The next month should be good for comet watchers, with two visitors visible through backyard instruments, and perhaps even to the naked eye. One comet's flyby will be among the closest passes to Earth ever recorded, while the other represents an increasingly rare example of a discovery made by amateurs.
The orbit of comet 46P was plotted in 1948 and it has been returning every 5.4 years, so its arrival is neither rare nor surprising. Often, however, it approaches the Sun when the Earth is on the other side. Not so this time. On December 16 it will pass 11.5 million kilometers (7.2 million miles) from the Earth, the closest it has come since it was 20th on the list of closest recorded comet approaches and 30 times the distance to the Moon.
Predictions for cometary brightness are unreliable – many highly anticipated comets have broken amateur astronomers' hearts. Nevertheless, so far 46P is brightening steadily, and on track to be bright enough to see with the naked eye around New Year's Eve. However, a consequence of it being so close is that 46P will take up a large chunk of the sky. When a comet’s light is so spread out, even objects that are cumulatively bright take very dark skies to be visible.
Right now, 46P is in the southern constellation of Fornax, but it is heading north as it brightens, and its peak will come when its high in the northern sky. The same day it makes its closest approach to Earth, it passes two degrees from the Pleiades star cluster in the sky. Five days earlier it will pass Uranus, so start your jokes now.
More details on 46P's path can be found at Universe Today, and anyone keen to take part in a global project to collate observations should join the University of Maryland's mailing list.
In contrast to the long-expected 46P, C/2018 V1 (Machholz-Fujikawa-Iwamoto) was a complete surprise. Not only is it a long-period comet whose orbit we have not had a previous chance to track, but it somehow was missed by all the professional sky-tracking projects that now detect almost all the new comets visiting the inner Solar System. Its name comes from the three amateurs who found it first.
Although the initial discoveries were made with telescopes well beyond the range of most amateurs (an 18-inch reflector for first discoverer Don Machholz) C/2018 V1 (Machholz-Fujikawa-Iwamoto) is now bright enough to be seen with binoculars in the early morning sky. We know even less about how bright it will become than we do for comet 46P. The dramatic increase of recent days bodes well, but it will be lost in the Sun's glare by the end of the month.
