There have been many milestones in the history of astronomy: foremost among them, of course, being Nicolaus Copernicus's daring suggestion that the Sun, not the Earth, was at the centre of the Universe. But since the triumph of the Copernican Revolution, there has been no more sensational event in astronomy than William Herschel's unexpected discovery of the planet Uranus in 1781. Ever since antiquity, it had been taken for granted that there were five planets beyond the Earth - Mercury, Venus, Mars, Jupiter and Saturn. Although Giordano Bruno had proposed in the late sixteenth century that stars beyond the Sun might have planets of their own, it never seems to have occurred to anyone that there might be other planets in the Solar System. At a stroke, the Solar System was doubled in size: even more shocking was that an amateur astronomer had made a discovery that had eluded the finest professionals. In addition, not only was Herschel's discovery sensational in its own right, it also directly contributed to more discoveries: notably the asteroids, because of a mathematical formula which the discovery of Uranus seemed to vindicate, and to the eighth planet, Neptune, as a result of irregularities in Uranus's orbit. In short, the discovery of Uranus, just like the Copernican Revolution, fundamentally changed our view of the Solar System.
The Discoverer
Friedrich Wilhelm Herschel was born in Hanover on 15th November 1738: his father Isaac came from a Jewish family, originally from Moravia in what is now the Czech Republic, that had converted to Lutheranism in the seventeenth century. Isaac Herschel was an oboist in the Hanover Military Band, and Wilhelm and elder brother Jakob followed in his footsteps, being engaged as oboists in the Hanoverian Guards. In 1757, after Hanover's forces were defeated by the French at the Battle of Hastenbeck, Isaac Herschel sent his two sons to Britain - then in personal union with Hanover - for their own safety. Wilhelm quickly learned English, going by the English version of his name, Frederick William, and forged a successful career as a musician and composer. In 1772, he was joined by his sister Caroline, ultimately to become a distinguished astronomer in her own right.
Herschel began to take an interest in astronomy in the 1770s, building telescopes to observe the night sky. He was constantly assisted in his activities by Caroline. His discovery of Uranus made his reputation: he was awarded the Copley Medal and elected a Fellow of the Royal Society, and in 1782 was appointed the official astronomer to King George III (not, note, the Astronomer Royal) and awarded an annual stipend of £200 (£24,000 in today's money, though less than he had earned as a musician) on condition that he move to Windsor to enable the royal family to look through his telescopes. He gave up his musical career to dedicate himself to his new position. In the remainder of his career, he discovered more than 800 binary or multiple star systems, six galaxies (though they were not recognised as such until two centuries after his death), two moons each of Saturn and Uranus, the motion of the Sun through space, and infrared light in sunlight. In 1816, he was knighted. On 8th May 1788, he married the widow Mary Pitt (nee Baldwin) (causing a bitter rift with Caroline, who lost her position in her brother's household as a result). The couple had one child, John, who would become a notable scientist in his own right. William Herschel died in Slough on 25th August 1822.
The Discovery
On the night of 13th March 1781, Herschel - uncharacteristically working alone rather than with Caroline - was searching for binary stars (using the telescope seen on the left) from the garden of his house in Bath, when he noticed a curious object in the constellation of Taurus. In his journal, he recorded "either [a] Nebulous star or perhaps a comet. Four days later, he records, "I looked for the Comet or Nebulous Star and found that it is a Comet, for it has changed its place". As Herschel noted, the object could not be a star, as it had moved: as no one expected there were any planets other than those already known, and as the asteroids had not yet been discovered, a moving object could only be a comet. On 26th April, Herschel presented his discovery to the Royal Society and also notified Nevil Maskelyne, the Astronomer Royal, still not suspecting that the object he had found was anything other than a comet. However, on 23rd March, three days before Herschel described the "comet" to the Royal Society, Maskelyne wrote back, noting that the object apparently had no coma (surrounding cloud of gas and dust) or tail, and suggesting that the object might be a planet.
