But on the clear Tuesday evening of March 13, 1781, as 42-year-old William Herschel hunkered down at the eyepiece of his 6.2-inch Newtonian reflector, he saw something he did not expect.
Between 10 and 11 o’clock, “while I was examining the small stars in the neighbourhood of H Geminorum,” Herschel wrote, “[my attention was drawn to] one that appeared visibly larger than the rest.”
Struck by the difference in both magnitude and apparent size, Herschel first thought he had discovered a new comet. But four nights later, he spotted the object again, which thickened the plot. It seemed odd, Herschel mused, for any comet to possess such a well-defined disk — much less the complete absence of a long, gaseous tail in its wake.
He reported his find to the Royal Society, as well as his close friend, Astronomer Royal Nevil Maskelyne. “In the quartile near Tauri,” we can imagine Herschel writing in his journal by the dim glow of a candle, “the lowest of two is a curious, either nebulous star or perhaps a comet.”
The solar system gets a new planet
It soon became obvious that the aquamarine object was no comet. And suspicions on the part of both Herschel and Maskelyne were made clear in their letters.
“I am to acknowledge my obligation to you for the communication of your discovery of the present comet, or planet, I don’t know which to call it,” Maskelyne wrote. “It is as likely to be a regular planet moving in an orbit nearly circular round the Sun as a comet moving in a very eccentric ellipsis.”
Over the next several months, Swedish astronomer Anders Lexell and French astronomers Jean-Baptiste Saron and Pierre Laplace worked to show that the object’s orbital characteristics were indeed more akin to a planet than a comet.
The significance of discovering yet-to-be-named Uranus was profound. For millennia, even the most educated people knew of only six planets — from tiny, Sun-hugging Mercury to ring-bedecked and distant Saturn. And calculations of Uranus’ orbit indicated that this seventh planet lay twice as far from the Sun as Saturn. In a single night, Herschel essentially doubled the size of the known solar system.
The new planet was not just distant, either. It was enormous. Herschel suggested an equatorial diameter of 34,000 miles (55,000 kilometers), roughly four times wider than Earth. This estimate turned out to be impressively close to the 32,000 miles (51,500 km) diameter derived from Voyager 2 spacecraft data.