Comet tale

Comets, according to comet-hunter David Levy, are like cats. They both have tails; and they both do whatever they want.

People my age remember Comet Kohoutek, which came with a tremendous build-up in 1973 and turned out to be a major disappointment. It had been discovered far out in its orbit, with about a year’s lead time; we had assumed that anything bright enough to be seen so far in advance would probably be spectacular by the time it arrived near Earth. It wasn’t. (I actually did see Kohoutek and, as comets go, it was small but pleasant. The only thing wrong with it was the hype it had received.)

Among comet circles, there has been fear that this year’s Comet Ison (named for the International Scientific Optical Network telescope near Kislovodsk, Russia, where it was discovered) would suffer the same fate. It likewise was spotted almost a year in advance of its closest approach, and it too has been the victim of hyping. Over the past few months, as it failed to brighten up the way the naive first guesses predicted, we all nodded our heads knowingly. Then, last week, overnight it increased its brightness sixfold.

According to a popular science teaching website, the sign of good science is that it is “not based on authority; testable; repeatable; universal; measurable; observable”. By those criteria, hardly anything about comets would count as good science. No comet is universal; each one is different. By the time your observations of a particular comet are published, that comet is gone and no one can go back to check your results. If we are to believe your results, we just have to trust your authority as an observer. Indeed, we have sent spacecraft to observe half a dozen different comet nuclei; even close up, each one has shown important characteristics that are different from all the others.

That list of criteria for “good science” (especially the “not based on authority” bit) finds its roots in Galileo’s famous book on the philosophy of science, The Assayer. Ironically, his motivation for writing that book in 1623 was to prove that comets were not really astronomical objects at all, but mere refractions of light in the upper atmosphere. He thought that it would be impossible for something to orbit in space the way that comets were observed to move, and still fit into the Copernican system he so loved; he did not understand or accept Kepler’s idea of elliptical orbits. His assertions against comets were based on his own authority; he never actually saw a comet himself.

Meanwhile, on Thursday, 28 November, Comet Ison’s highly elliptical orbit passes very close to the Sun; so close that in fact the comet may break into dozens of small bits that will boil away before they have a chance to reappear in our skies. Our best guess at the moment is that it will survive, but we cannot be sure. Comets do whatever they want.

The first days of December will tell the tale when, if we’re lucky, Comet Ison will become visible to the naked eye. Assuming the weather cooperates, look just after sunset (or again in the dawn just before sunrise) and you might see a fuzzy streak of light, a bit north of where the Sun has set (or north of where it will rise).

And meanwhile, without hype, unheralded Comet Lovejoy has also been making its appearance in the morning sky. So far, it has actually been brighter than Comet Ison, easily spotted with binoculars just off the handle of the Plough (or Big Dipper).
Comets are like saints. Sometimes the really good ones don’t get the publicity.