“It should be an excellent year to carry a small scope to any holiday gatherings ...”
December should be a lovely month for shallow sky observing, with all the gas giants in the sky at once, Mercury back in the early evening sky, plus a nice close dance between Venus and Mars in the morning sky — assuming the weather permits, of course.
Saturn reaches opposition on the night of December 16-17; that means it will be well placed all night. In addition, it's about as far north as it ever gets, which means it'll be high in the sky most of the night, up where the seeing is steady. It should be an excellent year to carry a small scope to any holiday gatherings you might be attending, and share the sight of this lovely planet, with its ring system tilted at a generous angle (my satmoons program says about 26 degrees), the gaps in the rings showing nicely (see last month's column), and nice subtle color on the planet's disk. Both photographers and visual observers take note: on January 4-5, Saturn will pass in front of the Crab nebula, a very rare event. Let's hope for clear skies that night!
Trailing Saturn by a few hours is mammoth Jupiter, not yet at opposition but close enough and high enough to show tremendous detail through a telescope. In addition, there's always plenty to watch in the motion of Jupiter's moons and the shadows they cast on their parent planet's disk, and it's a particularly good time right now, because the plane of Jupiter's equator is nearly edge-on to us, which means that we can see the unusual spectacle of mutual satellite events (eclipses and occultations). A Sky & Telescope article this month has more detail on this sort of phenomenon. On the night of December 19th, look for an annular eclipse of Io by Europa starting at about 8:48pm. I've never seen an annular eclipse of a Jovian moon, where one moon casts a shadow on the middle of another. Will we be able to see Io as a ring? How big a telescope will it take to see the ring? If you have clear skies, take a look, and let me know! Keep watching after the occultation, and almost exactly two hours later Europa will occult Io, passing in front of it. Will we be able to separate the two by color, brightness, or size, even when there's no dark space between them? I have no idea, and I can't wait to find out!
There are lots more mutual events mentioned in the Sky & Tel article. Check my site, shallowsky.com — I'll work up a table of those events that should be visible from here, and make them available there. Or check skypub.com for the scoop from Jean Meeus, the Belgian astronomer who's famous for calculating these things (and for writing books showing other people how to calculate them). If you're interested in contributing to science, here's an area where an amateur can help out by timing the events : check http://www.bdl.fr/Phemu03/phemu03_eng.html for details on how to contribute. They also have software for predicting the events. I'd love to hear what local observers saw — post a note to sjaa-chat or the shallow-sky list, or (I'm sure our esteemed editors won't mind my suggesting this) write something up for next month's Ephemeris!
Farther out,, Uranus and Neptune are still within our reach, a few hours behind the sun. Pluto is in conjunction on the 9th, so it's not visible to us this month.
In inner solar system news: Mercury is back in our evening skies, in Sagittarius, going from near-full to third quarter over the course of the month. It reaches maximum elongation on Christmas Eve (perhaps another fun thing you can show your friends and relatives). Venus and Mars make a close pass in our skies, remaining less than five degrees apart all month, with the party getting started on the first when the slim crescent moon joins them to make a trio. Venus is showing a nice crescent through the first half of the month; it should be a lovely sight, especially if you have a telescope with a field wide enough to show both planets while still showing Venus' phase. Poor Mars really looks dim with Venus outshining it, magnitudes +1.5 and -4.7 respectively. But the color contrast is great when you can see them together like that. Take a look, in binoculars or whatever you have available.
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