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Photo of the Week. Clearing sky.

Astronomy news for the week starting Friday, December 18, 2009.

Busy week. And that is just for the Moon as it plies its crescently path toward first quarter next Friday, December 24, Christmas Eve, about the time of Moonrise in North America.

We begin the week with the cold Moon invisibly passing north of Mercury. Far better to watch the Moon be symmetrically split by Jupiter, the crescent down and to the right of the planet the evening of Saturday the 20th and then up and to the left of it the following evening (closest passage taken during the day). It does the same with Neptune, but see below. Then the evening of Wednesday the 23rd, it's Uranus's turn for a visit, the Moon passing six degrees north of it. While we are at it, note that the Moon goes through apogee, where it is farthest from Earth, on Sunday the 20th, not that that makes much difference in our companion's appearance.

Planets, planets, planets! You might look for Mercury in southwestern evening twilight, as this smallest planet (barring Pluto, but that is another story) goes through its greatest eastern elongation relative to the Sun on Friday the 18th. It's at its best a couple nights later, when it sets just short of the end of twilight. Then its Jupiter's turn as it glides just 0.6 degrees south of Neptune the night of Saturday the 19th. The Boss of the planetary system is wonderful for early- evening viewing in the southwest just after sunset. Except for the Moon, it's the brightest thing out there, and thus easy to find. But look early, as it's down by 9 PM. Try to note Capricornus's Deneb Algedi just below it.

Next up? Mars. Rising around 8:30 PM in far western Leo just across the border from Cancer, known as Ares in Greek, the reddish planet hits a milestone by entering retrograde (westerly) motion against the stellar background on Monday the 21st as the Earth prepares to pass (late next month) between it and the Sun. And don't forget Saturn, which is now rising in the east shortly after midnight in advance of Spica.

At the very end of this week's tale sits our own Earth. On Monday the 21st, at 11:47 AM CST (12:47 PM EST, 9:47 AM PST, and so on), the Sun -- as seen from Earth -- passes the Winter Solstice in Sagittarius to mark the beginning of astronomical winter in the northern hemisphere. On that date, with the Earth's axis tilted toward the Sun, old Sol is as far south of the celestial equator as possible (23.4 degrees), giving us our shortest day and longest night, and rising and setting as far south of the east and west points of the horizon as possible.

With the Sun at the Winter Solstice, the Summer Solstice in classical Gemini rides high at midnight, with Orion below and to the right of the celestial Twins. Far below them is the sky's brightest star, Sirius, the luminary of Canis Major, the Larger Dog. The early evening has its own charms, among them Pegasus, the Flying Horse, recognizable by the Great Square. Well down and to the right of it lies the much smaller and fainter box that represents Equuleus, the Little Horse.
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