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Astronomy Picture of the Day

Cloud shadows

Photo of the Week.. Clouds throw spectacular shadows into bright air.

Astronomy news for the short week starting Friday, December 6, 2002.

The Moon, having passed its new phase last week, waxes through its crescent phase the early part of this week, and reaches its first quarter, when we see half the lunar daytime and half the lunar nighttime sides, on Wednesday, December 11. Look to the west in twilight for the slim crescent, the nighttime side of the Moon glowing with light reflected from the Earth, the night of Friday the 6th. While the shortest day of the year takes place when the Earth passes the Winter Solstice in Sagittarius (this year on December 21), the earliest sunset and evening twilight occur some two weeks earlier, during the two- week period centered on December 7, the result of the eccentricity of the Earth's orbit and the 23.5 degree tilt of its axis. By the time of the Solstice passage, the evening drive home after work is noticeably brighter.

We begin the week with Venus glowing at its very brightest in its current morning appearance. As it climbs yet a bit higher in the sky, the planet will dim some, but the fading will be so little for now that it will be hardly noticeable. Venus is making an extended month-long visit with much dimmer reddish Mars, which lies nearby, just to the west. Though Venus quickly approached Mars, both are now moving to the east against the starry background, and Venus is moving the faster, so the two do not come into actual conjunction. Closest approach, when they are only 1.5 degrees apart (three times the angular diameter of the Moon), takes place on Friday, the 6th. By contrast with the morning sky, which contains the two planets that flank the Sun, the evening sky is now graced with the two giants of the Solar System, Jupiter and Saturn, the ringed planet rising just after sundown, Jupiter just after 9 PM. Far beyond them, the three outer planets make a "hidden" appearance, as the crescent Moon passes 5 degrees north of Neptune on Sunday the 8th, and the same angle north of Uranus just over a day later, both planets in direct easterly motion in Capricornus, Uranus close to the Aquarius border. In supreme invisibility, Pluto then passes conjunction with the Sun on Monday, the 9th.

Andromeda and Cassiopeia take center stage in early evening, the famed "W" of the celestial Queen high overhead for those in mid-northern latitudes, Andromeda due south. People in the southern tropics would see Cassiopeia swinging just above the northern horizon, while those in mid-southern latitudes would see brilliant blue Achernar, the star at the end of the River Eridanus, over their heads, Achernar quite invisible from most of the US and all of Canada. Between the two lies the celestial equator, which in early evening is embraced by the fainter figures of Pisces and Aquarius. Crossing the meridian to the south in early evening is the Vernal Equinox, the point passed by the Sun on the first day of spring, the Equinox just down and to the left of Pisces' obvious (if one has a dark sky) "Circlet."
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