The Tibetan Astro. Science spread in Tibet is mainly of two kinds: Astronomy (skar-rtsis) and Astrology ('Byung-rtsis).
The basic for this system is there in Tibet from the very early time which our ancient farmers and nomads make examination of the constellation to forecast regarding the changes of four seasons and rainfall for their agriculture purposes and pastures for the cattle.
However, this system brought into develops and is more systematic way after the translation of Kalachakra tantra into Tibetan in the year 1027A.D. which contains five chapters. Our subject is more related to the external Kalachakra tantra regarding the formation of the Universe which comes under the first chapter.
The Lord Buddha Shakyamuni first taught this Kalachakra tantra on his 81st years of age (881 B.C.) at the stupa of Shri Dhanakataka or Paldern Drepung in Amravati, South India on the full moon day of 3rd month of a Metal-Dragon Year. This tantra contains information such as the natural state of the Universe, inhabitance and especially the planetary or the cosmological influence and about how comes the four seasons were clearly shown.
The rCTA material of the Kalachakra tantra presents the sources of all the calculations and interpretation used for determining the position of the Sun and Moon and other planets for making the calendar or almanac. Much of our calendar and almanac is based on principles derived from the same.
This Kalachakra material also includes of various activities according to planetary aspects, and contains the text known as "Arising Vowel", which correlates the characteristics of Sanskrit alphabet and phonemic systems of vowels and consonants with Astrological interpretations from the positions of the planets.
Though the rudiment for this system presents in Tibet during the pre-Buddhist religion Bon, it has just been passing down only through Oral transmission. However, later during the time of gNam-ri-srong-btsan in the 6th century, the translation of elemental astrology form China early started and helped bringing development in the same.
This is one of the systems used to make predictions regarding the joy and sorrows that one way experience in one's lifetime by using elemental laws of relations and methods. This doesn't require any mathematics calculation as required in astronomy. The elemental system is believed to have taught by the lord Manjushri in China during 837 B.C.
In brief, the Kalachakra tantra contains all the information and calculation used in Astrology and Astronomy. Therefore, Kalachakra is the main source of our practice.