Neptune Examined: William Lassell, a Satellite, and Neptune’s “Ring”

Abstract

One ‘discovery’ about Neptune proved to be nothing of the sort. William Lassell, one of the foremost British astronomers of the nineteenth century, rose to fame during the months after the telescopic finding by discovering Triton, Neptune’s chief moon—important for calculating the planet’s mass and its influence on Uranus. But another feature also aroused Lassell’s suspicions; the telescopic image of Neptune suggested to his eye the presence of a ring system. This time Lassell, as he would later come to accept, was wrong. Lassell’s notebooks held at the Royal Astronomical Society and other archive materials reveal that Lassell’s ring resulted from an issue or issues with his telescope, possibly astigmatism that originated in flexure of the speculum-metal mirrors of his 24-inch reflector. Indeed, the history of Neptune’s supposed ring system is a well-documented example of instrumental failure that provides rich insights into astronomical practice during the mid-nineteenth century as well as the great interest in observing Neptune following its discovery.