Voyager 2 image of Uranus

Five planets have been known since prehistoric times. Discovered by no individual, they're there for all to see. So why was the sixth one – now called Uranus – not known until 1781?

A telescopic object
Formerly the outermost known planet, Saturn is only half as far away as Uranus. The discovery of Uranus doubled the size of the known Solar System. Does its great distance mean that you need a telescope to see it? Certainly that would have kept it from view until the 17th century.

But no. Although Uranus [YUR-uh-nis] was discovered telescopically, it's often visible to the unaided eye. There's even indirect evidence that Greek astronomer Hipparchus (190-120 BC) saw Uranus and included it in his catalog as a star.

Uranus the star
In 1612, Galileo saw Neptune with his primitive telescope. He labelled it as a star. Since Neptune is farther away than Uranus, surely someone also saw Uranus through a telescope. In fact, we know that several someones saw it.

John Flamsteed (1646-1719) was the first Astronomer Royal at Greenwich. His star catalog, published posthumously, was preeminent for many decades. In December 1690 he observed a faint star – or so he thought – which he catalogued as 34 Tauri. It was Uranus. Many years later, he noted Uranus as an unnamed star in a series of observations.

Tobias Mayer (1723-1762), a professor in Göttingen, Germany, was one of the most accomplished of the 18th century astronomers. One of his observations at the beginning of 1756 included a faint star, now known to be Uranus.

James Bradley (1693-1762), the third Astronomer Royal, saw Uranus three times. He recorded it as a faint star in Capricornus in 1748, and in Aquarius in 1750 and 1753. Bradley didn't realize that it was three sightings of the same object.

Pierre Le Monnier (1715-1799) was a notable French astronomer, and the only person to see Uranus before it was officially discovered, and then live to see it accepted as a planet. He observed it a dozen times between 1750-1769 and recorded it as a star. Six of the observations were made in a 4-week period.

Why didn't anyone realize what they were seeing?
You can distinguish a planet from a star by seeing it with enough magnification to show the planet's disk. Whatever the magnification, a star remains a point of light. William Herschel was the first to have a telescope that showed Uranus as more than a point of light.

Unlike stars, planets move. Le Monnier was unlucky to have missed the movement. Planets farther away from the Sun than Earth orbit more slowly, sometimes appearing to hold still. This is known as a stationary point, and Uranus was at such a point during Le Monnier's observations.

Flamsteed's series of observations occurred at a good time to see Uranus. However William Blitzstein, who made an extensive analysis of Flamsteed's observations, suggested that Flamsteed simply didn't notice. He “did not often review a night's observations soon after they were made. Most of the reductions were made many years after the observations!”

One obstacle applied to all the observers, including Herschel. No one expected to see a new planet.

March 13, 1781
In Bath, England, noted musician William Herschel was in the garden at work on his sky survey. He was making detailed observations of the sky at high magnification in search of double stars. The date is of historical interest, but Herschel didn't rush to share the 18th century equivalent of “OMG. Just found a new planet.” He described the unusual find in his observing journal as “a nebulous star or perhaps a comet”.

Flamsteed had been collecting data for a star catalog, but Herschel was taking a keen interest in what was in the sky. Therefore when he next saw the nebulous object, it was obvious to him that it had moved. It wasn't a star, so it must be a comet.

Comet → planet
Via his friend William Watson, Herschel's discovery made its way to the Royal Society and beyond. Some observers were skeptical because they couldn't see the comet in their telescopes. Herschel had his own way of observing, and it was hard to turn his observations into standard positional measurements.

Yet it got astronomers talking, and they were starting to say that it wasn't a comet. As data was collected in order to plot the object's orbit, the Astronomer Royal Neville Maskelyne thought it might be a new planet. The great comet hunter Charles Messier also made it clear that it was nothing like any comet he'd ever seen. It moved too slowly, its orbit was too circular.

When it became obvious to everyone that Herschel had discovered a new planet, German astronomer Johann Bode searched for and found the previous sightings listed above.

It's a planet – name it!
Herschel's friends told him to name the planet, or else the French would. He named it Georgium Sidus (George's Star) after King George III who became Herschel's patron. This was fine in England, but it didn't go down well elsewhere, especially in the traditional-enemy France and the newly-independent America.

Bode suggested Uranus, the Latinized form of Greek Ouranos [oo-RAH-nus] who was the personification of the Sky. Since Ouranos was the father of Cronos (Saturn), who was the father of Zeus (Jupiter), it seemed an appropriate choice, and followed the mythology of the other planets.

References:
(1) William Blitzstein, W (1998), The seven identified observations of Uranus made by John Flamsteed using his mural arc, The Observatory, Vol. 118, p.219
(2) Bourtembourg, R (2014), Seeing Uranus in antiquity? An amateur astronomer's hypothesis about what Hipparchus saw c. 128 BC. The Free Library. Retrieved 2018-05-27
(3) Mark Littmann, M (2004), Planets Beyond: Discovering the Outer Solar System