And Those Of Neptune

Similar occultation observations involving Neptune were made during the 1980s. These also provided a hint that the planet has rings, but with a difference.

In the decades after Uranus was spotted, astronomers followed its progress in order to chart its orbit. Because that planet takes 84

FIGURE 12-4. The rings of Uranus photographed using the Hubble Space Telescope in 1998. The rings were discovered through occultation observations 20 years before. This is the true orientation, because the spin axis of this planet is tipped over, and the rings orbit above the equator. Several of the Uranian moons can be seen, along with bright areas on the cloud-canopied planet itself.

FIGURE 12-4. The rings of Uranus photographed using the Hubble Space Telescope in 1998. The rings were discovered through occultation observations 20 years before. This is the true orientation, because the spin axis of this planet is tipped over, and the rings orbit above the equator. Several of the Uranian moons can be seen, along with bright areas on the cloud-canopied planet itself.

years to circuit the Sun, less than three Uranus years have yet to elapse since it was discovered (and it is sobering to note that Pluto has not completed even one-third of an orbit since it was found in 1930). Astronomers quickly realized that there was a problem with Uranus, because it didn't seem to behave as calculated, wavering from the path expected if only the Sun and the known planets affected its motion. By the 1840s it was obvious that something was wrong, and two astronomers—Urbain Le Verrier in Paris, France, and John Couch Adams in Cambridge, England—inde-pendently predicted the mass and position of another planet beyond Uranus. The idea was that the gravitational tugs of this yet-unseen planet would explain the anomalous orbit of Uranus. While British astronomers dithered, Johann Galle and Heinrich d'Arrest, using the Frenchman's predicted positions for the new planet, spotted Neptune from Berlin in September 1846.

This provoked uproar in Britain, as claims were made for parity between Adams and Le Verrier in terms of credit for the prediction. The brunt of the responsibility for letting the discovery slip away needed to be borne by the professionals at the Royal Greenwich Observatory and within the universities, particularly at Cambridge. If amateurs with good equipment, such as James South in Kensington, had been privy to Adams's prediction then perhaps British honor might have been saved and Neptune discovered from within its shores.

Stung by all this, various amateur astronomers leapt into action. One of them was William Lassell, who had an excellent private observatory situated near Liverpool, later removing to the clearer climes of Malta. Like William Herschel before him, Lassell was skilled at constructing large reflecting telescopes, and with his champion he quickly discovered Triton, the massive moon of Neptune. But Lassell went further. Before long he was claiming that a ring like that of Saturn accompanied this new planet. That ring seems to have been either a figment of Lassell's imagination or a spurious image produced by his homemade instrument. Eventually rings around Neptune were discovered, but only a couple of decades ago, and they are much too tenuous to bear any relation to Lassell's claim.

Again these rings were identified through the tracking of occultations. Astronomers in the 1980s witnessed dips in their traces of stellar intensity before and after the star passed behind the planet itself, as with Uranus, but in this case the dips were not symmetric about Neptune. A strong decrease on one side was not repeated on the other, and when a pair of dips did occur they were not equally distant from the planet. This left a bit of a quandary for the astronomers: had they identified Neptunian rings or not?

By this time a dark, thin ring about Jupiter had been spotted using the Voyager spacecraft, leaving Neptune the odd man out of the gas giants if it lacked a ring system. Thus the betting was on rings being confirmed when Voyager 2 at last reached Neptune in 1989. Sure enough, those rings were found in accord with the occultation data, and the reason for the ambiguity became obvious: rather than having complete circular rings, the dust orbiting Neptune seems to be concentrated in short arcs, as in Figure 12-5. The occultation observers by chance had intersected some arcs, but not others, producing their puzzling results.

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