William Lassell and the Imaginary Rings of Neptune
As with many stories in the history of astronomy, the discovery of Neptune in 1846 was a major stepping stone for the field. A planet had been “found” using nothing more than mathematics and follow-up observations, but the finding opened new questions such as if more planets were out there and what was the nature of Neptune. Through some mysterious circumstances, a feature of Neptune was spotted that shouldn’t have been possible with the equipment of the time. Yet the amazing thing was that it ended up being right!
This odd tale begins with John Herschel, who was a friend of both Adams and Le Verrier aka the big players in the Neptune discovery. Through his correspondences with Adams on the subject, he assigns William Lassell, an expert on telescope technology, to look for moons around Neptune in a letter written on October 1 of the same year of the planet’s discovery. By the 12th, Lassell writes back saying he will look for moons as well as rings, despite those not being mentioned in the correspondence. How did he get the idea for rings? After all, only Saturn was known at the time to have them, and Neptune’s wouldn’t be formally found until June 10, 1982. Was he somehow predisposed to an idea before actually finding evidence or did he already seem to spot something and simply mentioned it offhand in his letter? (Baum 68-9)
The latter seems likely, for Lassell started observations on October 2, but a full moon obstructed much of the light. However, he thought he spotted a moon as well as a ring around the planet, and the next night seemed to see it again. But weeks would go by without observations as clouds obstructed the sky and likely Lassell’s brewery career. It wouldn’t be until October 20 that Lassell got another chance to see Neptune, but he didn’t see a ring that night. But after several more observations where he did see a ring and a moon, he finally brings other astronomers to use his telescope on November 10 and draw what they see. All of them ended up having Neptune drawn with both features and he would report in the Times that the planet looked like a miniature Saturn (Baum 76-7, Smith 3-4).
Of course, Lassell realized that his 24 inch telescope could be producing a faulty image. After all, John Russell Hind at the South Villa Observatory had looked at Neptune on September 30 and after looking through a 7 inch Dolland equilateral refractor, he didn’t note any rings nor a moon. But on December 11, he hears about the supposed features and gives the planet another glance. Now, he thinks he does see something. And on January 19, 1847 Lassell writes to Challis, one of the astronomers involved in the Neptune debacle, about a fellow astronomer named De Vico who talked about his observations. Said astronomer was the director of the observatory at the Collegio Romaro Observatory and also someone who thought they spotted moons or rings around the planet over an extended period of time. Other astronomers who felt they too saw rings were Maury and W.C. Bond (Baum 77-80, Smith 4).
Challis was intrigued and so made some observations of Neptune starting on October 3, 1846. Using a 11.25 inch Northumberland refractor, Challis collected data until January 15, 1847. Sadly, much of that period was cloudy for him but he did get a good look on January 12 as well as January 14. Both days he feels like he sees either an elongation of the planet or rings. He brings his assistant to draw what he sees and he too observes the same features. Challis was able to show that the elongation had a 3:2 ratio to the diameter of the planet, according to his tables. But something was amiss, he decided. After all, he had made several prior observations of Neptune during the discovery phase and hadn’t seen anything then, so why now? He postulated that maybe some atmospheric disturbances are at play, but he wrote to Lassell even with tips on the best type of scope and magnification setting for optimal results in the ring viewing (Baum 80-1, Smith 5).
Regardless, Lassell now feels confident in his findings after hearing so many other astronomers seeing the same thing. And that is that, right? Wrong. In a letter written to Challis from a fellow astronomer named Dawes dated April 7, 1847, said astronomer points out how the orientation of Neptune’s supposed rings varies from drawing to drawing and doesn’t match what Challis found either. Challis admits this is a big concern but Lassell feels he can show that everything is in agreement, it’s just how the drawings were presented. But Challis knows better and mentions that to go from a 20 degree declination to a 25 degree declination isn’t a matter of perspective. Clearly, more data was needed and so Lassell begins his observations again on July 7, 1847 after waiting for the planet to again become visible in his latitude. The moon was indeed confirmed to exist and was given the name Triton but Lassell didn’t mention the ring because the weather was not conducive to seeing them (Baum 81-3, Smith 4-5).
Finally, September 8, 1847 was a clear enough night and Lassell along with Dawes go ring hunting. Turning their 24-inch telescope to the sky, they looked for the rings and sure enough they saw them again. Even after rotating the telescope by as much as 30 degrees, the rings were still there and in the right orientation. Writing about this to the Times, he does mention that all the observations with a positive ring sighting have happened with clouds in the area with a max of 3-4 hours for observation. As far as Lassell was concerned though, many different telescopes saw the rings and the potential for human error was eliminated (Baum 84, Smith 6-7).
Not for Challis. He couldn’t make many observations over the following year because of weather but he wanted to get observations from opposition to make sure that the rings really did check out. He also attempted to rotate the actual lenses to make sure a defect in them wasn’t altering the light coming in the telescope. Lassell did have that chance but failed to note anything about the rings, instead finding Hyperion, another moon in the solar system, on September 18, 1848. Later, on August 21, 1849 William along with friends again looks at Neptune and finds the rings still there. Same story in 1851. Surely, the matter should be done now, for over a period of years the rings were still being seen (Baum 85-6, Smith 8).
But then something odd happened. In the fall of 1852 Lassell made some upgrades to his 24-inch telescope and moved it to Valetta, Malta where observation windows were more conducive for nighttime gazing. On October 5, 1852 he trains the telescope on Neptune and sees his rings. Again he repeats this on November 4, 10, and 11. But when he compares his data, something is wrong. He finds that the declination the rings varied greatly with values of 60, 49, 46.19, and 76.45 degrees measured. He can only attribute this to the telescope, for no way could the rings move about so much in such a short span of time. Then he stopped seeing them altogether and was unable to find them again. He gives up the case for the rings (Baum 87-88).
But this leaves us with a big mystery. Sure, we can get that Lassell’s telescope was faulty but how can we explain away all those other astronomers who felt they saw something? And why did it take so long for the telescope to give such wild and different angle measurements? Maybe it was indeed atmospheric disturbances, for at the time Neptune would have been close to the horizon during observations. Plus, psychology may have come into play, with some feeling like they should see it but this doesn’t explain the people who viewed the rings without any prior knowledge of them. Maybe it’s just bits of all of this, working to provide us with a tale to share with other astronomers (89-91).
Baum, Richard. The Haunted Observatory. New York: Prometheus Books, 2007. Print. 68-9, 76-91.
Smith, R.W. and Baum. “William Lassell and the Ring of Neptune.” Journal for the History of Astronomy, Vol. 15:1, No. 42, P. 1, 1984. Print. 3-6.
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