See a Rare Comet-Moon Conjunction Tonight

I want to alert you to a rather unusual event occurring this evening.

Many of you already know about the triple shadow transit of Jupiter’s moons Io, Europa and Callisto. That’s scheduled for late tonight.

Earlier, around nightfall, the crescent moon will lie 1° or less to the south-southwest of comet 15P/Finlay. No doubt lunar glare will hamper the view some, but what a fun opportunity to use the moon to find a comet.

Finlay underwent a flare in brightness last week when it became easily visible in binoculars.

The farther south you live, the closer the moon will approach the comet tonight. This diagram shows the view from Tucson, Ariz. at nightfall when less than 1/2° will separate the two. At about the same time (~7 p.m. local time) the moon will occult or cover up a 6th magnitude star (seen poking out from its left side). Source: SkyMap
The farther south you live, the closer the moon will approach the comet tonight. This diagram shows the view from Tucson, Ariz. at nightfall when less than 1/2° will separate the two. At about the same time (~7 p.m. local time) the moon will occult or cover up a 6th magnitude star (seen poking out from its left side). Source: SkyMap

Though a crescent moon isn’t what you’d call a glare bomb, I can’t predict for certain whether you’ll still see the comet in binoculars tonight or need a small telescope instead. Most likely a scope. Finlay has faded some since its outburst and now glows around magnitude +8.5.

You can try with a 10×50 or larger glass, and if you don’t succeed, whip out your telescope; a 4.5-inch or larger instrument should handle the job.

Just point it at the moon at star-hop a little to the north-northeast using the map until you see a fuzzy spot with a brighter center. That’s your comet. The tail won’t be visible unless you’re using more firepower, something closer to 10-inches.

Comet Finlay in outburst on January 20, 2015 shows a beautiful parabolic-shaped head. Credit: Joseph Brimacombe
Comet Finlay in outburst on January 20, 2015 showing a beautiful parabolic-shaped head. Credit: Joseph Brimacombe

By the way, the father south you live, the closer the moon approaches Finlay. From the far southern U.S. they’ll be just 1/2° apart. Keep going south and parts of Central and South America will actually see the earth-lit edge of moon approach and then occult the comet from view!

UPDATE: Although light clouds marred the view I had difficulty finding the comet this evening in my 10-inch scope. It’s possible it’s further faded or my conditions weren’t optimal or both. No luck BTW in binoculars.

Comet Finlay in Bright Outburst, Visible in Small Telescopes

Short-period comet 15P/Finlay, which had been plunking along at a dim magnitude +11, has suddenly brightened in the past couple days to +8.7, bright enough to see in 10×50 or larger binoculars. Czech comet observer Jakub Cerny and his team photographed the comet on December 16th and discovered the sudden surge. Wonderful news!

While comets generally brighten as they approach the Sun and fade as they depart, any one of them can undergo a sudden outburst in brightness. You can find Finlay right now low in the southwestern sky at nightfall near the planet Mars. While outbursts are common, astronomers still aren’t certain what causes them. It’s thought that sub-surface ices, warmed by the comet’s approach to the Sun, expand until the pressure becomes so great they shatter the ice above, sending large fragments flying and exposing fresh new ice. Sunlight gets to work vaporizing both the newly exposed vents and aerial shrapnel. Large quantities of dust trapped in the ice are released and glow brightly in the Sun’s light, causing the comet to quickly brighten.

Some comets flare up dramatically. Take 29P/Schwassmann-Wachmann. Normally a dim bulb at 17th magnitude, once or twice a year it flares to magnitude 12 and occasionally 10!

Animated movie showing the expansion of the coma of Comet Holmes over 9 nights during its spectacular outburst in November. Credit: 3.6-meter Canada-France-Hawaii telescope on Mauna Kea / David Jewitt
Animated movie showing the expansion of the coma of Comet Holmes over 9 nights during its spectacular outburst in November 2007. Credit: 3.6-meter Canada-France-Hawaii telescope on Mauna Kea / David Jewitt

Whatever the reason, outbursts can last from days to weeks. It’s anybody’s guess how long 15P/Finlay will remain a relatively easy target for comet hungry skywatchers.  While not high in the sky, especially from the northern U.S., it can be seen during early evening hours if you plan well.

By pure good chance, Comet Finlay will track with Mars through December into early January. They'll make a remarkably close pair on the evening of December 23rd. This map shows the nightly position of the comet from Dec. 18th through Jan. 12th. Mars location is shown every 5 nights. Positions plotted for 6:15 p.m. (CST) 1 hour and 45 minutes after sunset. Stars shown to magnitude 8. Star magnitudes are underlined. Click to enlarge and print. Source: Chris Marriott's SkyMap software
By good luck, Comet Finlay will track with Mars through December into early January. On December 23rd, they’ll come together in a remarkably close conjunction. This map shows the nightly position of the comet from Dec. 18th through Jan. 12th. Mars’ location is shown every 5 nights. Positions plotted for 6:15 p.m. (CST) 1 hour and 45 minutes after sunset. Stars shown to magnitude 8. Star magnitudes are underlined. Click to enlarge and print for outside use. Source: Chris Marriott’s SkyMap software

Comet Finlay was discovered by William Henry Finlay from South Africa on September 26, 1886. It reaches perihelion or closest approach to the Sun on December 27th and was expected to brighten to magnitude +10 when nearest Earth in mid-January at 130 million miles (209 million km). Various encounters with Jupiter since discovery have increased its original period of 4.3 years to the current 6.5 years and shrunk its perihelion distance from 101 million to 90 million miles.

Comet Finlay appears considerably fainter in this pre-outburst photo taken on December 14th. Credit: Alfons Diepvens
Comet Finlay appears considerably fainter in this pre-outburst photo taken on December 14th. Credit: Alfons Diepvens

Looking at the map above it’s amazing how closely the comet’s path parallels that of Mars this month. Unlike Comet Siding Spring’s encounter with that planet last October, Finlay’s proximity is line of sight only. Still, it’s nice to have a fairly bright planet nearby to point the way to our target. Mars and Finlay’s paths intersect on December 23rd, when the duo will be in close conjunction only about 10? apart (1/3 the diameter of the Full Moon) for observers in the Americas. They’ll continue to remain almost as close on Christmas Eve. Along with Comet Q2 Lovejoy, this holiday season is turning out to be a joyous occasion for celestial fuzzballs!

To give you a little context to make finding Comet FInlay easier, use this wide-view map. A line from bright Vega in the western sky left through Altair will take you directly to Mars and the comet. This map shows the sky at nightfall tonight when the comet will be about 15 degrees high in the southwestern sky. Source: Stellarium
To give you a little context to make finding Comet FInlay easier, use this wide-view map. A line from bright Vega in the western sky left through Altair will take you directly to Mars and the comet. This map shows the sky at nightfall tonight when the comet will be about 15° high in the southwestern sky. Source: Stellarium