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The Sky This Week, 2015 December 8 - 15

The year's best meteor shower!
Jupiter with Europa and Io, imaged 2015 DEC 6, 11:39 UT

The Moon leaves the pre-dawn sky and returns in the early evening this week. New Moon occurs on the 11th at 5:29 am Eastern Standard Time. If it’s clear see if you can spot the very thin waning Moon before dawn on the 10th. She should appear in the southeastern evening sky in the fading twilight of the 12th. Over the next few evenings she will move into the sparse starfields of the autumnal sky.

We’re still experiencing the earliest sunsets for the year, but by next week Old Sol begins to slowly take back the night. Sunset on the 15th will occur one minute later than it does on the 8th. The total length of night is still increasing, though, as the Sun has yet to reach his latest rise time. That will occur during the first week of January, when he’ll rise 10 minutes later than he does right now. The longest night of the year will fall on the night of the winter solstice on the 22nd. A detailed account of sunrise/sunset phenomena around the times of the solstices may be found here.

These longest nights of the year offer many treats for the intrepid skywatcher. This week we will be treated to the year’s most consistent annual meteor shower, the Geminids. These tiny bits of cosmic fluff encounter the Earth’s atmosphere for the next couple of weeks, with a peak of activity on the night of December 13/14. If the sky is clear and you find yourself in a dark location away from city lights you can expect to see about 100 "shooting stars" per hour. Even observers in suburban sites can see up to 50 per hour. They appear to emanate from a point in the sky near the stars Castor and Pollux in the constellation of Gemini, the Twins. This "radiant" point is high above the eastern horizon by the late evening, so unlike most of the annual meteor showers they can be seen well before local midnight. Your best view will be between about midnight and 3:00 am, when the radiant is almost straight overhead. The one drawback to the Geminids is that they occur in December, when the idea of lying on your back looking up at the sky doesn’t have quite the same cachet as doing so for the Perseids, which occur in mid-August. Staying warm is the key to enjoying this celestial show, so dress in your warmest clothes and have a Thermos full of your favorite hot beverage handy. The meteoroids originate from an interesting asteroid object known as (3200) Phaethon. In addition to being the first asteroid discovered by a space probe, it passes closer to the Sun than any other named asteroid. It also may be the nucleus of a defunct comet, still sputtering small bits of itself along its orbit which collide with Earth every year in mid-December. Whatever its origin, Phaethon’s dusty wake produces one of the best sky shows you’ll see in the winter sky.

While the morning parade of planets doesn’t benefit from interesting conjunctions with the Moon, the later times of sunrise mean that getting up at 6:00 am will find them in a still-dark sky. You’ll find Jupiter high in the south at this hour approaching the meridian. This is an excellent time to view the giant planet in a telescope through generally steady air, and the bracing chill of the early morning ensures that you’ll be wide awake for the rest of the day. Halfway between Jupiter and the very bright Venus you’ll see the ruddy glimmer of Mars. His disc is still quite tiny in the eyepiece, but moments of steady air will show his bright north polar ice cap. Mars will have a much better showing as we move into spring.

Comet Catalina may be found a few degrees north of Venus for the first few days of the week. I was able to spot it from suburban skies with binoculars over the past weekend. It resembles an out-of-focus greenish star. It will continue to drift northward toward the bright star Arcturus over the course of the rest of the month. This will be its only visit to the inner solar system; its hyperbolic orbit will sent it out to the far reaches of the Oort Cloud next year.

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