A manv-antara is the period controlled by one Manu. The reign of fourteen Manus equals the length of one day (twelve hours) in the life of Brahma, and the night of Brahma is of the same duration. These calculations are given in the authentic astronomy book known as the Surya-siddhanta. This book was compiled by the great professor of astronomy and mathematics Bimal Prasad Datta, later known as Bhaktisiddhanta Sarasvati Gosvami, who was our merciful spiritual master. He was honored with the title Siddhanta Sarasvati for writing a commentary on the Surya-siddhanta, and the title Gosvami Maharaja was added when he accepted sannyasa, the renounced order of life.