Alan MacRobert
Sky & Telescope
Friday, November 28, 2008

Anyone looking southwest in evening twilight as November turns to December will witness a close pairing of the two brightest planets, Venus and Jupiter. On November 30th and December 1st they'll be separated by only 2° — about the width of your finger held at arm's length. And on the evening of December 1st, skywatchers in the Americas will see the crescent Moon joining the two planets to make a remarkably compact celestial triangle. It's sure to turn heads. The illustration at right shows the scene.It's plotted for observers in the middle of North America (and the alignment of the Venus-Jupiter pairing is exact for December 1st), but it's close enough to give a good idea of how the scene will look from anywhere in North America on the evenings in question. Although the three objects appear close together, appearances in astronomy are deceiving. The Moon is only 252,000 miles away, less far than you may have driven in your lifetime. Right now Venus is 370 times farther away, at 94 million miles. And Jupiter, at 540 million miles, is nearly six times farther than Venus. If the sky is cloudy for this event, don't despair. Venus and the crescent Moon will have another spectacular twilight pairing (though without Jupiter) on December 31st, New Year's Eve. This week's planet conjunction marks the second time this year that Venus and Jupiter have mimicked a brilliant "double star" in our sky. Back on February 1st, they appeared together in the eastern sky before dawn. At that time they nestled even closer together — only ½° apart. The last time these two planets were paired in the evening sky and easily seen was September 2005, when they appeared about 1½° apart. They won't be this close together and well placed for evening viewing again until May 2013 (1° apart.) Special Treat for Europeans Across much of Europe, even people who never normally look up should watch the western twilight on Monday evening, December 1st. The Moon will be right next to Venus and Jupiter. But the real show happens when the Moon's dim, earthlit edge occults (covers) dazzling Venus itself. The second act happens roughly an hour or more later, when Venus reemerges into view from behind the crescent's bright edge. At locations where the twilight sky has darkened enough, all you'll need are your eyes. But binoculars or a telescope will give a much grander view — and will also allow you to watch the occultation where it happens in a bright sky, even before sunset. Actually, with Venus a full 43° from the Sun, you may be able to pick it up in daylight with your unaided eye, depending on the clearness of the air. Venus will appear small and gibbous in a telescope, 69% sunlit and 17 arcseconds across (a hundredth the diameter of the Moon). Because of Venus's significant angular size, its disappearance and reappearance will each be gradual, taking 30 seconds or more.

Don't miss the best naked-eye sky spectacle of the year! (These scenes are always drawn for the middle of North America. European observers: move each Moon symbol a quarter of the way toward the one for the previous date. In the Far East, move the Moon symbols halfway. For clarity, the Moon is shown three times actual size.) Sky & Telescope diagram By Friday the 5th Jupiter and Venus have drawn 5° apart, about three finger-widths at arm's length. Sky & Telescope diagram On December 1st, the apparent path of Venus behind the crescent Moon depends on your location in Europe. (Sky & Telescope diagram)

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Source: "Sky & Telescope: http://www.skyandtelescope.com/observing/home/35217944.html

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