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University of Minnesota

Starwatch May 2011

April 17, 2011

mercury1-small 300

The MESSENGER spacecraft has sent back intriguing images of Mercury's surface, including this one.

Photo: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

By Deane Morrison

During May the sun climbs almost as high as it will get, bringing balmy but shorter evenings to watch the heavens. This year stars get most of the attention, since all but one of the bright planets are now clustered in the predawn sky.

Luckily, the lone bright evening planet is Saturn. Coming out in the east after sundown, the majestic orb traverses the night sky above the bright star Spica, in Virgo. Its rings are tilted only about eight degrees from horizontal, but through a telescope the planet's golden color never fails to delight.

Two other bright stars accompany Saturn in its journey. To the northwest, Regulus, the heart of Leo, anchors the Sickle of stars around the lion's head. And to the northeast, brilliant Arcturus leads its kite-shaped constellation, Bootes the ploughman (or herdsman), across the sky.

Just east of Bootes hangs Corona Borealis, the northern crown. Its brightest star, Alphecca, also called Gemma, sets off this celestial tiara. Moving east once more, we come to the upside-down form of Hercules with its signature hourglass of stars.

Looking to the north, the Big Dipper "spills its water" down toward the Little Dipper. On the other side of the Big Dipper (between its bowl and Leo) see if you can find three evenly spaced pairs of stars known as the Three Leaps of the Gazelle. If each pair of stars is a landing spot, it appears the lion-leery gazelle leapt along a line heading northwest from a spot just above Leo's hindquarters.

In the morning sky, Venus is slowly sinking, Mars and Jupiter are ascending and Mercury pops in for a short visit. All four planets will be up by about half an hour before sunrise, but besides taking place at an unearthly hour, the spectacle unfolds so close to the horizon that it will be hard to see any of the players. By the end of the month, however, Jupiter will have climbed high enough to see and Mars, ascending in Jupiter's wake, may be marginally visible.

Mercury's foray into the predawn sky may not dazzle us, but new images of the tiny planet from NASA's MESSENGER spacecraft are making a splash. They show the planet's heavily cratered surface and mysterious deposits near its north pole. Among other questions, astrophysicists hope MESSENGER will shed light on how Mercury, which has a large, metal-rich core, came to be the densest planet and why Mercury and Earth possess magnetic fields while neighbors Venus and Mars do not.

The Milky Way hugs the horizon during evenings in May. But if you stay up and watch until 1 or 2 a.m., you'll see it lift up in the east and move westward behind the spring constellations.

Algonquin Indians knew May's full moon as the flower moon, the corn planting moon or the milk moon. It shines the night of the 16th-17th and will be closest to perfect fullness when it sets at 5:24 a.m.

The University of Minnesota offers public viewings of the night sky at its Duluth and Twin Cities campuses. For more information and viewing schedules, see:

Duluth, Marshall W. Alworth Planetarium:

Twin Cities, Department of Astronomy (during fall and spring semesters):

4/20/11 Contact: Deane Morrison, University Relations, (612) 624-2346,

Find U of M astronomers and links to the world of astronomy at


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