Surprise! An obscure comet that only amateur astronomers would pay attention to experienced a sudden outburst in brightness earlier this week putting it within range of telescopes as small as 4.5 inches.
Comet 15P/ Finlay hunkered along at a dim 11th magnitude successfully avoiding the limelight until about December 16th. That's when Czech comet observer Jakub Cerny and his team took a photo revealing the comet had surged in brightness by some 8 times to magnitude 8.7. Suddenly, Finlay became a minor celebrity.
It still remains in outburst visible low in the southwestern sky very close to the planet Mars, which serves as a convenient "pointer" to help us find it. From mid-northern latitudes, Comet Finlay is best viewed as soon as evening twilight ends, when highest in the sky. The moon, ever a spoiler when it comes to viewing fuzzy stuff like comets and galaxies, won't be over-bright until after Christmas, leaving us almost a nearly week-long window for finding and tracking the comet before it fades.
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Use the general map to take you to Mars and then the more detailed version to pinpoint the comet's location. Finlay will look like a small, fuzzy spot with perhaps a faint tail to the east visible depending on the size of your telescope.
Planet and comet paths are currently converging toward conjunction. On December 23rd and 24th Mars and Finlay will be separated by just 10 arc minutes or 1/3 the diameter of the Full Moon. By December's end they'll gradually separate as each follows its own orbital path around the Sun. The close pairing is line of sight only.
We've known about Comet Finlay since September 26, 1886, when William Henry Finlay happened across it with his 7-inch telescope from the Cape of Good Hope in South Africa. He described humanity's first view of the object as round, 1 arc minute in diameter and "very slightly more condensed towards the centre."
Finlay 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). This month's outburst may change that prediction.
What causes a comet to quickly and unpredictably surge in brightness still baffles astronomers. Unlike most rocky asteroids, comets are friable creatures. They crumble easily. It's thought that sub-surface ices, warmed by a comet's approach to the Sun, vaporize, creating pressurized pockets of gas that break through the overlying ice above, sending fragments flying and exposing fresh new ice.
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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 suddenly brighten.
So now we have two fairly bright comets during the holidays - Finlay and Comet C/2014 Q2 Lovejoy. Lovejoy has been steadily brightening and now glows at around magnitude 5.5, bright enough for observers with dark skies to see it with the naked eye. Click HERE for more information and a finder chart for it.
While this means we have two cometary gifts under the Christmas tree, by all means, don't wait until Christmas to unwrap either. Have at them the next clear night!
