Astronomy news for the week starting Friday, April 27, 2001.

The Moon, in its perpetual journey around the Earth, passes through its first quarter this week, when it is 90 degrees to the east of the Sun, on Monday, April 30, thereafter waxing toward full. Two days after the quarter, it passes perigee, its closest point to the Earth (typically 363,000 kilometers, or 226,000 miles, 5.5 percent closer than average).

As May begins the giant planets slip away to the west. On Sunday the 29th, Saturn sets just as twilight ends around 9:30 PM daylight time, though Jupiter lingers another hour. Mars, however, is quickly rising ever-earlier, and is now up in the southwest in Sagittarius by midnight. In the morning, Venus rises in the east just minutes after twilight commences. On Friday May 4th, this closest of all planets reaches greatest brilliancy for its current morning appearance. Since Venus has just passed between us and the Sun, we are still mostly seeing its nighttime side, and the planet appears in the telescope as a small crescent.

Twilight is caused by sunlight that illuminates the Earth's atmosphere after sunset or before sunrise. There are three definitions. As the Sun sets, "civil twilight" ends when the Sun reaches 6 degrees below the horizon and it becomes too dark for ordinary outdoor activities. At 12 degrees we reach the end of "nautical twilight," when the seagoing navigator can no longer see the ocean horizon (against which to measure the altitudes of stars to determine latitude and longitude). At 18 degrees, the end of "astronomical twilight," the sky becomes as dark as it can get. That is the time for the astronomer to get to work. The sequence is repeated backwards in the morning. Professional observatories run on "-18 degrees."

As twilight ends, the Big Dipper of Ursa Major is now high overhead for those in the mid-northern hemisphere, the Little Dipper of Ursa Minor pointing up to meet it. If far enough north, above 35 degrees north latitude, you can see Cassiopeia swing beneath the pole opposite the Dipper. To the west of the Dippers and around toward Cassiopeia is an immense area of sky so devoid of bright stars that the ancients invented no constellations within it. Not until after 1600 did astronomers need to organize the sky better and fill in the blanks, giving us (among many others) the obscure and large northern figures of Camelopardalis ("the Giraffe," which lies between Auriga and the Pole) and Lynx (the obvious Lynx), which sprawls in a long line from east of Auriga to nearly under the Big Dipper.
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