(Updated December 3, 2013: Comet ISON is no more! What many experts feared finally happened: its nucleus broke apart and completely disintegrated during its close encounter with the sun on Novermber 28. A cloud of debris seemed to survive its close brush with the sun, but it rapidly dissipated over the following hours and days. Consequently, comet ISON will not be visible in the early morning sky during December, as we had hoped. Our view of a spectacular comet will have to wait!)
Hyakutake was the first bright comet I ever observed, and though it was seventeen years ago I still remember it well. In March 1996, the majestic comet’s diffuse coma and long, straight blue tail were easy to see above the horizon.
It’s the kind of observation that marks, and redefines, our relationship with nature. Suddenly, there’s a bright new object in the sky, one we can assimilate not just intellectually but with our senses as well. The comet is really there; it becomes part of our lives, as if another Moon was added to the sky.
I’m sharing this story because, right now, there’s another comet near the Sun — one that has the potential to thrill us all: Its name is Comet ISON. It came from the distant reaches of our solar system and is experiencing its first encounter with Sun.
So what can we expect to see in December? ISON was predicted to pass within 1.2 million kilometres of the Sun on November 28; at that distance, temperatures reach over 2800° C. As of this writing, we don’t know if the comet’s 5 kilometre-wide, icy nucleus will have survived the encounter: It might break apart or vaporize completely.
But if ISON’s nucleus does manage to survive this hellish ordeal, it could provide a spectacular end-of-autumn show.
Early morning is the best time to observe the comet. Around 7 A.M on December 1, just before sunrise, ISON will be low on the east-southeast horizon. A thin crescent Moon will appear to its upper right, along with Mercury and Saturn as a bonus.
Throughout December, Comet ISON will move farther from the Sun and become fainter. At the same time, it will gain altitude and become visible against a dark sky earlier before daybreak. ISON will also move closer to Earth and appear progressively larger. It is therefore hard to predict the best time to observe the comet. If the most optimistic predictions are correct, ISON should be visible in the glow of dawn, sporting a blue-green coma and a 35 degree-long tail that spans the equivalent of 70 lunar diameters!
If its brightness persists until mid-December, the comet could then become visible in the evening sky, low on the western horizon just after sunset. On December 26, ISON will pass closest to Earth at a distance of 64 million kilometres.
The period surrounding the New Moon, on December 2, will offer a good opportunity to observe ISON among the pre-dawn stars. Though later in the month, around December 17, the light of the Full Moon might hinder observations of the comet.
Now for the planets…
Comet ISON is not the only celestial show this month. December nights are long and ideal for astronomical observing. This year, several planets are visible to the naked eye. If you have a telescope, you can take advantage of what these distant worlds have to offer…
Venus is visible in the west after sunset, however, it gradually approaches the Sun this month so early December is the best time to view the dazzling planet. On the 5th, a thin crescent Moon will appear 7 degrees to the upper right of Venus.
As Venus sets in the west, Jupiter rises in the east among the stars of Gemini, where it will remain visible all winter. This month, the giant planet culminates high in the south around 1:30 A.M. On December 18, a nearly full Moon will appear just 5 degrees to the lower right of Jupiter. Jupiter and the Moon, the brightest objects in the sky at the time, will no doubt compete for our attention.
At the beginning of the month, Mercury remains visible in the southeast just before dawn, but the furtive planet quickly closes in on the Sun and becomes increasingly harder to see.
In December, Mars rises due east around 12:30 A.M. and gains altitude as the night progresses. About 4:30 in the morning, it’s Saturn’s turn to rise and join Mars and Jupiter, which are already well up in the wee hours of the day. On the 22nd, a waning gibbous Moon will come between Mars and Jupiter: On that date, Saturn, Mars, the Moon and Jupiter will be spread more-or-less evenly along the ecliptic. Since the Moon and planets orbit the Sun on nearly the same plane as the Earth, their position in the sky is always close to the ecliptic.
A meteor shower
The Geminid meteors are one of the most spectacular showers of the year. But contrary to the Perseids, which occur in mid-August when nights are warm, the Geminids occur from December 12 to 14, under far, far colder conditions. If you are brave at heart, the show is well worth it: You can expect to see over 60 meteors an hour… and they are usually very bright.
This year the Moon will be present; however, it sets late at night, so the best observations will have to wait until then. The meteors (the technical term for “shooting stars”) radiate from a point in the constellation Gemini, hence their name. But you can see them anywhere in the sky: Looking toward the bright stars, Castor and Pollux, meteors will more plentiful but their trails will be short; farther afield, there will be fewer meteors but their trails will be much longer.
Don’t forget to dress very warmly; and bring some food and a good ground-pad if you want to try your luck!
In closing, keep in mind that this year the winter solstice occurs on December 21 at 12:11 A.M. It marks the moment when the Sun reaches its lowest point in the sky (when viewed from the northern hemisphere). So as of 12:12 A.M. the days will start getting longer!
Clear skies and good comet observing to all!
