The Tantra of Kalachakra is the basis of Tibetan astronomy.
It explains some phenomena in a similar manner as modern astronomy science. Hence, Sun eclipse is described as the Moon passing between the Sun and the Earth.
In 1318, the 3rd Karmapa received vision of Kalachakra which he used to introduce a revised system of astronomy and astrology named the "Tsurphu Tradition of Astrology" (Tibetan: Tsur-tsi) which is still used in the Karma Kagyu school for the calculation of the Tibetan calendar.
^An Introduction To Tibetan Astro. Science Archived February 3, 2009, at the Wayback Machine
^The Third Gyalwa Karmapa, Rangjung Dorje, site of Karma Lekshey Ling Institute[dead link]
^Staff. "Kagyu Lineage: The Third Karmapa Rangjung Dorje (1284 - 1339)". Kagyu Office of His Holiness the 17th Gyalwang Karmapa. Retrieved 2010-02-04.
Alexander BerzinTibetan Astrology and Astronomy. in Maitreya Magazine (Emst, Holland), vol. 11, no. 4 (1989).
Berzin, A Tibetan Astro Studies. in Chö-Yang, Year of Tibet Edition (Dharamsala, India), (1991).