Description of Planetary Motion

For Hooke, gravity was one of the universal principles. Among his very first experiments as curator of the Royal Society at the end 1662 he carried out experiments as to whether bodies at different heights had measurable differences in weight, but could not show any difference however. Almost three years later, he repeated similar experiments in a deep well at Banstead in Sussex, which was also without any significantb results. A further attempt in 1666 to measure a height dependence of gravity using the periodic time of a pendulum led to no measureable results.

As an astronomer Hooke was interested in how the observed motion of the planets could be explained. On 23rd. May 1666, he proposed to the assembled members an interpretation for the planetary orbits described by Kepler. The motion of the planets could be interpreted as a superposition of a straight line motion with a deviation caused by the attraction of the Sun at the center. In his Cutler Lecture of 1674, Hooke firmed up his conception on the effect of gravitation further. The effect between the celestial bodies become stronger the nearer they are to each other.

Isaac Newton

At the end of November 1679 Hooke, in his capacity as the new secretary of the Royal Society, renewed contact with Newton and requested him to take up correspondence with the Society again. Incidentally Hooke asked Newton what he thought of his thesis that planetary motion consisted of a tangential motion and an attractive motion towards the central body. Newton replied that he had no knowledge of Hooke's thesis ans was not interested at the moment in any exchange of scientifiv ideas. In spite of Newton's initial dismissive attitude, he developed between 24th. November 1679 and 3rd. December 1680 a seven-letter excahnge of ideas counting as the most influential in the history of physics in the couse of which Hooke related Newton in 1680 "My assumption is however that the attraction acts proportionally to the reciprocal of the square of the distance from the center

In January 1684, Hooke, Wren and Halley met in a coffee house after a meeting of the Royal Society and discussed the issue of whether the elliptical form of a planetray orbit could be produced by a force which decreased by the square of its distance from the Sun. Hooke maintained to both that it could prove it so.

When Halley, half a year later, was a guest of Newton in Cambridge, he asked him the question as to what form an orbit would have under the influence of such a force. Newton answered immediately "an ellipse" and told Halley that he had carried out the required calculations. he promised to send them to Halley. In November Halley received Newton's nine-sided paper "De motu corporum in Gyrum (on the motion of bodies)", which is a forerunner of Newton's 'Principia Mathematica' which appeared in Summmer 1687. Halley, who finaced the publication of Newton's Principia and supervised the printing, told him in May 1686 , that Hooke expected an appropriate recognition for this discovery.