At the end of the last section Isaac Newton was still a student, who had embarked on a six-year period of intensive study teaching himself the modern analytical mathematics, the basics of mechanics and optics.In 1666 during the phase when he was learning mechanics, principally from the works of Descartes and where like Huygens he corrected Descartes theories of elastic collision and Galileo’s false value for g, the acceleration due to gravity, he had his legendary flash on inspiration, possibly inspired by the equally legendary falling apple, in which he asked himself if the force that causes an object to fall to the ground is the same as the force that prevents the Moon from flying off at a tangent, as the law of inertia, acquired from Descartes, said it should. Newton made a back of an envelope calculation, which gave an interesting correlation but was somewhat inaccurate due to inaccurate input data. Newton dropped the line of enquiry and didn’t take it up again for almost twenty years. However, one aspect of his calculation was very important for the future. In order to calculate the force holding the Moon he plugged Kepler’s third law into Huygens’ formula for centripetal force, which led to the inverse square law of gravity.

In 1669, on the recommendation of Isaac Barrow the retiring incumbent, Newton was appointed Lucasian Professor of Mathematics at Cambridge University.The appointment was not as impressive as it appears today and Newton remained still largely under the radar, although the mathematics fan John Collins (1625–1683) had circulated some of his mathematical manuscripts awaking the world to his immense mathematical talent. This changed in the early 1670s when he presented the world with his reflecting telescope, the first functioning one, and published his first paper on the nature of white light. A new leading natural philosopher had arrived on the European stage.

In 1680 and 1681 two new great comets lit up the skies and once again the astronomers all turned their attentions into trying to determine their flight paths. The 1680 comet was discovered by the German astronomer Gottfried Kirch (1639–1710) from Coburg, who lived from writing and publishing almanacs, on 4 November.

It was the first ever comet to be discovered by telescope, that is before it became visible to the naked eye. It remained visible until 7 December when it disappeared. The comet of 1681 first appeared on 20 December. One astronomer, John Flamsteed (1646–1719), who had been appointed Astronomer Royal for the new Royal Observatory at Greenwich in 1675, had the bright idea that these were not two separate comets but one single comet on its way to and from the sun (modern designation C/1680 V1). Unsure of his assumption Flamsteed turned to Isaac Newton to ask his opinion. Flamsteed did not know Newton personally so the contact, by letter, was initially through a mutual acquaintance at Cambridge.

Flamsteed’s hypothesis was that the comet turned in front of the Sun upon reaching it; he, echoing Johannes Kepler, suggested that the comet was attracted to the Sun magnetically and then through a change in polarity as it neared the Sun repulsed. In two letters in February 1881 Newton dismantled Flamsteed’s hypothesis, concentrating on his magnetic argument but also not accepting that the two comets were actually just one. Newton had applied the inverse square law of gravity to a theoretical system consisting of a single planet and the Sun, a year earlier, but did not apparently consider applying it to the comet at this point in time. However, in a draft of his second letter to Flamsteed, which he never sent, he did sketch a dynamic system of the comet circling behind the Sun but in terms of magnetic attraction.

Later in the year Newton received new observational data on the comet from an old school acquaintance, Arthur Storer (c. 1648–1686) an amateur astronomer, who had emigrated to Maryland in 1679. He also later sent Newton data on the 1682 comet (Comet Halley), which he was amongst the first to observe in North America and which was named after him there for some time. Edmond Halley (1656–1741), an excellent astronomer and mathematician, who observed the comet of 1680/81, whilst travelling in France, also believed, like Flamsteed, that the two comets were one. In 1682 he came to Cambridge to visit Newton and the two of them discussed the comets.

Newton observed the comet of 1682 and at some point after 1680 he systematically collected together data on all recorded comets and decided that comets did indeed obey the inverse square law of gravity just like planets, their paths being oval if they returned and hyperbola if not. This was possibly the point where Newton’s thoughts on gravity became a universal theory of gravity. Comets and their flight paths would go on to play a significant role in the *Principia*. Newton apparently didn’t think to inform Flamsteed of his change of mind and acknowledge that Flamsteed had been right, at least in principle, until 1685.

Newton and Flamsteed were not the only people to reconsider the flight paths of comets in the early 1680s and Newton was not the only person to think that the inverse square law of gravity applied to them, Newton’s rival Robert Hooke also did so. Robert Hooke had been investigating the effects of gravity for many years and had discovered the inverse square law for himself and became convinced of a universal gravity. He thought that the flight paths of comets, like planets, were determined by gravity and that the inverse square law also applied to them. However, unlike Newton he didn’t do the mathematics. This mutual independent discovery of universal gravity would lead to renewed conflict between the two natural philosophers, who had already crossed swords over the nature of light.