Shallow Sky In December

Akkana Peck

Jupiter and Saturn will be well placed for observing all month. If you get together for the holidays with family or friends who may not be aware of the goings-on in the sky, why not set up a telescope and share the joys of planet watching?

Jupiter shows a wealth of festoons this year, long streamers reaching from the north equatorial belt into the equatorial zone. The Great Spot Formerly Known As Red is not very prominent this year, and appears most often as a break in the southern equatorial belt, but on nights of excellent seeing, it's possible to see some swirls within the spot, and a train of white ovals preceding and following it.

Saturn remains a glorious sight, with the rings tilted generously open. It's easy to see the Crepe ring (the shadowy translucent ring inside the A and B rings), and Cassini's division (the wide gap dividing the outer A from the middle B ring). There's an obvious shading in the B ring, from bright at the outside edge next to Cassini's division to dark at the inside near the Crepe ring. I don't remember seeing this shading so easily in past oppositions.

My impression is that the gaps in the A ring (variously named after Encke, Keeler, deVico and various other observers, since there's no consensus on who first recorded the A-ring gap) are much easier to observe this year than they were at the last opposition; I've been able to see evidence of an A-ring gap in telescopes as small as a C-5, whereas last year I was hard pressed to see this mark even in telescopes of 8" or larger, and then only on nights of excellent seeing. I'm not sure if this is due to practice (I spent a lot of time last opposition looking for that darned A-ring gap) or that it's more obvious with the wider ring tilt we see this year. Probably both. Anyway, get out and take a look, and let me know how you do looking for this gap and what sort of telescope you're using.

Mars remains low in the southwest in early evening, and its gibbous disk continues to shrink as our distance from the red planet increases. Not much detail will be visible on Mars this month, but observers may enjoy watching a conjunction with Uranus on December 14, when the two planets, separated by roughly half a degree of sky, should be visible in the same low-power eyepiece field. It makes another close pass, with the moon, on the 12th; observers on the eastern US and in South America get to see an occultation, but here in California we'll miss it, alas.

Neptune is west of Uranus, and therefore from our vantage point will be closer to the sun and more difficult to observe this month; faint Pluto is even closer, and will be very difficult to observe.

The earth crosses its winter solstice on the 21st. On December 22 the moon reaches its closest perigee of the year, at 356,753 km. Since that day is also the full moon, the full moon will be unusually large that day, and tides will be unusually high. Two weeks earlier, on the 8th, the moon reaches its farthest apogee of the year (406,624 km).

Early risers will get lovely views of Venus this month, especially in the first few days of December when the crescent moon is nearby. Below them, almost hidden in the sunrise glare, hangs Mercury, which should remain visible all month. It's at greatest Western elongation on the first, and sinks lower as the month progresses.

Akkana Peck; last updated: October 03, 2007 Prev Next