SKYLIGHTS
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.
