Venkatesh Ketakar

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Venkatesh Ketakar or V.B.Ketakar (? - Aug 1930) was an astronomer who lived in India. Almost forgotten now even in his native country, his claim to fame is that he correctly predicted some parameters of the orbit of Pluto - nearly 20 years before it was actually discovered by Clyde Tombaugh. Though Tombaugh was unaware of Ketakar's work, the latter's calculations for Pluto turned out to be much more correct than those of Percival Lowell and William H. Pickering, who are still remembered for their significant contributions to astronomy. By the time Pluto was discovered in February 1930, Ketakar had suffered a severe paralytic stroke and died soon after.

Around the turn of the 20th century, astronomers thought that the orbit of Neptune was being disturbed by the gravitational pull of a yet-undiscovered planet. Neptune itself had been discovered because it disturbed the orbit of Uranus. Therefore many astronomers including Lowell and Pickering started analyzing Neptune's orbit in the hopes of discovering another planet. So did Venkatesh Ketakar.

Ketakar published his calculations in the May 1911 issue of Bulletin of the Astronomical Society of France. Unlike Lowell and Pickering, he modeled his computations after those of Pierre-Simon Laplace who had analysed the motions of the satellites of Jupiter. Ketakar computed that the heliocentric longitude of the unknown planet (which he called Brahma) would be 114.23 degrees and that its mean daily movement was 14.6364 arc-seconds which meant that its orbital period was 242.28 years. Using the third law of Johannes Kepler, Ketakar estimated that the new planet's mean distance from the Sun was 38.95 A.U. Ketakar also postulated the existence of another planet beyond Brahma at 59.573 A.U away from the Sun. He called it Vishnu. No such planet has been discovered and its existence is very doubtful.

On February 18, 1930, Clyde Tombaugh actually discovered Pluto after analyzing thousands of photographs of the night sky. It was found in the zodiacal constellation of Gemini at a heliocentric longitude of 136.28 degrees. Soon it was known that its mean daily motion, orbital period and mean distance from the Sun were 14.283 arc-seconds, 248.475 years and 39.52 A.U., respectively. These values are very close to what Ketakar had calculated. Pickering's and Lowell's estimates of the same parameters turned out to be far less accurate. A simple extrapolation using Ketakar's figures show that on the date of discovery, Pluto's heliocentric longitude would have been 137.45 degrees - just about a degree away from the true location!

A major weakness in Ketakar's work is that he did not compute the other co-ordinate needed to find Pluto, i.e., the inclination of the planet's orbit to the ecliptic. Pickering and Lowell estimated it to be 21 and 10 degrees respectively. At 17.15 degrees, Pluto's orbital inclination is larger than that of any other planet. Thus, Pickering had an inkling about Pluto's unusual orbital inclination. His calculations dated January 1, 1909 also gave Pluto's heliocentric longitude as 105.8 degree which accords very well with the actual value of 105.59 degrees on that date. However, since Pickering's estimate of mean daily motion was only 9.5 arc-seconds, his work would not have helped a future astronomer find Pluto.

In an ironic footnote to the history of the search for Pluto, modern measurements show that there are, in fact, no perturbations in the orbit of Neptune. Nineteenth century astronomers had erroneously ascribed a wrong value to the mass of Neptune, hence had thought that Neptune was being pulled by the gravity of a planet beyond it. Therefore, all the calculations of Pickering, Lowell, Ketakar and others were based on wrong data! In any case, after the discovery of Pluto's satellite Charon in 1978, the planet is now known to be too small to influence Neptune's orbit. Thus, the accuracy of Ketakar's predictions must be regarded as sheer coincidence!