स्फुटनिर्णयतन्त्रम्: Sphutanirnaya-Tantra of Acyuta with Auto-Commentary (An Old and Rare Book)

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Introductory
The Sphutanirnaya-Tantra, critically edited here with auto- commentary, is a comprehensive work on an important aspect of Hindu astronomical computation, to wit, the accurate determination (nirnaya) of the True Positions (sphuta) of the planets. The author, Acyuta (A. D. 1550-1620) was an astute astronomer hailing from Kerala in South India. He was a versatile scholar and original thinker on astronomy who enunciated, for the first time in Indian astronomy, the correction called 'Reduction to the ecliptic', in the work edited here, composed before 1593, and later explained its rationale in another work Rasigolasphutaniti, composed specially for that purpose. The Sphutanirnaya belongs to that class of astronomical texts called Tantra, which are characterised by the enunciation of the planetary revolutions etc. in terms of the aeon (yuga) and the commencement of the calculations from the beginning of the current Kali-yuga.

Sphutanirnaya
This is a compact work in ninety verses, divided into five chapters devoted, respectively, to: (1) the enunciation of the astronomical constants as fixed by the author; (2) the computation of the Mean Positions of the planets in their respective orbital circles; (3) the reduction of the said Mean Positions to the Manda-epicycles; (4) the further reduction of the corrected Mean Positions to the celestial sphere (bhagola); (5) the reduction even of these to the centre of the earth (bhugola); and (6) the still further reduction thereof to the earth's surface (bhugola-prstha). The last will give the True Position (sphuta) of the planets as seen by an observer stationed on the surface of the earth.