Though Herschel continued to describe his discovery as a comet, other astronomers shared Maskelyne's suspicions. Not only did the object lack a coma and tail, as Maskelyne had pointed out, but also for a comet to be as bright as this one, it would have to be very close to the Sun, and a comet so close to the Sun would be moving much faster than this object. Maskelyne called on astronomers across Europe to examine the object, and they duly took up the challenge. While travelling through Europe, Swedish-speaking Finnish astronomer Anders Johan Lexell made preliminary calculations of the orbit of the mysterious object, based on Herschel's and Maskelyne's observations. On returning to Russia, where he was based, Lexell made a more precise calculation of the object's orbit, but due to the orbit's length it was not clear whether the object had a near-circular orbit, like a planet, or a highly elliptical one, like a comet. However, Lexell subsequently found the record of a star observed in the constellation of Pisces in 1759 by German astronomer Christian Mayer but which was not catalogued and had seemingly disappeared from the sky by the time another German astronomer, Johann Elert Bode, sought it. Lexell assumed that this was an earlier sighting of Herschel's "comet": he was thus able to calculate its orbit, which turned out to be regular, establishing that the object was in fact a planet. He also accurately calculated the new planet's size, aided by the fact that Mars happened to be close to it at the time. Independently of Lexell, Bode also made orbital calculations. After consulting older star charts, Bode realised that Herschel's object had been catalogued as a star several times: for example, Britain's first Astronomer Royal, John Flamsteed, had listed it at least six times as 34 Tauri in his 1690 catalogue. These earlier observations gave Bode the necessary information to calculate a precise orbit. By now, there could be no doubt about it: William Herschel had indeed discovered a new planet.
Choosing a Name
It was truly an unprecedented moment in the history of astronomy: the first five planets beyond the Earth had been known to all civilisations since time immemorial, and no individual could claim to have "discovered" them, in the same way that no one can be hailed as the discoverer of fire, or the inventor of telling the time. Since the invention of the telescope in 1609, many major discoveries had been made: moons around Jupiter and Saturn, Saturn's rings, Halley's Comet and numerous stars, but never a new planet.
As soon as the seventh planet's existence was confirmed, a heated debate arose about what name should be given to it. The names of the other planets derive from classical gods: Mercury, the one that moves the swiftest across the sky, after the messenger god with wings on his sandals; Venus, the brightest planet, after the goddess of love and beauty; Mars, in view of its blood-red colour, after the god of war; Jupiter, the largest planet, after the king of the gods; and Saturn, the slowest moving planet, after the god of time. To us then, it might seem obvious that this precedent should be followed in naming the seventh planet, and eventually it would be: however, due to the unprecedented nature of the situation, it would be nearly 70 years before a name would be universally accepted. Also, at the time there was no clear pattern for naming newly discovered objects in the Solar System. Jupiter's four largest moons, known collectively as the Galilean moons after their discoverer, Galileo Galilei, are well known today under the names Io, Europa, Ganymede, and Callisto, after lovers (of different sexes) of the god Jupiter: however, Galileo himself never used these names. He instead named the moons Cosimo, Francesco, Carlo and Lorenzo, after his patrons, the Medici family. The names we use today were proposed by the now largely forgotten German astronomer Simon Marius, who had independently discovered the moons at the same time as Galileo. Out of spite, Galileo refused to use Marius's names, and after his own proposed names failed to catch on, preferred to number the moons in order of their distance from Jupiter: the names Io, Europa, Ganymede and Callisto would not be widely used until the twentieth century. Saturn's largest moon, Titan (so called because in mythology Saturn was king of a race of giants called the Titans) was discovered in 1655, but would not be named until 1847 (incidentally, it was named by John Herschel, son of the discoverer of Uranus), over 190 years later.
In 1783, William Herschel, having finally accepted that his discovery was a planet, received a letter from Maskelyne, urging him to "do the astronomical world a faver [sic] to give a name to your planet, which is entirely your own, which we are so much obliged to you for the discovery of". Herschel proposed that his planet be named Georgium Sidus ("George's Star"), in honour of George III, successfully hoping that this would encourage the King to honour him and enable him to become a full-time astronomer. He justified this choice in a letter to Joseph Banks, president of the Royal Society:
In the fabulous ages of ancient times the appellations of Mercury, Venus, Mars, Jupiter and Saturn were given to the Planets, as being the names of their principal heroes and divinities. In the present more philosophical era it would hardly be allowable to have recourse and call it Juno, Pallas, Apollo or Minerva [though incidentally all four of the above names would later be given to asteroids], for a name to our new heavenly body. The first consideration of any particular event, or remarkable incident, seems to be its chronology: if in any future age it should be asked, when this last-found Planet was discovered? It would be a very satisfactory answer to say, "In the reign of King George the Third" [At the time it was common for major events to be dated by reference to the monarch in whose reign they occurred]
This name caught on in Britain (and, under today's naming rules which assign exclusive naming rights to the discoverer, would have had to have been accepted), but was never accepted elsewhere: the French were especially averse to honouring their traditional enemy. French astronomer Jérôme Lalande suggested naming the planet "Herschel": though the idea of naming a planet after its discoverer may seem quaint now, it should be remembered that this is still how comets are named, and has been ever since Halley's Comet became the first to have its orbit calculated. Swiss mathematician Johann Bernoulli favoured "Hypercronius" or "Transsaturnis" both meaning "above Saturn", while others made the case for Cybele, based on a dubious belief that the word means "dumb-bell", befitting a planet that had for so long remained silent. In a similar jesting vein, German astronomer Hofrath Lichtenberg suggested "Austräa", after the last Greek goddess to flee from the Earth, but he soon acknowledged the unsuitability of this name as Austräa was one of the goddesses identified with the constellation of Virgo. Swedish astronomer Erik Prosperin proposed "Neptune", presumably in reference to the planet's blue colour, Neptune being the Roman god of the sea. Lexell proposed, as a compromise, calling the planet "Neptune de George III" or "Neptune de Grande-Bretagne". Some British astronomers were attracted to this proposal, seeing it as a way of honouring Britain's naval victories. Prosperin's name eventually fell by the wayside, but it pointed to where the naming debate was eventually heading: towards the name of a classical god.
