On the evening of 13 March 1781, an organist from Bath was working through a survey of double stars from his back garden when he noticed an object that did not look right. Unlike a star, it showed a small disc, and it grew larger when he increased the magnification. Four nights later it had moved against the background stars.
William Herschel had found the first planet discovered in recorded history, and he was not an astronomer. He was a professional musician who ground his own mirrors because he could not afford to buy a decent telescope.
The Musician
Friedrich Wilhelm Herschel was born in Hanover in 1738 into a family of military musicians and joined the band of the Hanoverian Guards as an oboist. After the French occupation of Hanover in 1757 he left for England, anglicised his name, and spent nearly twenty years as a working musician: a teacher, performer, composer of some two dozen symphonies, and eventually organist at the Octagon Chapel in Bath.
His interest in astronomy grew out of the mathematics of musical harmony, which led him to optics, which led him to want a telescope. The commercial instruments available were expensive and poor. He decided to build his own.
This was a formidable undertaking. Reflecting telescope mirrors were cast from speculum metal, a brittle alloy of copper and tin, and had to be ground and polished by hand into a precise curve. Herschel became obsessive about it. His sister Caroline recorded feeding him by hand while he polished, since he could not stop for meals: interrupting a polishing run of sixteen hours would spoil the figure of the mirror.
He got extremely good at it. By 1781 his instruments were better than anything at the Royal Observatory, and he eventually made and sold around four hundred telescopes. His reflectors belong among the instruments that changed what humans could see, since they were the first to reach far enough to add an object to the solar system.
The Object
Herschel was engaged in a systematic survey of the sky, examining every star down to a certain brightness to look for double stars. The programme was not glamorous and it meant that he examined objects most astronomers had no reason to look at twice.
The telescope he used on 13 March was a reflector of six and a quarter inches, and the object was in Taurus. His journal records it as a curious either nebulous star or perhaps a comet.
The key observation was the disc. Stars remain points of light at any magnification, since they are too distant to resolve. This object grew with magnification, so it had a measurable angular size, which meant it was close and large.
He reported it to the Royal Society as a comet, which was the reasonable assumption. Nobody had discovered a planet, and there was no expectation that one existed to be found.
The orbit calculations, by Nevil Maskelyne, Anders Lexell and others, ruled that out within months. The path was nearly circular, not the elongated ellipse of a comet, and it lay about nineteen times further from the Sun than the Earth. There was no coma and no tail.
It was a planet, and it sat roughly twice as far out as Saturn. In a single observation the known solar system had doubled in size.
The Naming Problem
Herschel, a German immigrant dependent on English patronage, named it Georgium Sidus, the Georgian Star, after King George III.
The gesture worked domestically. George III granted him a salary of two hundred pounds a year as the King’s Astronomer, which allowed him to give up music entirely and observe full time.
It did not work internationally. Astronomers outside Britain declined to name a planet after a British monarch, and various alternatives circulated, including Herschel, which the French preferred. Johann Elert Bode proposed Uranus, on the grounds that Saturn was the father of Jupiter and Uranus was the father of Saturn, which fitted the established pattern. The name gained ground slowly and was not universal until around 1850.
It Had Been Seen Before
Uranus is just at the edge of naked eye visibility under excellent conditions, and it had been recorded repeatedly without being recognised.
John Flamsteed observed it in 1690 and catalogued it as a star, 34 Tauri. Pierre Charles Le Monnier recorded it on at least a dozen occasions in the 1760s and 1770s, including several nights in a single month, and never noticed that it moved.
Those earlier sightings turned out to be useful, since they extended the observational record backwards by ninety years and made the orbit far easier to determine.
They also make a point about observation. Herschel’s advantage was not merely a better telescope. It was that he was conducting a systematic survey with attention to the appearance of every object, rather than checking positions against a catalogue. The others saw it. He looked at it.
The Forty Foot Telescope
With royal funding, Herschel built progressively larger instruments, culminating in 1789 in a reflector with a mirror four feet across and a tube forty feet long, erected at Slough. It was the largest telescope in the world for over fifty years and became a tourist attraction.
It was also awkward. The mirror tarnished quickly and had to be repolished, the mounting was cumbersome, and Herschel did most of his best work with his smaller and more manageable twenty foot instrument. Even so, in its first week of operation he discovered two moons of Saturn, Mimas and Enceladus.
What Else He Found
Uranus is the discovery everyone knows, and it is not the most important thing he did.
- Infrared radiation, 1800. Herschel split sunlight with a prism and placed thermometers in each colour to see which carried the most heat. A thermometer positioned just beyond the red end, where no light was visible, recorded the highest temperature of all. He had found radiation outside the visible spectrum, which was the first evidence that light is part of something larger
- Binary stars. He had begun observing double stars hoping to measure parallax. Instead, over twenty years, he found that many pairs are physically orbiting each other. This established that Newtonian gravitation operates outside the solar system, which had been an assumption rather than an observation
- The motion of the Sun. By analysing the apparent drift of stars in different directions, he determined that the solar system itself is moving through space, towards a point in the constellation Hercules
- Nebulae. He catalogued some 2,500 nebulae and clusters, and speculated about their nature and about the structure of the Milky Way as a system of stars with a definite shape
Caroline
Caroline Herschel came from Hanover to keep house for her brother and became an astronomer in her own right. She recorded his observations, performed the reduction calculations, and swept the sky herself with a small telescope he made for her.
She discovered eight comets, catalogued nebulae, and corrected and extended Flamsteed’s star catalogue. In 1787 the King granted her fifty pounds a year as her brother’s assistant, which made her the first woman in Britain paid for scientific work. In 1828 the Royal Astronomical Society awarded her its Gold Medal.
The family enterprise continued with William’s son John, who took the telescopes to the Cape of Good Hope and extended the survey of nebulae to the southern sky.
What Uranus Led To
The new planet did not behave. Once enough observations had accumulated, its motion showed small deviations from the path Newtonian gravitation predicted, even after the perturbations of Jupiter and Saturn were taken into account.
Two mathematicians, John Couch Adams in England and Urbain Le Verrier in France, independently concluded that an undiscovered planet further out was pulling on it, and calculated where that planet must be. Neptune was found in 1846, within a degree of Le Verrier’s prediction, on the first night of searching.
Herschel found Uranus by looking. Neptune was found by calculating, and the difference marks a change in what astronomy had become in the intervening sixty five years.
The instrumental tradition that made both possible, from the first telescopic surveys to the great observational catalogues, is collected in our six book pack on the history of astronomy, including Tycho Brahe’s account of the instruments he built to reach the limits of what the unaided eye could do.
The Amateur
Herschel had no formal training in astronomy, no university position, and no access to a professional observatory. He was forty two when he found Uranus, and until that night his income came from music lessons and church organ playing.
What he had was a systematic observing programme, instruments he had built to a standard nobody else could match, and a willingness to spend entire nights outdoors examining objects that everyone else had already catalogued and dismissed. Two of those three are still available to anyone.
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