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Contrail sunrise

Photo of the Week. Jet contrails dominate a spectacular dawn sky, reflecting a Sun still below the horizon. The foreshortening of the more distant trails near the horizon reveals the atmosphere to be a flat layer close to the ground.

Astronomy news for the week starting Friday, December 15, 2006.

Our Moon fades away in its waning crescent phase during most of the week, getting closer and closer to the eastern dawn horizon until Wednesday the 20th, when it passes new and makes its transition to the evening sky. The evening of Thursday, December 21st, you might get your first glimpse of the slim waxing crescent in southwestern twilight shortly after sunset. On Monday the 18th, the crescent passes south of Jupiter, then at almost the same time both Mars and Scorpius's Antares, but in the afternoon and evening, with the planets out of sight.

Overwhelmed with the light of dawn, Jupiter and Mars are pretty much out of sight anyway, although a dedicated observer with a clear eastern horizon might still spot them, Jupiter leading the way, Mars much fainter. And too bad, since on Monday the 18th, Mars will pass north of its stellar look-alike and namesake, Antares (the name meaning "like Ares," the latter the name of the Greek god of war). Saturn remains the planet of choice, rising ever-earlier, this week around 9 PM. Look for it just to the east of Regulus in Leo, the separation slowly increasing, the planet now in retrograde motion. At the other extreme of planetary visibility is Pluto, which passes conjunction with the Sun on Monday the 18th.

The biggest planetary news is of Earth . At 6:22 PM on Thursday the 21st (7:22 EST, 5:22 MST, 4:22 PST), the Sun crosses the Winter Solstice in Sagittarius and formal astronomical winter begins in the northern hemisphere (summer in the southern, the celestial point clearly named by northerners). On that date, the Sun will lie as far south (and for northerners as low) as possible (23.4 degrees south of the celestial equator), and northern days will be shortest. It then also rises and sets at its extreme southeasterly and southwesterly points, and will pass overhead at the Tropic of Capricorn. Though beginning to move slowly north, days will continue to get colder as it will take the Sun some time to get high enough for its heating power to gain the upper hand on the cooling ground and waters.

As evening twilight draws to a close, look just west of south about halfway up the sky to find the "Y" of stars that makes the heart of Aquarius, the Zodiacal Water Bearer, most of the constellation sprawling to the southeast of it, though two fairly bright stars (Sadalmelik and Sadalsuud, respectively Alpha and Beta Aquarii) lie to the west. Down and to the right is Capricornus (the "Water Goat," while up and to the left is Pisces (the Fishes), the three marking an ancient "wet quarter."
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