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Ohio River sky

Photo of the Week. The Ohio River reflects an autumn sky. (See the water's beauty at full resolution.)

Astronomy news for the week starting Friday, December 5, 2008.

The week is bookended by lunar "phases of the moment," beginning with first quarter on Friday, December 5, which is passed in daylight with the Moon climbing the eastern afternoon sky. The quarter is then followed by a week of the waxing gibbous, which ends with the full Moon on Friday the 12th. With the Sun approaching the Winter Solstice in Sagittarius, the full Moon will be just shy of the Summer Solstice in Gemini and thus -- for those of us in mid-northern latitudes -- very high in the nighttime sky. The only planetary passage is with Uranus on Saturday the 6th, the Moon going to the north of William Herschel's planet.

On the same day as the full phase, the Moon will also pass its perigee, where it is closest to the Earth. Given that we are also approaching perihelion (where the Earth is closest to the Sun) early in January, coastal tides will be especially high (the size of the tide responsive to the inverse cube of the distances between Earth and Moon or Sun).

This week marks the run-up to the Geminid meteor shower, which comes from the flakings of Comet Phaeton, once thought to be an asteroid. Though the shower peaks the night of Saturday the 13th (the morning of the Sunday the 14th), you might see some meteors by the end of the week (the bright Moon unfortunately making a bit of a mess of things).

Mars, lost in western twilight for the past couple months, is finally officially gone from evening, as it passes conjunction with the Sun on Friday the 5th. We'll not catch it in the morning skies for some time. The evening still, of course, belongs to Venus and Jupiter. These two brightest planets (Venus by far the brighter) have switched places, Venus now to the east of the giant planet. While we start the week with both in northeastern Sagittarius, Venus passes across the border into Capricornus on Wednesday the 10th. Both now glorious in the southwest after sunset, Jupiter does not set until after 7 PM, well after the end of twilight, Venus not 'till around 7:30. Then Saturn (in Leo) enters the scene, as it makes a transition to rising before midnight later in the week.

While the shortest day of the year occurs when the Sun passes the Winter Solstice, which it will do on December 21, this week marks the time of the earliest sunset, which takes place around Sunday the 7th. The difference is caused by the slight eccentricity of the Earth's orbit and the 23.4 degree tilt of its axis, which at this time of year causes the Sun to get ahead of its average position, which is used for steady timekeeping.

This is a glorious time for Cassiopeia, the Queen, as she and her royal Chair are passing high in the northern sky in early evening. Just to the east lie the glories of Perseus, the Hero's constellation marked by the central Alpha Persei Cluster, the Double Cluster, and the fascinating eclipsing double star, Algol. A bit farther to the east find Auriga, the Charioteer, with bright Capella, the most northerly among the first magnitude stars.
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