It was Bode, in a March 1782 treatise, who was the first to propose the name Uranus. He argued that the planet should be named after a classical deity so as not to stand out from the other planets. Why Uranus? In Greek mythology, Uranus was a personification of the sky: he was both the son and the husband of Gaia - the Earth. They were the parents of the Titans, the Cyclopes (one-eyed giants) and the Hecantonchires (hundred-handed giants). However, Uranus hated all of their children and imprisoned them in Tartarus, deep within the Earth, which was painful (in more ways than one) to Gaia. Thus, she encouraged all of her sons to castrate Uranus using a great flint-bladed sickle that she had forged. Only the youngest of the Titans, Cronus (Saturn to the Romans), accepted the challenge, cutting off his father's genitals and throwing them into the sea. From Uranus's genitals was born the goddess Aphrodite (Venus to the Romans), and from his blood, which spilled onto the Earth, sprang the Giants, the Furies and the Meliae (ash-tree nymphs). Cronus and the Titans then ruled the Universe until they were deposed by Cronus's son Zeus (called Jupiter by the Romans). It was not, however, the rather gruesome nature of the Uranus myth that inspired Bode's suggestion: rather it was the belief that since Saturn was the father of Jupiter, so the new planet should be named after the father of Saturn. In support of Bode's choice, his Royal Swedish Academy colleague, German chemist Martin Klaproth, gave the name uranium to a newly discovered element in 1789 (this precedent would be followed subsequently, with the element neptunium being named after the newly discovered Neptune, and cerium and plutonium being named after the dwarf planets Ceres and Pluto, both of which were initially regarded as planets). Uranus became the most popular choice for the planet's name, though "Georgium Sidus" remained widely used in Britain for many decades: Bode's name finally gained universal approval in 1850, when HM Nautical Almanac Office switched from "Georgium Sidus" to "Uranus". Uranus is, incidentally, the only planet named after a Greek rather than a Roman deity (Caelus was the Roman name of Uranus).
More Discoveries
Calculating Uranus's orbit, and choosing its name, were not the only contributions made by Bode to astronomy. In 1766, German astronomer Johann Daniel Titius had devised a mathematical formula that accurately predicted the average distances from the Sun of the six planets then known to exist. In 1772, Bode, then aged just 25, advanced the same formula: in later years, he acknowledged Tititus's priority. The Titius-Bode law, also known simply as Bode's law, was initially dismissed by most astronomers as an interesting coincidence, but perceptions of it were altered dramatically by the discovery of Uranus, which lay at exactly the right position for a trans-Saturnian planet. The law was now apparently vindicated: what was more, it predicted the existence of an undiscovered planet in between Mars and Jupiter. Bode, who could have been forgiven for feeling not a little triumphalistic, called on his fellow astronomers to search for this new planet. In 1800, requests were sent to 24 experienced astronomers, dubbed the "celestial police", to search for a fifth planet. One of those was the Sicilian astronomer Giuseppe Piazzi: however, on 1st January 1801, before receiving the message, Piazzi had already discovered a small orbiting body which he named Ceres, again in the position predicted by Titius and Bode. Ceres was immediately accepted as the missing fifth planet, but just a year later, German astronomer Heinrich Olbers discovered another body, Pallas, in the same orbit as Ceres: it had always been assumed it was impossible for two planets to share an orbit. Herschel pointed out that Ceres and Pallas were much smaller than the other seven planets: so small that even in the best telescopes they only appeared as small pinpricks of light, like the stars, instead of fully formed discs, like the major planets, and proposed that they be referred to as "asteroids", meaning "star-like". Initially, Herschel's terminology was not accepted, and Ceres and Pallas continued to be referred to as planets, as did Juno, discovered in 1804 by Karl Harding, and Vesta, discovered by Olbers in 1807. Thus, for nearly 40 years, the Solar System had 11 planets. However, between 1845 and 1851, 11 more bodies orbiting between Mars and Jupiter were discovered: these too were originally classified as planets, and as such assigned astronomical symbols like all other planets. By 1851, there were so many planets that the astronomical symbols were becoming increasingly difficult to draw, so instead the asteroids were given numbers in order of discovery, thus implicitly classifying them apart from the planets, and gradually they ceased to be referred to as planets. An important precedent had been set: ever since the Copernican Revolution all objects orbiting the Sun, other than comets, had been classified as planets, but now the principle had been established that small objects, and those that shared their orbits with other bodies, did not count as planets, though the word "planet" remained without a formal definition. The discovery of Uranus had led to the discovery of a completely new set of Solar System bodies, and to a limitation in the type of objects regarded as planets: both changes to our view of the Solar System as significant as Uranus's discovery itself.
