Celebrating Neptune's first birthday with Chris Linton

Voyager image of Neptune
(Courtesy NASA/JPL-Caltech)

Prof. Chris Linton (Loughborough) opened his talk in Loughborough University's Schofield Building with the above picture of Neptune, taken by the Voyager spacecraft in 1989. Earlier this year, on 12th July 2011, Neptune arrived for the first time at the same heliocentric longitude as when it was discovered on 23rd September 1846. In this sense, then, 12th July 2011 was one Neptunian year since the discovery of the planet.

Chris argued that the interesting element of this story to mathematicians was that Neptune was discovered first through theory. Chris recounted the story of the discovery of Uranus by Herschel in 1781. Uranus followed an orbit that differed quite quickly (by 1788) from predictions. The predictions were recalculated to take into account the perturbations due to Jupiter and Saturn and this helped for a while, but still Uranus deviated from the predicted path by the early 1800s. Unless Newton's Laws were wrong, there must be a planet outside Uranus which was affecting its orbit.

Chris explained how this novel theoretical challenge was taken up by John Couch Adams and Urbain Le Verrier, giving some detail of the models formed and simplifications used to arrive at a result. Both independently predicted limits for the position of this extra-Uranian planet and Chris gave an account of several searches that were undertaken. The first of these to succeed in observing the new planet and recognising it as such was Johann Galle. Chris spoke about a row that erupted over the discovery but in the end tempers cooled and the new planet was named Neptune.

Chris also briefly outlined the example of Vulcan, the theorised planet between Mercury and the Sun that was causing a discrepancy in Mercury's orbit. In fact, this was found to be a limitation of Newton's theory; the discrepancy eventually being accounted for by Einstein's general relativity in 1915.

The recent Nobel Prize in Physics was awarded for the observations that the rate at which the universe is expanding is accelerating, a discrepancy from theory that led to the theory of dark energy and dark matter. Chris left us with the food for thought: is dark energy a Neptune situation (something out there we can't yet see) or a Vulcan situation (a limitation of current theory)?

Overall this was a highly enjoyable talk presented to a large audience, including a sizable contingent of sixth form students. I hope this brief account, skipping much of the detail, has given some flavour of the experience.

The next East Midlands Branch talk is From Sylvia Plath to Bad Sex: uses of mathematics in fiction by Tony Mann (University of Greenwich/British Society for the History of Mathematics) on 15th November 2011 at the University of Leicester.