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A procession of planets: each of the bright planets makes an appearance over the course of an April night.

MERCURY IS unusually high at dusk in the first half of April, floating lower right of brilliant Venus. Fading Mars hangs very high in the south at nightfall all month. Saturn is highest later in the evening, and Jupiter doesn't appear until dawn.


Mercury puts on its best evening display of 2010 (for mid-northern latitudes) in the first half of April. From April 1st to 15th the flighty planet is at least 10[degrees] above the west-northwest horizon for observers looking a half-hour after sunset. Note, however, that it fades eight-fold in those two weeks, from magnitude -0.9 to +1.4, so it will be difficult to spot without binoculars by mid-April.

Mercury shines at magnitude +0.1 on April 8th, the day it reaches greatest elongation from the Sun. Telescopes then show its tiny disk 40% lit and less than 8" wide. The planet sets more than 1 1/2 hours after the Sun during the first half of April, lingering above the horizon just after the end of astronomical twilight (at 40[degrees] north latitude) for the only time this year.


Venus is the planet you'll see low in the west well before Mercury. It flames at magnitude -3.9, providing a handy guide for locating Mercury to its lower right. The minimum separation between Venus and Mercury occurs on April 4th, when the two are 3[degrees] apart. This is what Belgian calculator Jean Meeus has dubbed a "quasi-conjunction": when two bodies come within 5[degrees] of each other without ever sharing the same right ascension or ecliptic longitude. It's the first quasi-conjunction between bright planets since 2006.

By April 15th, when the lunar crescent hangs just above greatly dimmed Mercury, the separation between Venus and Mercury has grown to 7[degrees]. On April 24th and 25th, wait at least an hour after sunset to see the Pleiades poised beautifully about 3V2[degrees] right or upper right of Venus (binoculars help). Aldebaran and the Hyades are somewhat farther to the upper left or left of Venus on those nights.


Venus remains small and roundish in telescopes this month. But from early April until early September, Venus will be at least 10[degrees] above the horizon 45 minutes after sunset (though never very high), and during that period we will see its disk grow and enter its crescent phase.


Mars, in Cancer, is highest in the south during evening twilight and remains high enough for good telescopic viewing all evening. On the other hand, Mars continues to fade and shrink as it falls far behind speedier Earth in its orbit. Another month, another halving in brilliance: in April it falls from magnitude +0.2 to +0.7. The Martian globe shrinks from 9" to 7", too small to show surface features in most telescopes. But the eastward motion of Mars carries it just 1[degrees] north of the center of Messier 44, the Beehive Star Cluster, from April 16th through 18th, providing pretty sights for telescopes, binoculars, and unaided eyes.

Saturn was at opposition on the night of March 21-22, so it's visible just about all night in April. It's not very high in the east-southeast at dusk, so wait until later in the evening to study its 19"-wide globe and very thin rings. Late last month, when Saturn was closest to Earth and the rings were tilted more than 3[degrees] from edgewise, Saturn shone at magnitude +0.5. Saturn fades slightly in April as it moves farther from Earth and the rings tilt narrower (toward their minimum of 1.7[degrees] from edgewise in late May). But the planet still outshines both Regulus and Spica, the bright stars that bracket the large swath of the zodiac that Saturn has passed through for the last two years. Saturn now floats just above the celestial equator and the ecliptic, in the head of Virgo, not far from the richest region of the Virgo Galaxy Cluster.




Neptune and Pluto are high enough to observe as morning twilight starts but will be better positioned for viewing this summer.

Jupiter shines at magnitude -2 but is quite low in the east at dawn. It comes up about an hour before the Sun at the start of April, about 2 hours by month's end.

Uranus was at conjunction with the Sun on March 17th, so it's hard to observe in the dawn glow even by the end of April.


The Moon is gibbous at dawn on April 3rd, when it's only about 1[degrees] from Antares for North America.

At dusk on April 15th, a sliver of lunar crescent floats about 1[degrees] above or right of Mercury--though both will be difficult to see without binoculars. On the 16th the Moon is well above Venus but just below the Pleiades (use binoculars). And on April 17th, it's well to the upper right of Aldebaran.
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Title Annotation:Sun, Moon, and Planets
Author:Schaaf, Fred
Publication:Sky & Telescope
Geographic Code:1USA
Date:Apr 1, 2010
Previous Article:Planetary almanac.
Next Article:Lessons in basinology: the youngest lunar impact basin can teach us much about older impacts.

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