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Now we mention some books written in Kerala. 1. Deva Keralam, Sukra Keralam, of about the seventh century A.D.
2. Sarkarānārayaneeyam, by Sarkaranārāyana of the ninth century A.D. This book refers to an astronomical observatory and its instruments established by a contemporary King Ravi Verma.
3. Venwaroham by Mahādevan, discusses trigonometry and planetary motions.
4. Karanapaddhati written in 1431 AD. 5. Sadratnamāla by Sakara Vaiman, Written in about 1530 AD. 6. Tantra Sangraham by Nilakantha, Written in about 1500 AD. 7. Yuktibhāṣā by Brahmadatta, Written in 1639, AD.
The last five books are in verse form and give infinite series expressions for sin x, cos x and tan x about two and half centuries earlier than in Europe. The series known as Gregory's series. For tan' x was also known in India more than two centuries earlier than in Europe.
Since astronomy is a practical subject, its success depends on the use of instruments. Ancient Indian astronomers were aware of this aspect and designed suitable instruments for astronomical measurements. In almost every book on astronomy, a chapter was devoted to the construction and use of such instruments.
The earliest instrument available in vedic times was Tūrya consisting of a quadrant circle with a movable tube through the centre and resting on the arc of the quadrant, which is graduated in degree and minutes. Through the tube, the heavanly bodies were observed, and the angle made by the tube with the vertical gave the zenith distance called Natansk and its complement unantash or attitude. During the Vedamga Joytişa period, a water clock was developed.
Chapter XIII of Sūryā Siddhānta discusses the construction of an auxiliary sphere, a Sankuyantra or gnomon, the Cakrayantra, Mayura yantra, Vanayantra, etc. Brahmagupta discusses, in Brahmasphuta Siddhānta, seventeen instruments by which observation could be made. Bhaskarācārya gave the description of an additional instrument called Phalakyantra. Another instrument called Dhruvmrey Yantra was devised by Padmanābhan in the fourteenth century. Yantra Cintāmani, written by Cakardan, was a separate book on instruments.
An observatory appears to have been constructed at Ujjain of Avanti; it may have been at the same place where the present observatory stands. Another observatory, Man Mandir, was constructed by Raja Mansingh at Varanasi and a number of instruments were added to it by Raja Jai Singh who also built the observatory at Jaipur as well as the city Jaipur around it. He described all
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