why is Aryabhata considered to be one of the greatest of astronomers?

Aryabhata is the famous Indian mathematician and astronomer,who was born in 476 AD.He studied at the university of Nalanda.Later,he became the head of the University.
  Aryabhata is considered to have changed the course of mathematics and astronomy to a great extent.He was the first to deduce that the earth is round,and that it rotates on its own axis,creating day and night.He declared that the moon is dark and shines only because of sunlight,and that the eclipses are caused by the shadows cast by the earth and the moon.
  His most famous works are the Aryabhatiya, and the Arya Siddhanta.Aryabhata was the first in the line of great mathematician-astronomers from the classical age of the Indian mathematics and astronomy.India's first satellite is named after him.