WILLIAM HERSCHEL (1738-1822)
It's interesting to make some comparisons between Charles Messier and William Herschel. Their working lives overlapped. Herschel was just eight years younger than Messier, although his astronomical worked started some twenty years after that of Messier. Perhaps because of that his work seems to belong to a different age. Messier was a professional astronomer, Herschel was not - or at least, he didn't start out as one.
Of the two, Messier is probably the most well remembered now, yet he achieved comparatively little compared to Herschel. Messier was an observer. Herschel went further and added scientific meaning to his observations.
Messier discovered or codicovered about twenty comets and listed less than the 109 deep sky objects in his catalogue.
Herschel discovered a planet and two of its moons, two moons of another planet, he correctly surmised the workings of binary star systems and the structure of galaxies, he listed 2500 deep sky objects, he discovered the infra red - so important in present day astronomy - and he built his own telescopes.
So why does Herschel seem best remembered for his music? Probably all less than expert astronomers are familiar with Messier's catalogue, yet how many could answer the question 'Whose work lies at the foundations of the New General Catalogue, the NGC objects?' The answer. of course, is William Herschel, his sister Caroline and son John.
Friedrich Wilhem was born in Hannover, one of six children. His father was a military musician who passed on his music to all his children, but he also introduced them to the night sky. William has a brief period in the military, but his health was a little delicate, but during this time, in 1756, he visited England. The following year he made his home there, though quite why seems to be one of life's little mysteries. The French had occupied Hannover and there was fighting there. One version of event suggests that William 'escaped' to England while his father was away, another that his father helped him leave. One brother went with him and returned to Germany, but another apparently helped build one of his telescopes.
What is not in dispute is that after a while he settled in or around Bath, 'in or around' because he seemed to move house quite frequently. He made a living as a musician. He was organist at the Octagan Chapel, he composed, he gave concerts and he taught music. And he obtained the odd book on astronomy and became interested, interested enough to obtain a telescope. It was at about this time that he was joined by his sister Caroline, who learned music and mathematics from him, who performed at concerts with him, who started out as his housekeeper but became not just his assistant but also an astronomer in her own right.
His first telescope proved too small, but on his musician's pay William couldn't afford to buy another, so he chose to make his own. Whereas Messier had used mainly refractors, Herschel developed a method of producing what were for the time high quality mirrors for a reflector. He mainly used an 18.8 inch, twenty foot focal length reflector.
In March 1781 he made his most famous discovery, and one which he is most definitely remembered for. Observing with the intention of adding to a list of double stars, his pet project of the time, he found an interloper. At first he thought it was a comet, but after several observations had been made and the results passed to the mathematicians Laplace in France and Lexell in St Petersburg, it proved to be a planet, subsequently named Uranus.
After this, of course, Herschel had it made. He was elected a Fellow of the Royal Society, he gained the patronage of the king, George III, and the astronomer royal, Nevil Maskelyne, visited and befriended him. He tried to have his planet named for King George, but common sense overrulled him, but probably most significantly for us, he was presented with a copy of Messier and Mechain's list.
The following year and at the behest of the king, the Herschels left Bath for Datchet, near Windsor. Wiliam became court astronomer on the princely salary of £200 a year. Caroline as his assistant received £50. It wasn't a huge sum, but William started a business manufacturing telescopes which augmented his royal retainer quite nicely. At this point music seems to have been relegated to a back seat, and who can blame him, but he continued moving house, first nearer Windsor then to Slough.
Herschel started looking at the objects in Messier's list, and in one of the understatements of all time surmised that 'several nebulae might yet remain undiscovered'. And then he started looking for them. His first discovery, that same year, was the Saturn Nebula, NGC7009, an 8th magnitude planetary nebula in Aquarius. Between 1786 and 1802 he published three catalogues containing between them 2500 objects, all of them termed at the time 'nebulae'. Herschel, like everyone else of the time, didn't know what he was looking at. But he did more than this.
Herschel used an 18.8 inch diameter reflector with a focal length of 20 feet. Compare that with a modern half metre reflector. Same diameter, but the focal length would be somewhere around five feet. The reason? Modern optics. Herschel was skilled at making mirrors, but they were metal, not the glass we use now, and they suffered all the abberations ours don't. The huge focal length was necessary to sharpen the image enough to make it usable. Herschel also built a 48 inch diameter reflector. It had a focal length of 40 feet and he found it too unwieldy to use much. As you can see from the drawing of the smaller of Herchel's reflector here, these were enormous beasts and very difficult to move. Astronomers of the time tended to set the telescope up and observe what moved past it rather than the other way round. But he did use the 48 inch to discover Saturn's moons Enceladus and Mimas. He also discovered two moons of his own planet Uranus.
And he didn't stop there. He maintained his interest in double stars and catalogued nearly one thousand of them. While studying them he found evidence for the existance of binary systems. In 1785 he put forward the disc theory of stellar systems and galaxies that we now know to be true. He also studied the Sun, and this had an important offshoot. He was using filters to observe the Sun and wanted to know how much heat passed through them. He passed the light through a prism to split it into its component colours and measured the heat given by each of them, then he went on to measure outside the visible spectrum and realised that light doesn't stop at what we can see, that it was emitting in other wavelengths that we can't see, and in particular outside the hot red end, what we now call infra red.
Like Messier before him, Herschel married almost as an afterthought. It was in 1788, when he was fifty years old, that he married Mary Pitt, a widower. They had a son, John, four years later.
With Herschel senior's marriage Caroline must have felt a bit left out and looked to her own studies for a while, until once again they took a back seat to John and his education. John went to Cambridge, became a mathematician, and carried on his father's work, but he left England for South Africa. In 1864, after his father's death, John published the General Catalogue of Nebulae. It contained 5097 objects, incuding for the first time southern hemisphere objects, and 4630 of them were discovered by the Herschels. For this achievement and the other scientific work the name of Herschel should stand way above that of Messier, yet it is overlooked. Why? Because only twenty two years after the publication of John's General Catalogue of Nebulae, it was superceded by the New General Catalogue of Nebulae, the NGC, and this is the reference we all use today. Messier's original 109 objecs were honoured with keeping their M numbers within the NGC, but the Herschels' contribution is not honoured in any such way.
William and Caroline were, however, honoured.
William was already part of the Royal Society, and when the Royal Astronomical Society was formed in 1820 he became its first vice president, then its president a year later. He died in 1822. Caroline had worked hard for him and continued to do the same for her nephew, however she still found the time to discover eight comets and to cross reference and correct Flamsteed's star catalogue. She was awarded a gold medal by the Royal Astronomical Society in 1828 and in 1835 she gained honorary membership of the Royal Society. She lived to be 98 years old.
So what of the Herschels' unsung legacy, the New General Catalogue? Well, that's another story, involving an even bigger telescope, another foreign astronomer and a bankrupt observatory.
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