Hindu Astrology or Jyotish was divided into three sections (Tri-Skandha Jyotish) by ancient masters. For example, none other than Varahamihira himself divided Jyotish into Ganita, Hora and Angavinischaya.
The science of Astrology has many branches within its scope and is mainly divided under three heads or sections termed Skandhas viz. Ganita or Tantra, Hora and Angaviniscaya. The treatment of this subject in its entirety has been designated as Samhita, Collection or compilation, by the Sages. The ﬁrst section or Skandha is called Tantra as it deals with the movements of planets through the signs of the zodiac that are arrived at by means of mathematical calculations. The second section treats Hora or horoscopy. The third and the last is termed Angaviniscaya — determination of the limbs — as it explains the method of ascertaining by calculation the ascendant and other houses in a natal chart. - Brihat Samhita I.9
Thus, we see that initially the mathematical part was divided into two separate sections. Calculating planetary positions were part of Ganita and Calculating Lagna was part of Angaviniscaya.
Sage Garga also divided Jyotish into three sections. The last section was named by him simply as Shakha (instead of Angaviniscaya).
A much later classic Prasna Marga (1649 AD) divided Jyotish into three sections, namely Ganita, Samhita and Hora.
Ancient astrological science is divided into three Skandhas or six Angas. The three skandhas are Ganita, Samhita and Hora. - Prasna Marga I.5
However, the author of Prasna Marga (a Namboodiri Brahmin from Kerala) mentioned another classification scheme. He divided Jyotish into six limbs.
Sages have classified the great science of Astrology into six Angas, viz., Jataka, Gola, Nimitta, Prasna, Muhurta and Ganita. - Prasna Marga I. 6
Notice the similarities here. There are six limbs of the Vedas and now the Jyotish also has six limbs - Jataka (natal horoscope), Gola (spherical astronomy - planets), Ganita (mathematical part), Nimitta (Omens, natural phenomenon, weather, forecasting of immediate rain, nature of future crops etc), Prashna (casting horoscope based on the time when the question was asked) and Muhurta (fixing up auspicious times for different undertakings).
The relationship between Skandhas and Angas are explained in the next verse.
Ganita Skandha deals with Gola and Ganita. Hora Skandha deals with Horoscopy, Prasna, Muhurta and a part of Nimitta. Samhita Skandha deals elaborately with Nimitta. - Prasna Marga I. 7
In another verse of the same chapter, the author proposed another classification scheme.
Astrology can also be divided into two, viz., Pramana and Phala. Ganita Skandha comes under Pramana while the other two Skandhas go under Phala. - Prasna Marga I.9
Pramana is basically the Ganita section. Thus we get the two-fold classification: Ganita Jyotish and Phalita Jyotish. The Ganita Jyotish now comprises the entire mathematical part, and Phalit Jyotish is basically the Hora and Samhita sections.
Notice that the terminologies and their meanings got changed over time.
- Brihat Samhita with English Translation, Exhaustive Notes and Literary Comments by M. Ramakrishna Bhat, Motilal Banarsidass Publishers, Delhi
- Prasna Marga English Translation with Original Text in Devnagari and Notes by Bangalore Venkata Raman, Motilal Banarsidass Publishers, Delhi