Herschel's observation of comets

To say that Woody Sullivan is interested in William Herschel would be quite an understatement.

“I have dressed up as Herschel for Astronomy 101 half a dozen times,” said Sullivan, a professor emeritus of astronomy at the University of Washington. He started a file on Herschel, the 18th and 19th Century astronomer, some 30 years ago, but can’t exactly pin down why he was so drawn to him.

“I have eclectic interests,” Sullivan said. “I’m always looking for what I call astronomy on the edges: astronomy and music, astronomy and astrology, history, literature, sundials.”

“Herschel was that way to some degree,” Sullivan added. “Perhaps that was it; I saw a fellow traveler there.”

Prof. Woody Sullivan at the May
meeting of the Seattle Astronomical
Society. (Photo: Greg Scheiderer)

Sullivan noted that it was nine years ago that he started doing more serious research into Herschel with the intent to write a biography. While there have been many penned, including a couple in the last decade or so, Sullivan noted that none have been particularly scholarly, and so that’s a void he’s aiming to fill.

After all of that research, the actual writing has begun.

“I do need to get on because I’m getting on,” Sullivan quipped. He spoke about his work at the most recent meeting of the Seattle Astronomical Society, discussing Herschel’s work on comets, about which few biographers have gone into much detail.

While his sister Caroline Herschel discovered eight comets, six of which bear her name, William never found one, though he came close a couple of times. He once reported a comet discovery, but the French astronomer Jean-Louis Pons had already found it a month before. Then in 1781 Herschel reported another comet discovery. But after six or eight months of observation, astronomers more skilled in the calculation of orbits found this new object to be in a nearly circular one well beyond Saturn’s. It was a new planet: Uranus.

First to “discover” a planet

“It’s hard to think about what a new planet means. What planets did we have before? We had the same planets that we had had since Ogg the caveman,” Sullivan noted. “Herschel was the first one to find a planet telescopically, and this made him instantly famous.”

William Herschel. (Photo: Public domain)

Herschel parlayed that into a gig as the court astronomer for King George III. It was actually a pay cut from Herschel’s work as a professional musician in Bath, but he supplemented his income by building and selling telescopes, and by marrying a rich widow. Herschel was mostly interested in deep-sky objects, but comets came to his attention on occasion, in part because he was interested in change.

“A comet is change par excellence,” Sullivan said. “It just appears in the sky, it’s different every day, you never know what’s going to happen.”

While Caroline wanted to discover them, William aimed to understand what they were. Sullivan noted that this wasn’t what most astronomers were doing then.

A different sort of astronomer

“Astronomy at that time was measuring accurate positions of things; planets and their moons and comets and stars for catalogs,” he said. “That’s why you had the Greenwich Observatory. The government was paying for that, not because they loved astronomy but they loved the navy, and you needed that for navigation.”

Herschel’s observations of the great comets of 1807 and 1811 were interesting. Sullivan pointed out that astronomers at the time thought there might be a planet or other object at the nucleus of a comet. Herschel was the first to claim he’d spotted one. When others couldn’t find it, Herschel chalked it up to the superior optics of his telescope. By the 1811 comet, he was trying to figure out if the nucleus reflected light from the Sun, or generated its own light. Herschel declared that the nucleus of this comet was perfectly round, and thus self-illuminated, because if it reflected light it would show phases. Sullivan, after poring through Herschel’s logs, concluded that he had fallen into a trap that scientists need to avoid.

“There’s just no doubt that he was picking and choosing the observations that fit into his concept,” Sullivan said. It was a bit of a reach to claim to be able to determine the roundness of an object of perhaps an arcsecond in width within the fuzzy coma of a comet.

“He’s getting all of his theory and observations mixed up,” Sullivan said. “This can get you in trouble.”

Though Herschel missed on this particular analysis, Sullivan noted that Herschel made some interesting conclusions, particularly in describing the tail of a comet as its atmosphere being pushed away by pressure from the Sun. Though it’s not the atmosphere, but dust and gasses, nobody to that point had really postulated that the Sun might be pushing on things. Other descriptions Herschel made of the mechanics of comets are not so far off from what is held true today.

Sullivan’s presentations are always interesting, and we look forward to the completion of the book and to learning about William Herschel, a fascinating character in the history of science.