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University of Minnesota
University of Minnesota

Starwatch March 2012

February 21, 2012

Colliding galaxies

This image, cataloged as Arp 147, shows two galaxies interacting. The blue ring in the galaxy on the right is an area of intense new star formation. The pair lies in the constellation Cetus, more than 400 million light-years away.

By Deane Morrison

In March we get to watch as four planets group into two pairs. Three of the planets are in the sky and one is right under our feet.

Mars reaches opposition on the 3rd, the day Earth passes it and it appears directly opposite the sun. The Red Planet will be up all night, burning its ruddy fire below the belly of Leo, the lion. The bright star west of Mars is Regulus, the brightest in Leo.

On the 5th, Mars will sweep closest to Earth and shine at maximum brightness for this visit. If it seems strange that our two worlds make their closest approach after Earth has lapped Mars, remember that both orbits are noncircular, with points of perihelion and aphelion when the planets are nearest and farthest, respectively, from the sun. Earth is heading out toward aphelion while Mars, having passed its aphelion in February, is now closing in. These motions continue to bring our two planets closer until two days after Earth has left the Red Planet behind.

Unfortunately, being so near aphelion means Mars won't come nearly as close as it did in 2003, when its opposition and perihelion fell only two days apart. This time it will be about 63 million miles away, compared to only 35 million miles in 2003.

Also in Mars news, recent data from the European Space Agency's Mars Express probe suggests that a large ocean once covered the planet's northern plains. The probe's radar picked up evidence of marine-like sediments in locations already suspected to have been ancient shorelines. Researchers speculate that the ocean enjoyed a rather brief life some 3 billion years ago.

The second planetary pairing happens on the 13th, when Jupiter drops past Venus on its way down into the sun's afterglow. The king and queen of planets come within three degrees of each other, then separate as Jupiter continues its freefall. 

On the 25th, a thin crescent moon comes out close to Jupiter. The next night, a slightly fatter lunar crescent appears next to Venus, just as the brilliant planet reaches its highest point before beginning a dramatic fall. With the Pleiades star cluster hovering just a few degrees above Venus, this is a night to grab your binoculars.

Saturn is well up in the east by midnight all month. Just southwest of the ringed planet is its companion, the bright star Spica in Virgo. Above and left of the pair is Arcturus, the brightest star in Bootes, while Mars shines high to their upper right.  

Algonquin tribes called the full moon of March the worm moon, for the casts of earthworms that now begin to appear on the softening earth. In more northerly areas, it was known as the crow moon, for the cawing of these feisty birds, or the crust moon, because during this season snow acquires a crust from cycles of melting and refreezing. This year it rises the night of the 7th and reaches perfect fullness at 3:39 a.m. on the 8th. 

High in the south during the evening is Procyon, the brightest star in Canis Minor, the smaller of Orion's hunting dogs. Procyon is actually a double star consisting of a star twice as wide as the sun and nearly seven times as bright and a white dwarf star that packs about 60 percent of the sun's mass into an orb smaller than Earth. Procyon isn't especially bright; it just looks that way because it's only about 11.5 light-years from Earth.

Procyon forms one point of the Winter Triangle of stars. The other points are Sirius—below Procyon and only 9 light-years away—and the gigantic red star Betelgeuse, which forms Orion's northeast shoulder and burns brightly despite being some 600 light-years away.

Spring arrives with the vernal equinox at 12:14 a.m. on the 20th. At that moment the sun crosses over the equator into the northern sky and the Earth—the other side of it, of course—will be lighted from pole to pole.

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, Minnesota Institute for Astrophysics (during fall and spring semesters):

2/21/12 Contact: Deane Morrison, University Relations, (612) 624-2346,

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Star map: White on black

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