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Skylog
April 2008
By Joe Rao
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Twenty-five-mile-wide crater, center, overlies a spidery pattern of troughs discovered on Mercury. For more images see the MESSENGER Web site.
Image by NASA/John Hopkinst U. Applied Physics Laboratory |
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This past winter, observers were treated to close-up imagery of the planet Mercury courtesy of a space probe known by the acronym MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging). In Roman mythology, of course, Mercury was the messenger of the gods. The only probe to visit Mercury previously was Mariner 10, which swept past it three times in the 1970s. Launched in August 2004, MESSENGER zipped by Mercury in January as part of an elaborate interplanetary dance. It will settle into orbit around Mercury in March 2011 for at least an Earth year's investigation of the rocky little world's geologic history, the chemical composition of its surface, the size and state of its core, the nature of its very thin envelope of gas (known as the exosphere),
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Joe Rao is a broadcast meteorologist and an associate and lecturer at the Hayden Planetar- ium in New York City. |
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and the strength and dynamics of its magnetosphere. The first pictures of a previously unexamined region reveal smooth surfaces suggestive of volcanic lava flows, corrugations formed by the planet's shrinking (thanks to its cooling core), and a spidery scar that one scientist said is “unlike anything we've seen anywhere in the solar system.”
From an Earthly standpoint, Mercury is in superior conjunction with the Sun (on the opposite side of the Sun) on the 16th of April. It begins to swing into view during the following week, and should be easily observable by the 23rd—shortly after sunset, very low in the west-northwest. This is the start of Mercury's best evening apparition of the year for those of us who live in the mid-northern latitudes, because the planet is moving in orbit north of the Sun's equator even as it moves to the Sun's east (it will be farthest east of the Sun on the evening of May 14th). By the last day of April, Mercury sets as much as 85 minutes after the Sun.
April Nights Out
1 Saturn, the “Lord of the Rings,” is on the local meridian (the line running north to south), and thus as high as it gets in the sky during the night, at about 10:30 p.m. It arrives there four minutes earlier each day thereafter, in excellent position for observers all month.
5 The Moon is new at 11:55 p.m. eastern daylight time (EDT).
8 A thin crescent Moon hovers just above the Pleiades star cluster (distinguishable through binoculars) in the west-northwest sky, about an hour after sunset.
10 Mars forms an isosceles triangle with Castor and Pollux, the brightest stars in the constellation Gemini, the Twins.
11 Appearing to move at roughly its own diameter each hour, the Moon slowly approaches Mars from the lower right.
12 The Moon waxes to first quarter at 2:32 p.m. EDT.
20 The Moon becomes full at 6:25 p.m. EDT.
23 Look for Mercury low in the west-northwest sky about a half hour after sundown, a little to the left of where the Sun has vanished. Binoculars may reveal the Pleiades 4 degrees directly above it.
28 The Moon reaches last quarter at 10:12 a.m. EDT.
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Copyright © Natural History Magazine, Inc., 2008
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