For the second time in as many months, the periodic comet 15P/Finlay has surged in brightness. Spot it soon — before the Moon interferes — using our exclusive sky charts.
It's not often that we get a chance to spot two fairly bright comets at once, but that's the case right now.
You've probably already heard about Comet Lovejoy, C/2014 Q2, which has been a delight in binoculars and a dazzling photographic target as it dances its way northward through the stars of Taurus and Aries. It's still an easy grab at 4th magnitude, roughly 10° west of the Pleiades for now and gliding northward toward Almach in eastern Andromeda. (A detailed finder chart is here.)
Comet Lovejoy has performed better than expected, but the real surprise is Comet 15P/Finlay. It returns to the vicinity of Earth's orbit every 6½ years. However, its current apparition (perihelion was December 27th) has kept it rather far from Earth. No one expected it to brighten much better than 10th magnitude.
But Comet Finlay abruptly brightened in mid-December, climbing to a little better than 9th magnitude before dropping nearly two magnitudes in the weeks afterward. So observers were stunned to see this interloper in outburst a second time, just a few days ago, that made it even brighter. This time the surge brought its total magnitude to roughly 7.0 — rather easy pickings for binoculars or a small telescope.
It's faded a bit in the past few days, but you can still glimpse it near magnitude 8 to 8½. Veteran observer Alan Hale, who saw Comet Finlay two nights ago, reports that the coma had faded slightly and was a bit less condensed, though the tail was more pronounced. Some photos show a "spine" structure in the tail, and he noticed that visually. "I've seen it on two previous returns, neither of which was favorable geometrically," Hale tells me, "and it was pretty well-behaved on those occasions."
Use the charts here if you want to try to spot Comet Finlay in the coming week. But don't wait too long — the Moon is returning to the evening sky by week's end.
In fact, the comet is dramatically close to the Moon, especially for the East Coast (roughly one lunar diameter away) on the evening of January 23rd. So get your cameras ready — though it might be challenging to capture both the relatively bright Moon and relatively dim comet well.
William Henry Finlay discovered this comet from South Africa in 1886. Its orbit extends from Earth's vicinity to out past Jupiter. Interestingly, the comet's orbital inclination is low, meaning it's never far from the orbital planes of the major planets. Consequently, it's been subject to occasional gravitational torquing by Jupiter. Each nudge by the big planet changes the perihelion distance a bit, putting it sometimes inside Earth's orbit and sometimes just outside of it.
A great resource for learning more about this (or any) comet is Gary Kronk's Cometography.com