When Lexell first calculated Uranus's orbit, he noticed that its orbit was being perturbed (pulled out of its predicted path), and suggested that there could be other, more distant, planets, responsible for the perturbation. He also predicted that, based on his data on various comets, the size of the Solar System could be up to 100 astronomical units (i.e. 100 times the distance between the Earth and the Sun) or more, a much bolder estimate than most others at the time. In 1821, French astronomer Alexis Bouvard published astronomical tables of Uranus's orbit, predicting its future position based on Newton's laws of motion and gravitation: however, the planet deviated very significantly from these predictions, causing Bouvard, as Lexell had done, to hypothesise the existence of a eighth planet disturbing Uranus's orbit. In the mid-1840s, British astronomer John Couch Adams and Frenchman Urbain Le Verrier independently computed a position for the eighth planet, and Le Verrier sent his calculations to German astronomer Johann Gottfried Galle at the Berlin Observatory. Galle received Le Verrier's letter on 23rd September and immediately began searching, assisted by his student Heinrich Louis d'Arrest. Just after midnight on 24th September 1846, Galle and d'Arrest observed Neptune after less than an hour of searching, less than one degree from Le Verrier's predicted position. Galle wrote to the French astronomer, "the planet whose place you have [predicted] really exists" (emphasis in original). Once again, the discovery of Uranus had led directly to a major new discovery, this time of another planet. (Incidentally, Neptune's discovery also discredited the Titius-Bode law, as its position did not conform to the law.)
But the discovery of Neptune was not to be the end of the story. Observations of Neptune's mass appeared to indicate that it alone was incapable of perturbing Uranus's orbit, as well as to apparent irregularities in Neptune's own orbit, leading to predictions of a large ninth planet. In 1930, Pluto was discovered by American astronomer Clyde Tombaugh, and was immediately accepted as a new planet, but it turned out to be far smaller than any other planet - and, more significantly, far too small to influence the orbits of Uranus and Neptune. As a result, several astronomers, notably Robert Sutton Harrington, predicted the existence of a giant 10th planet, but no such planet was ever found. In 1992, E Myles Standish recalculated Neptune's effect on Uranus, based on data from the Voyager 2 space probe, which had revised Neptune's mass downward by 0.5 per cent - this might not sound like much, but in the context of a planet as huge as Neptune this is comparable to the entire mass of Mars. With this new information, the irregularities in Uranus's and Neptune's orbits disappeared: in other words, there was not another giant planet in the Solar System. Also in 1992, astronomers began to discover that Pluto, just like Ceres, is not alone but belongs to a very large group of small icy bodies beyond Neptune's orbit known as the Kuiper belt. The discovery in 2003 of Eris, a distant body 27 per cent more massive than Pluto (though now known to be slightly smaller in diameter, contrary to what was initially thought) led the International Astronomical Union, in 2006, to devise a formal definition of the word planet for the first time. It determined, in keeping with the precedent set in relation to the asteroids in 1851, that a planet is a round body in orbit around the Sun that has "cleared the neighbourhood" around its orbit. Round bodies orbiting the Sun that have not "cleared their neighbourhoods", such as Pluto, Ceres and Eris, were to be classified as "dwarf planets". Although it cannot be said that the discovery of Uranus directly contributed to this decision, Uranus was indirectly one of the reasons that the search which uncovered Pluto began: its discovery thus can be said to have begun the process that led to the 2006 definition. In addition, this definition, as noted above, conformed with a precedent set with the asteroids, whose discovery was a consequence of the discovery of Uranus.
Conclusion
The discovery of Uranus was, in its own way, as significant a turning point in the history of astronomy as the publication of Copernicus's De Revolutionibus Orbium Caelestium. If the latter altered forever our conception of the Earth's - and by extension our own - place in the Universe, the former was the first indication that the Solar System is much bigger that had been assumed. It was the beginning of a process that would lead to the discovery of Neptune, the asteroids and the Kuiper belt, and eventually to a formal definition of the word planet for the first time. Of course, all these developments were inevitable, but it is still remarkable to think that it all began with a completely unforeseen discovery made by an amateur astronomer.
