In 1680, The Royal Astronomer, John Flamsteed, gathered observational data about a massive comet that passed Earth. At this time, astronomers thought that comets came in pairs: to the general observer, it appeared as though one comet would go past the earth and get lost in the sun, and then another would arrive from the opposite direction. Further, while many believed in the heliocentric schema of the solar system, it was commonly thought that the planets and other heavenly bodies orbited in circles, not ellipses. Astronomer Johannes Kepler (1571-1630) had previously argued for elliptical orbits, but this had not yet been proven.
John Flamsteed made extremely accurate observations of this new comet in 1680, and he became convinced that there was only one comet, not a pair of comets. Moreover, he thought that the comet did not move in a circular pattern, but rather, in an ellipse. However, Flamsteed incorrectly believed that the comet only approached the sun and was forcibly repelled by its cosmic rays, which sent it careening back the way it came. He did not think that it traveled around the sun.
Flamsteed was excited by his idea and contacted his colleague, Isaac Newton. At the time, Isaac Newton was the Lucasian Professor of Mathematics at Cambridge University in England. Newton was skeptical, but he asked for more data, which Flamsteed provided. Newton had also been working on a text that later became the basis of a new kind of physics, the Philosophiæ Naturalis Principia Mathematica. The Principia (prin-ki’-pia) as it became known, was published 5 July 1687.
When the Principia appeared in print, it became clear to Flamsteed that Newton had used his data in Book 3 ("The System of the World"). Newton developed calculations from the data to prove that the comet went around the sun in an elliptical orbit; the comet was not pushed away from sun, but rather swung around it, not completely unlike a ball on a string. Further, Newton’s data and diagrams showed that there was only one comet, not a pair of comets moving in opposite directions, just as Flamsteed had hypothesized. Perhaps most profoundly, Newton used the data associated with the comet’s elliptical path to introduce the idea of gravity. The Great comet of 1680 would also become known as "Newton's Comet."
Much to his dismay, Flamsteed was never credited for his contribution. And yet, Flamsteed’s data allowed Newton to prove Kepler’s law of parabolic orbits, create a new form of physics, and institute a massive restructuring in how we understand the mechanics of the universe.
To the Left is Newton's sketch describing the comet's path as it travels around the sun, marked as 'S' in the middle of the page.
Flamsteed to Crompton [for Newton] (15 December 1680) and Flamsteed to Crompton [for Newton] (3 January 1680/1) in Flamsteed, Correspondence, vol. 1, 749, 750.
Newton to Crompton [for Flamsteed] (28 February 1680/1) in Flamsteed, Correspondence, vol. 1, 764-769.
Flamsteed to Crompton [for Newton] (7 March 1680/1 in Flamsteed, Correspondence, vol. 1, 771-778.
Newton to Flamsteed (19 September 1685) in Flamsteed, Correspondence, vol. 2, 245-246.