Telescopes from the Ground Up

1781: A new planet

On March 13, 1781, the British amateur astronomer and telescope-maker William Herschel was surveying the night sky. He moved his telescope methodically from one object to another, looking for “double stars” – two stars that are so close together that, without proper magnification, they look like one star.

He stopped on something unexpected: not a point of light, but a fuzzy disk.

He initially thought it was a comet, but observations over the next several days showed something amazing. A comet that bright would be quite close to the Sun, and thus move rather quickly against the background stars. This object was moving, but too slowly to be a comet. It was moving so slowly that its motion suggested it would have to be far from the Sun’s gravity, even farther away than Saturn – the farthest known planet. To be that bright and that far away, it had to be a planet.

“And I will name him George...”

Herschel had discovered the first new planet. Up until this point, astronomers had known of only five other planets in the solar system: Mercury, Venus, Mars, Jupiter and Saturn, which are visible to the naked eye.

Herschel tried to name the new planet after his benefactor, King George III of England. It was eventually named Uranus, after the father of Saturn, to fit with the mythological naming scheme of the other planets.

What we can see today: The Hubble Space Telescope looks at Uranus What we can see today — HST looks at Uranus

Astronomers had actually seen Uranus many times before. It had been noted as early as 1690, when it was recorded as a star in the constellation Taurus. But only Herschel had made a telescope powerful enough to see Uranus was not a star. When the amateur astronomer tried to compare his observations with two professional astronomers, neither had telescopes powerful enough to confirm his find.

Herschel’s discovery shook the astronomical world and thrilled the public. It earned him international fame and a paid astronomy job. People had imagined that other planets could exist in the solar system, but this was proof that more planets did exist. The solar system had just gotten bigger, and astronomy’s possibilities had expanded with it. The search for new planets was on.

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