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Sir Edmond Halley: His dog was even better at astronomy than him (Wikimedia Commons)

In 1682, the astronomer Sir Edmond Halley correctly predicted that the great comet now bearing his name would return to the skies in 1758. He was right. But unfortunately he didn’t live to see his prediction come true, and died in 1742, at the age of 85, some sixteen years before its return.

But if you think that’s an unfortunate story, in 2013 a team of researchers at Birkbeck University, London, discovered one that was potentially even more disappointing.

Halley’s extensive library of letters, notebooks, and three-dozen highly detailed diaries and astronomical journals are now housed in the archives of the British Library, and the Birkbeck team took it upon themselves to catalogue and index their contents. In doing so, they stumbled across what at first seemed like nothing more than a comedy anecdote. On the evening of 17 August 1720, Halley was about to begin his astronomical observations for the evening from his home observatory in Islington, his beloved pet dog, Sally, ran over to him, launched himself at his legs, and accidentally struck the tripod of his telescope, sending the telescope itself whirling around.

Having “reprimanded the witless beast most severely”, Halley wrote, he decided to “ascertain just how great an astronomer I now had curled up, looking most dejected, at my feet”. (FYI—he means the dog.)

Sally had knocked the telescope way off Halley’s original coordinates, but before he reset it, Halley decided to see if Sally had inadvertently spotted anything of interest. Peering through the eyepiece, to his surprise Halley saw—well, absolutely nothing. “Blankness,“ he wrote. “A smattering of dim and distant lights against the black.”

“Alas, my dear Sally is no greater an astronomer than DG!” he continued—making a none-too-disguised jab at the Scottish physician and mathematician David Gregory, who had beaten Halley to the post of professor of astronomy at Oxford after an acrimonious battle in 1691. But thinking nothing more of the accident, he returned the telescope to its original position and continued his observations, which at the time constituted a lengthy and exhaustive analysis of the surface of the Moon.

So far, then, so daft. After all, “Dog knocks over telescope and discovers nothing” isn’t exactly going to grab any headlines. But as it happens, Halley had the foresight to write down the coordinates of the position Sally the dog had randomly jostled his telescope. And the Birkbeck team had three centuries of astronomical research on their side.

They knew the date and the approximate time of Halley’s mishap. They knew he was at home in Islington, in his south-facing observatory. And, using a computer program developed by their colleagues at Cambridge, they knew the approximate position of everything in the solar system that night 293 years ago. So having crunched the numbers, they could work out with remarkable accuracy precisely what Halley would have seen through his Sally-knocked telescope that night. And far from being nothing more than “blankness,” Sally had discovered Uranus.

Unbeknownst to Halley, Sally the dog had knocked his telescope onto precisely the right coordinates for him to see the seventh planet from the Sun—some 71 years before William Herschel would discover it in 1791.

Of course, there are other factors at play here: telescopes and optical technology had developed massively by the 1790s, and we don’t know the weather conditions that night, nor the quality of Halley’s lenses, his eyesight, nor even whether he made any effort to focus his telescope correctly.

What we do know, however, is that without realising it, he was looking directly at Uranus—and moreover, that his dog had discovered it.

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