|Residence||Trikkandiyur, Tirur, Kerala|
|Other names||Kelallur Comatiri|
|Known for||Authorship of Tantrasamgraha|
|Notable work(s)||Golasara, Candrachayaganita, Aryabhatiya-bhashya, Tantrasamgraha|
Kelallur Nilakantha Somayaji (Malayalam: നീലകണ്ഠ സോമയാജി, Nīlakaṇṭa Sōmayāji ?, Sanskrit: नीलकण्ठ सोमयाजि) (1444–1544) (also referred to as Kelallur Comatiri) was a major mathematician and astronomer of the Kerala school of astronomy and mathematics. One of his most influential works was the comprehensive astronomical treatise Tantrasamgraha completed in 1501. He had also composed an elaborate commentary on Aryabhatiya called the Aryabhatiya Bhasya. In this Bhasya, Nilakantha had discussed infinite series expansions of trigonometric functions and problems of algebra and spherical geometry. Grahapareeksakrama is a manual on making observations in astronomy based on instruments of the time.
Biographical details 
Nilakantha Somayaji was one of the very few authors of the scholarly traditions of India who had cared to record details about his own life and times. So fortunately a few accurate particulars about Nilakantha Somayaji are known.
In one of his works titled Siddhanta-star and also in his own commentary on Siddhanta-darpana, Nilakantha Somayaji has stated that he was born on Kali-day 1,660,181 which works out to 14 th June 1444 CE. A contemporary reference to Nilakantha Somayaji in a Malayalam work on astrology implies that Somayaji lived to a ripe old age even to become a centenarian. Sankara Variar, a pupil of Nilakantha Somayaji, in his commentary on Tantrasamgraha titled Tantrasamgraha-vyakhya, points out that the first and last verses of Tantrasamgraha contain chronograms specifying the Kali-days of the commencement (1,680,548) and of completion (1,680,553) of Somayaji's magnum opus Tantrasamgraha. Both these days occur in 1500 CE.
In Aryabhatiya-bhashya, Nilakantha Somayaji has stated that he was the son of Jatavedas and he had a brother named Sankara. Somayaji has further stated that he was a Bhatta belonging to the Gargya-gotra and was a follower of Asvalayana-sutra of Rigveda. References in his own Laghuramayana indicate that Nilakantha Somayaji was a member of the Kelallur family (Sanskritised as Kerala-sad-grama) residing at Kundagrama, now known as Trikkandiyur in modern Tirur, Kerala. His wife was named Arya and he had two sons Rama and Dakshinamurti.
Nilakantha Somayaji studied vedanta and some aspects of astronomy under one Ravi. However, It was Damodara, son of Kerala-drgganita author Paramesvara, who initiated him into the science of astronomy and instructed him in the basic principles of mathematical computations. The great Malayalam poet Thunchaththu Ramanujan Ezhuthachan is said to have been a student of Nilakantha Somayaji.
The epithet Somayaji is a title assigned to or assumed by a Namputiri who has performed the vedic ritual of Somayajna. So it could be surmised that Nilakantha Somayaji had also performed a Somayajna ritual and assumed the title of a Somayaji in later life. In colloquial Malayalam usage the word Somayaji has been corrupted to Comatiri.
Nilakantha Somayaji as a polymath 
Nilakantha's writings substantiate his knowledge of several branches of Indian philosophy and culture. It is said that he could refer to a Mimamsa authority to establish his view-point in a debate and with equal felicity apply a grammatical dictum to the same purpose. In his writings he refers to a Mimamsa authority, quotes extensively from Pingala's chandas-sutra, scriptures, Dharmasastras, Bhagavata and Vishnupurana also. Sundararaja, a contemporary Tamil astronomer, refers to Nilakantha as sad-darshani-parangata, one who had mastered the six systems of Indian philosophy.
In his Tantrasangraha, Nilakantha revised Aryabhata's model for the planets Mercury and Venus. His equation of the centre for these planets remained the most accurate until the time of Johannes Kepler in the 17th century.
In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, Nilakantha developed a computational system for a partially heliocentric planetary model in which Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted this planetary model.
Works of Nilakantha Somayaji 
- Golasara : Description of basic astronomical elements and procedures
- Sidhhantadarpana : A short work in 32 slokas enunciating the astronomical constants with reference to the Kalpa and specifying his views on astronomical concepts and topics.
- Candrachayaganita : A work in 32 verses on the methods for the calculation of time from the measurement of the shadow of the gnomon cast by the moon and vice versa.
- Aryabhatiya-bhashya : Elaborate commentary on Aryabhatiya.
- Sidhhantadarpana-vyakhya : Commentary on his own Siddhantadarapana.
- Chandrachhayaganita-vyakhya : Commentary on his own Chandrachhayaganita.
- Sundaraja-prasnottara : Nilakantha's answers to questions posed by Sundaraja, a Tamil Nadu based astronomer.
- Grahanadi-grantha : Rationale of the necessity of correcting old astronomical constants by observations.
- Grahapariksakrama : Description of the principles and methods for verifying astronomical computations by regular observations.
- Jyotirmimamsa : Analysis of astronomy
See also 
- K.D. Swaminathan (1972). "Jyotisa in Kerala". Indian Journal of History of Science 8 (1 & 2): 28–36. Retrieved 3 February 2010.
- K.V. Sarma (editor); V.S. Narasimhan (translator). "Tantrasamgraha with English translation" (in Sanskrit and English). Indian National Academy of Science. p. 48. Retrieved 17 January 2010.
- Tantrasamgraha, ed. K.V. Sarma, trans. V. S. Narasimhan in the Indian Journal of History of Science, issue starting Vol. 33, No. 1 of March 1998
- P. Vinod Bhattathiripad; K.D. Nambudripad (3 May 2007). "Yaagam (Yajnam)". Namboothiri Websites Trust. Retrieved 4 February 2010.
- George G. Joseph (2000). The Crest of the Peacock: Non-European Roots of Mathematics, p. 408. Princeton University Press.
- K. Ramasubramanian, M. D. Srinivas, M. S. Sriram (1994). "Modification of the earlier Indian planetary theory by the Kerala astronomers (c. 1500 AD) and the implied heliocentric picture of planetary motion", Current Science 66, p. 784-790.
- A.K. Bag (1980). "Indian literature on mathematics during 1400 - 1800 AD". Indian Journal of History of Science 15 (1): 79–93. Retrieved 30 January 2010.
Further reading 
- R.C. Gupta. "Second order interpolation in Indian mathematics up to the fifteeth century". Indian Journal of History of Science 4 (1 & 2): 87–98.
- Shailesh A Shirali (May 1997). "Nilakantha, Euler and pi". Resonance: 28–43. Retrieved 1 February 2010.
- Ranjan Roy (Dec. 1990). "The discovery of the series formula for π by Leibnitz, Gregory and Nilakantha". Mathematics Magazine (Mathematical Association of America) 63 (5): 291–306. doi:10.2307/2690896. Retrieved 18 January 2010.