bathimpact advent calendar, December 10th 1684: Netwon’s theory of gravity read to the Royal Society


On this day in 1684 Edmond Halley or Comet to his friends read out Isaac Newton’s derivation of Kepler’s laws to the Royal society in London.

Kepler’s laws of planetary motion were observational improvements on those of Copernicus nearly 100 years before.  Kepler used Copernicus data set and expanded upon it to produce a detailed map of the locations of each heavenly body at a given date.

newtonKepler’s laws state:

  1. The orbit of a planet is an ellipse with the Sun at one of the two foci.
  2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

However even though these laws were generally accepted to be correct, the mathematical proof for such motion was still unproven. Essentially, we knew the what but not the why.

As with most scientific breakthroughs in the 17th century the story behind Newton’s involvement with Kepler’s laws is very, well, underwhelming. Following a visit for Halley, where they had a chat about problems plaguing the intellectual elite of the day, possibly over tea and cucumber sandwiches, Newton sat down and within less than a year wrote what is potentially the foundation for all space based technologies we use today. Simple really!

Of course Newton being Newton, he didn’t bother naming the thing so “De motu corporum in gyrum” is the name that for some reason we gave it. I personally think all scientific papers should follow the Die Hard naming sequence.  “Newton with a Vengeance” is a much better name in my opinion

The extract from “Newton with a Vengance” or “Planetary bodies in an orbit” Edmund Halley read out to the society, gave mathematical proof, that, according to Newton’s own laws of motion and Universal gravitation all three of Kepler’s laws could be used to good approximation as a model of our solar system.

This meant that the planets motions could be predicted mathematically, the best times to observe Venus or mars could be found out years in advance, the period of comets could be plotted and many more things astronomic things could be calculated… most of which wouldn’t really be that useful until the 20th century actually, but I’m sure Kepler didn’t mind.

Even now, when landing on comets or shooting rockets into space, Kepler’s laws are used as back of envelope calculations in order to check that complicated computer models are at least giving sensible answers.  Which is kind of impressive, I suppose.


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John Barlow is Editor-in-Chief (2015/16) and former bite Editor (2014/15) at bathimpact. He writes about society, pop culture, music and film. He also reports on a number of University of Bath and local politics issues.

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