In India, by the 6th century AD astronomers believed that comets were apparitions that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th-century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.