William Herschel, the ultimate observer.

Tuesday 17th February, 2015

  A Newtonian telescope by William Herschel at the Museum of the History of Science, Oxford

Our speaker was Christopher Taylor who is a founder member of both Stratford-upon-Avon Astronomical Society and the Society for the History of Astronomy as well as a Director of Hanwell Community Observatory. He came to talk to us about "William Herschel, the ultimate observer" and began with a quote from Herschel regarding his discovery of the planet Uranus.

"On Tuesday, the 13th of March, 1781, between ten and eleven in the evening, while I was examining the small stars in the neighbourhood of H Geminorum, I perceived one that appeared visibly larger than the rest: being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet."

On further investigation Herschel realised that he was not looking at a comet or star but a new planet as when he used higher magnifications the object's diameter steadily increased, which does not happen with stars that are much further away, and it lacked a distinctive cometary tail. After further investigations and the help of the Russian academic Anders Lexell, who computed the orbit, Herschel reached the conclusion that it must be a planet and called it the "Georgian star" or "Georgium sidus" in honour of the current monarch King George III. Unfortunately, the name was not universally accepted but this flattery led him to being appointed The King's Astronomer with an annual salary and access to other grants that would allow him to build larger and larger telescopes.

Herschel's very first recorded telescopic observation was on 4th March 1774 when he sketched the Orion Nebula. The year before he had started making his own telescopes as he could not afford to buy one at that time. These early telescopes of his were all Newtonian in design and moved around an alt-azimuth mounting. He was a dedicated and fastidious telescope manufacturer and spent up to 16 hours a day grinding and polishing the speculum metal primary mirrors for his telescopes. His telescopes even gained great praise from the Astronomer Royal of the day, the Revd Dr Nevil Maskelyne.

Herschel's early observational work from his back garden in Bath focussed on pairs of stars that appeared very close together. He was aiming to record the positional changes of the stars over time in order to measure their movement across the sky (otherwise known as their proper motion) as well as their distance from Earth. It was while he was cataloguing these that he came across the new planet Uranus.

Although he was unsuccessful in these primary objectives, from his observational data he put forward the theory that some of these double stars were actually in orbit about each other. He went on to catalogue over 800 double or multiple star systems that formed the basis for modern binary star astronomy. Among his list of achievements are the discovery of infrared radiation, finding two moons of Uranus and another two moons of Saturn as well as various deep sky catalogues.


This article was written for the club news column of the Stratford Herald. The actual lecture explained the subject at a deeper level.