Siddhanta Skanda is one of the aspect of Astrology.
As per Narada Samhita, astrology is a combination of three Skandas namely Siddhanta, Samhita and Hora Skandas. Later the five parts or Pancha Skandas namely Siddhanta, Hora, Samhita, Prasna, Sakuna put together formed into Astrology.
Siddhanta Skanda deals with the division of time periods as Yuga, Mahayuga, Solar period, Savana period, Lunar period and various time measurements and their uses. The movements of planets like Sun etc., the retrograde motion of five planets like Mars, etc., the planetary movements, the variation in sunrise & sunset depending on the place, Moudha (combustion), the period of planetary movements, revolvements, orbits of each planet, the distance of various planets from earth, particulars of place, time and their geometry.
The books related to these aspects in Siddhanta Bhaga help in writing of Panchanga / Ephemeris.
The books that mention calculation of planets from the beginning of Kalpa is called Siddhanta Granthas. Tantra Granthas deal with the calculation of planets from Kaliyuga. Karana Granthas accounts the calculations of planets from the beginning of Saka. Besides these there are separate books on the machinery for determination of time, cosmology, etc,.
In the Siddhanta there are famous books like Surya Siddhanta, Aryabhattiyam, Siddhanta Siromani, etc.. The movements of planets are well explained in these books.