William Herschel Discovers Uranus (1781): A New Planet Found

Uranus was discovered on 13 March 1781 by William Herschel, a German-born musician and amateur astronomer, observing with a home-made telescope from his garden in Bath, England. It was the first planet found with a telescope and the first new planet identified since antiquity. The discovery roughly doubled the known size of the Solar System and turned Herschel into one of the most famous scientists of his age.

Background: a musician turned astronomer

Friedrich Wilhelm Herschel was born in Hanover on 15 November 1738. Like his father, he became a musician in the band of the Hanoverian Foot Guards. In 1757, during the Seven Years’ War, he left for England, where he worked as a music teacher, composer and performer, anglicising his name to William.

In 1766 he became organist of the Octagon Chapel in Bath, a fashionable spa town. His sister Caroline joined him there in 1772. William’s interest in mathematics and optics led him to astronomy, and finding commercial telescopes too expensive, he began grinding and polishing his own metal mirrors. With Caroline’s help, the house became a workshop for building some of the finest reflecting telescopes of the time.

Herschel’s goal was ambitious: to survey the whole sky systematically, looking at every star bright enough to be seen with his instruments, in order to understand the structure of the heavens.

His telescopes were reflectors, which use a curved mirror rather than a lens to gather light. The mirrors were made of speculum metal, an alloy of copper and tin that tarnished quickly and had to be repolished regularly. Herschel spent long hours grinding them by hand, sometimes for many hours without stopping, while Caroline read to him or fed him. The quality of these mirrors gave him sharper images than most professional astronomers of the time could obtain.

The discovery

On the night of 13 March 1781, Herschel was carrying out his survey from the garden behind 19 New King Street, using a reflecting telescope about 7 feet (2.1 m) long with a mirror about 6.2 inches (16 cm) across. In the constellation of Taurus, near the border with Gemini, he noticed an object that appeared as a small disc rather than a point of light.

Using higher magnifications, Herschel saw that the disc grew larger, as a nearby object should, while true stars did not. Over the following nights he found that it was moving slowly against the background stars. He concluded that it was most likely a comet, and his paper, read to the Royal Society in London on 26 April 1781, was titled “Account of a Comet”.

From comet to planet

Herschel’s report reached the Astronomer Royal, Nevil Maskelyne, who observed the object and suspected it might be a planet. Astronomers across Europe began following it. The object showed no tail or fuzzy coma, unlike a comet, and its motion did not fit the long, elongated orbits typical of comets.

Mathematicians, notably the Finnish-born Anders Johan Lexell in St Petersburg, calculated that it followed a nearly circular orbit about 19 times farther from the Sun than Earth, well beyond Saturn. Johann Elert Bode in Berlin reached a similar conclusion. By around 1783 it was generally accepted that Herschel had found a new planet, and he himself acknowledged it.

Searches of older records showed that the planet had been seen many times before, including by John Flamsteed in 1690, who catalogued it as the star “34 Tauri”, and repeatedly by the French astronomer Pierre Charles Le Monnier. None had noticed its motion.

Uranus is just at the limit of naked-eye visibility under very dark skies, and through a small telescope it looks very like a faint star. Its slow motion, only a few degrees of sky in a whole year, is easy to miss unless an observer returns to the same spot repeatedly. Herschel’s success came from the combination of an excellent telescope, high magnification and the habit of examining every object in turn.

By the numbers

The naming debate

As the discoverer, Herschel had the right to propose a name. In 1782 he suggested Georgium Sidus, “the Georgian Star”, in honour of King George III, who had just granted him an annual salary. Astronomers outside Britain were unenthusiastic about a planet named after a British king.

Other names were proposed, including “Herschel”. Bode suggested Uranus, after the Greek god of the sky and the father of Saturn (Cronus), keeping the tradition of naming planets after classical deities. Uranus became standard in most of Europe, but British publications such as the Nautical Almanac used “Georgian” until about 1850.

Consequences for the Herschels

The discovery transformed William’s life. He was awarded the Royal Society’s Copley Medal and elected a Fellow in 1781. In 1782 George III appointed him “the King’s Astronomer” with a salary of £200 a year, allowing him to give up music. The Herschels moved to Datchet, briefly to Old Windsor and then, in 1786, to Slough, near Windsor Castle.

There William built ever-larger telescopes, culminating in a 40-foot (12 m) instrument completed in 1789. He discovered two moons of Uranus, Titania and Oberon, in 1787, two moons of Saturn in 1789, and thousands of nebulae and double stars. In 1800 he discovered infrared radiation. He was knighted in 1816 and died at Slough on 25 August 1822.

Caroline Herschel became a distinguished astronomer in her own right. She discovered eight comets, the first in 1786, and in 1787 the king granted her a salary of £50 a year as William’s assistant. In 1828 the Royal Astronomical Society awarded her its Gold Medal for her catalogue of nebulae.

Timeline

• 1738, 15 November: William Herschel is born in Hanover.
• 1766: Herschel becomes organist of the Octagon Chapel in Bath.
• 1781, 13 March: Herschel observes the new object from 19 New King Street, Bath.
• 1781, 26 April: His “Account of a Comet” is read to the Royal Society.
• 1782: Herschel proposes the name Georgium Sidus and is appointed King’s Astronomer.
• 1787, January: Herschel discovers Uranus’s moons Titania and Oberon.
• 1822, 25 August: William Herschel dies at Slough.
• 1986, January: NASA’s Voyager 2 makes the only spacecraft flyby of Uranus to date.

Legacy

The discovery of Uranus showed that the Solar System was not complete as known since ancient times, and that careful observation could still reveal major new bodies. Small irregularities in Uranus’s orbit later led astronomers to predict a still more distant planet, ending in the discovery of Neptune in 1846.

Uranus itself remained mysterious for two centuries. It takes about 84 years to orbit the Sun and is tilted on its side by about 98 degrees. Most of what we know about its rings and moons came from the Voyager programme, whose Voyager 2 probe passed the planet in January 1986. The house in Bath where the discovery was made is now the Herschel Museum of Astronomy.

Why it matters

Herschel’s discovery broke the ancient limit of six known planets and proved the power of the telescope for exploring the Solar System. It also launched the remarkable careers of William and Caroline Herschel, whose survey work helped found modern observational astronomy.