Uranus: the first planet discovered with a telescope

When Uranus, the seventh planet from the Sun, was discovered in 1781, it expanded the known limits of our solar system. It was also the first planet to be discovered using a telescope, as Mercury, Venus, Mars, Jupiter and Saturn were all bright enough to be easily visible to the naked eye.

In fact, because these planets had been known to people for millennia, Uranus was arguably the first planet in recorded history to have been ‘discovered’ at all.

William Herschel

William Herschel, astronomer and musician, is the man credited with this discovery. Strictly speaking, Uranus had been seen by many people before Herschel observed it through his telescope, but its dimness and small size led to it being classified as a star. Herschel himself was also misled initially, thinking the planet was a comet.

Friedrich Wilhelm Herschel—only later would he adopt the anglicised spelling of his name—was born in Hanover, now part of Germany, in 1738. Following the family tradition, he trained as a musician and joined the Hanoverian band of the foot guards. By the 1760s, work was scarce and Herschel moved to London to make a living, also harbouring ambitions to make his name as a composer. He pursued musical positions around the country, eventually settling in Bath in 1772. His younger sister Caroline joined him there from Hanover to train as a singer.

While in England, Herschel developed a passionate interest in astronomy. He devoured James Ferguson's book Astronomy explained upon Sir Isaac Newton’s principles, and made easy to those who have not studied mathematics, which, despite its unwieldy title, was a popular introduction to the subject.

Herschel had no intention of remaining an armchair astronomer, wanting instead to observe the sky for himself. Dissatisfied with the results he obtained with hired instruments, he decided to make his own reflecting telescopes.

The key component of a reflecting telescope is the mirror which gathers light from distant celestial objects. In the 1700s, before mechanised manufacturing techniques, producing mirrors was time consuming and required minute attention to detail. First, a 'blank' was cast from a shiny metal alloy known as speculum. Then the metal was ground and polished with ever finer abrasives, in order to achieve a perfect light-focussing curve and extreme shine.

In between his musical commitments, Herschel spent hours working at his speculum mirrors, before building them into telescopes and using them to study the sky. Caroline, despite her initial bewilderment at the house being converted into a workshop, soon embraced astronomical pursuits herself. She assisted Herschel both with the polishing and during their punishing nights of astronomical observations.

Herschel's perseverance paid off, and he began to acquire a reputation for the quality of the telescopes he made. During the 1770s he received visits from high-profile members of the scientific community, including the Astronomer Royal Nevil Maskelyne. In 1779, Dr William Watson, a Fellow of the Royal Society, encountered Herschel observing with his telescope in the street. They became firm friends, and Watson invited Herschel to be a founder member of the new Bath Literary and Philosophical Society.

A new planet

image shows william herschel holding a diagram depecting the orbit and position on the Gerogian planet — Portrait of Sir William Herschel shown holding a strip of paper entitled 'The Georgian Planets with its satellites' to celebrate his discovery

During observations on 13 March 1781, Herschel spotted a curious object in the sky, that he initially assumed was a star. A few days later he sighted it again, but found it had moved. He concluded from its movement that it was too close to Earth to be a star, and instead presumed it must be a comet. He informed Maskelyne, who after several unsuccessful attempts to spot the object himself, made a close study of its orbit. Further investigation confirmed that it wasn't a comet at all, but a previously unknown planet.

Herschel quickly became a celebrity both within and beyond the scientific world. In November 1781 he was awarded the coveted Copley Medal of the Royal Society. King George III was delighted by the new planet—not least because Herschel had strategically chosen to name it Georgium Sidus ('Georgian Star') after him—and soon offered Herschel a pension that allowed him to give up music and devote all his time to astronomy.

Naming the discovery after the King echoed a longstanding tradition in exploration of naming newly-encountered lands or geographical features after patrons or monarchs. Indeed Herschel's contemporaries celebrated his achievements as equivalent to those of the explorer Captain James Cook: 'He may justly be deemed among astronomers what Cook is among navigators, the first of his profession, the explorer of worlds unknown, and the illustrator of the celestial, as our great navigator was of the terrestrial, globe’ (The Gentleman's Magazine, and Historical Chronicle, Volume 55, Part I, 1785).

Unsurprisingly, other astronomers in Europe were much less keen that the heavenly body should celebrate George III. German astronomer Johan Elert Bode proposed the name Uranus instead; in Greek mythology Uranus was the father of Saturn and grandfather of Jupiter. Although this name was ultimately adopted, the British resisted changing official documents until the mid 1800s.

Uranus as part of the solar system

Small orrery on mahogany stand showing 6 planets out to Saturn. This example has a brass calendar base with a wooden pedestal stand and six planets out to Saturn on rod-arms. — Orrery made by Benjamin Martin

This miniature orrery, or planetary model, was made by Benjamin Martin, an instrument maker working in London in the mid 1700s. Look closely and you'll see it has seven planets orbiting the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn and Uranus.

Since Martin died in January 1782, at first glance it might appear that this instrument is a very early representation of the newly-enlarged solar system. However, a close examination of the internal gearwork reveals that the seventh planet was a later addition. The owner of this instrument must have 'corrected' it to reflect the latest discovery.

Because of its great distance from Earth, Uranus is difficult to study. However, over the last two centuries we have learnt much about its composition and characteristics. For example, that it is an 'ice giant', in contrast to the 'gas giants' Jupiter and Saturn. Our view of it was improved by the Voyager 2 space probe, which captured images of it as a featureless sphere in 1986.

Uranus didn't remain the furthest object in the solar system for very long; the discovery of Neptune followed only 65 years later. Tiny Pluto was spotted in 1930 and designated the outermost planet, but controversially relegated to dwarf planet status in 2006. The debate amongst astronomers rages on.

Herschel's achievements didn't end with the discovery of a new planet. Amongst his various accomplishments were the discovery of two of Uranus's moons, and experiments that showed that sunlight yielded heat beyond the visible red portion of the spectrum, which led to him subsequently being hailed as the discoverer of infrared light. He also sought to make larger and larger telescopes, which could gather more light and see more distant objects. His reflector with a 40-foot focal length, constructed at his home in Slough, was the largest telescope in the world for 50 years.

Caroline continued to work closely with William, and together they discovered thousands of previously unseen star clusters. She became a successful and respected astronomer in her own right, discovering eight new comets and rewriting star catalogues. Her achievements earned her the gold medal of the Royal Astronomical Society—she was the first women to receive the honour—and her own pension from King George III.

Suggestions for further research

Books and papers

Lemonick, Michael D. The Georgian Star: How William and Caroline Herschel Revolutionized our Understanding of the Cosmos. New York City: W.W. Norton Company, 2009
Holmes, Richard. The Age of Wonder: How the Romantic Generation Discovered the Beauty and Terror of Science. New York City: HarperCollins, 2008
Michael Hoskin, "Herschel, Caroline Lucretia", Oxford Dictionary of National Biography, 2005
Michael Hoskin, "Herschel, William", Oxford Dictionary of National Biography, 2008
J. R. Millburn, "Benjamin Martin and the Development of the Orrery", British Journal for the History of Science, volume 6 issue 4, 1973

Online

George III: A royal passion for science, Science Museum
Starry nights: astronomy in the popular imagination, Google Arts and Culture