902 miles.      

    Another of Aryabhatta's work, Aryabhatiya-Siddhanta, is only known through references to it another books.Among his most notable contributions to modern astronomy are: the explanation and computation of solar and lunar eclipses, the expounding of the heliocentric model of the solar system and the computation of the length of earth's revolution around the sun.

     We now go ahead in chronological order to the other great astronomers of ancient India beginning with Varahmihira (505 - 587 AD). He workedas one of the Navratnas or nine gems in the court of Chandragupta Vikramaditya. His book Panchasiddhantika (The Five Astronomical Canons), written in 575 AD gives us information about older Indian texts which are now lost. The work is a treatise on mathematical astronomy.

    Next,we come to, Brahmagupta (598-668 AD). He wrote two texts - Brahmasphutasiddhanta in 628 and the Khandakhadyaka in 665. Some of his important contributions are: methods for calculations of the motions and places of various planets, their rising and setting, conjunctions, and the calculations of eclipses of the sun and the moon.    
   

    Sripati(1019 - 1066 AD) was an Indian astronomre and mathematician, author of Dhikotidakarana (written in 1039 AD) a work on solar and lunar eclipses. He also wrote the Druvamanasa in 1056 AD for calculating planetary longitudes, eclipses and planetary transits. He also wrote a majr work on astronomy titled Siddhantasekhara and an incomplete mathematical treatise Ganitatilaka.
 
     Next, we take a look at Bhaskara (1114 - 1185). His main works are Lilavati, Bijaganti and Siddhanta Shiromani. He worked on the following subjects: mean longigtudes of the planets, true longitudes of the planets, the three problems of diurnal rotation, syzygies, lunar and solar eclipses,latitudes of the planets, risings and settings, the moon's crescent, conjunctions of planets with each other and the conjunctions of planets with the fixed stars, the paths of the sun and the moon. He is also credited with the near accurate calculation of the sidereal earth as 365.2588 days. The modern accepted measurement is 365.2596 days, an error of just one minute. He also wrote about the first visibilities of the planets,astronomical instruments, problems of astronomical calculations and the seasons.

     Here we end the great journey that began with Aryabhatta and ended with Bhaskara. I hope you can respect that the work that these great astronomers have done at so early a time. Their work was lost before being found. Theories are being discussed that the Arabs translated this work in Kerala and then made it available to the Europeans in the 15th century which introduced them to the works of calculus. This is only a theory and has not yet been proved.Studies on this matter continues till this date. There is also work on the translation of some of the major works into English and Hindi. But, the true beauty of these works can be recognised only when read in the language in which they were written -  Sanskrit.