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The Sky This Week, 2015 December 15 - 22

The season's colorful lights
Cylindrical projection map of Jupiter
compiled from observations made between 2015 December 4 - 8

The Moon spends this week waxing in the evening skies. First Quarter occurs on the 18th at 3:14 pm Eastern Standard Time. Luna adds light to the sparse starfield of the autumnal constellations. The Moon won’t be close to any bright objects until she rises with the bright star Aldebaran on the evening of the 22nd.

The time of sunset is now beginning to slowly trend a bit later on successive nights. Most of us probably won’t notice much of a difference in the time of sunset this week, but by Christmas Day it should become noticeable to anyone with a casual interest in the sky. At that time the Sun will set five minutes later than it does at the beginning of this week. However, the time of the latest sunrise is still moving later each morning, offsetting the later sunset times. The two trends "cross" each other on the date of the winter solstice, which occurs on the 21st at 11:48 pm EST. At that time the center of the Sun’s disc stands directly over the Tropic of Capricorn just west of the western coast of Australia. This will mark the shortest day of the year for residents of the Northern Hemisphere; here in Washington, DC we’ll have 9 hours and 26 minutes between sunrise and sunset.

As the Moon waxes, the few dim constellations of autumn that occupy the early evening sky become even more difficult to see. Capricornus, Aquarius, and Pisces have no stars brighter than third magnitude, and the sprawling pattern of Cetus has a single second magnitude star as its brightest luminary. From urban locations these constellations are virtually invisible under the best of circumstances, so a vast swath of the sky looks essentially empty. However, you don’t have to wait too long for brighter prospects to appear. By 9:00 pm nine of the 25 brightest stars in the sky will have cleared the eastern horizon. Anchored by the familiar outline of Orion, the Hunter, the Great Winter Circle provides a welcome touch of light and color for the longest nights of the year. The last of these stars to clear the horizon is Sirius, the brightest star in the sky. Easily found by extending an imaginary line southeastward from Orion’s famous "Belt Stars", Sirius flickers through all the colors of the rainbow as its light passes through the denser atmosphere near the horizon. Moving clockwise from Sirius you’ll pass Procyon, Pollux, Castor, Capella, Aldebaran, and wind up at Rigel, Orion’s "knee". Near the center of the circle is the red-tinted star Betelgeuse, which marks one of Orion’s "shoulders". By midnight these stars are crossing the meridian, blazing away over the winter landscape, unencumbered by the waxing Moon.

Early risers will find the Great Winter Circle settling below the western horizon, replaced by another rather bland area of the constellations of spring. Normally this wouldn’t be a show worth getting up early for, but this year we have a nice array of bright planets to spice up the view. You’ll find giant Jupiter near the meridian at 6:00 am, very well-placed for telescopic observation. Close to the horizon you’ll see the dazzling planet Venus. Last week Venus was occulted by the waning crescent Moon, but before winking out behind the lunar limb she was easily visible to the naked eye in broad daylight. This isn’t all that unusual; many people have mistaken Venus for a high-flying airplane during the daytime. Even my high-school astronomy teacher, who served as a gunnery officer on a convoy transport in World War II, once tried to shoot it down! Between Venus and Jupiter you’ll find ruddy Mars, easily distinguished by his color. The red planet will pass close to the bright star Spica by the week’s end.

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