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Starwatch February 2012

January 20, 2012


Planet Venus.

Cloud-shrouded Venus goes through phases and is lovely in all of them.
Image: NASA

By Deane Morrison

A climbing sun takes some of the chill off February, and nights are still long and dark enough to see lots of wonders.

This is the best month of the year to see Sirius, the brightest of stars, as it reaches its highest point in the south during the evening hours. Compare its brightness to Venus, now a high and brightening evening star in the west. Between them is Jupiter, very high in the southwest, but in the midst of a dramatic drop toward Venus. Watch the distance between these two planets shrink from 40 degrees to 12 degrees this month.

One of the year's best full moons rises on the 7th between 5:30 and 6 p.m., depending on your location. Coming less than two hours after the moment of exact fullness, moonrise will be a beauty. Algonquin Indians called this the full snow moon, but since February's heavy snows often interfered with hunting, some tribes called it the full hunger moon.

Earth is catching up to Mars in the orbital race, and that means the Red Planet is getting bigger and brighter. On the 1st, it rises in the east about three hours after sunset, but by March it comes up just after sundown. It glows like a ruby in the space below the triangle of stars in the hindquarters of Leo, the lion.

If you follow Mars from night to night, you'll see that it has begun moving westward against the backdrop of stars, toward Regulus, Leo's brightest star. This is called retrograde motion, because the planet is moving backward from the outer planets' usual eastward motion against the stars. This happens because Earth is about to lap Mars, just as a runner in an inside lane might lap somebody in an outer lane. As the inside runner passes, the outer runner appears to move sharply from left to right against the landscape, which translates to east to west in the case of planets.

Speaking of the orbital race, Saturn is ahead of Mars, and Earth will pass it, too, after leaving Mars behind. The ringed planet is now rising in the east shortly before midnight, but will come up about two hours earlier by month's end. On the 8th it begins a retrograde course toward its neighbor Spica, the brightest star in Virgo.

Groundhog Day began as the Celtic feast Imbolc, celebrating the lambing season. It was one of four cross-quarter days falling halfway between a solstice and an equinox. Supposedly, a cloudy, shadow-free day portended rains, a softening earth and an early start to spring planting. But a clear day meant six more weeks of cold, snow and frozen ground.

A new moon arrives on the 21st. On clear nights near that date, look for a faint, broad finger of light pointing back along the sun's path an hour or two after sunset. This is the zodiacal light, a reflection of sunlight off a disc of dust that stretches far into space in the plane of the solar system. If you see it, you're lucky, because it's a rare sight—especially for city dwellers.

In the news, NASA has just placed twin spacecraft called GRAIL in lunar orbit. Our moon is a lumpy little planet, with many dense regions that exert stronger gravity than other places. GRAIL's mission is to map the moon's gravity to give a better picture of its makeup, which in turn should yield clues to how the moon and even the Earth evolved.

Here's how GRAIL will work:

The two spacecraft will orbit in tandem. As the lead vessel approaches an area of stronger gravity, it will be accelerated forward and downward, increasing its distance from the trailing vessel. As it moves beyond the area, the extra gravity will exert a drag, and the inter-vessel distance will shorten. (The leading vessel will also regain its altitude.) The distance will change again as the second spacecraft passes the high-gravity area. Using radio waves to monitor minute changes in the inter-vessel distance, NASA physicists can map the moon's gravity and, thus, density. 

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: www.d.umn.edu/planet

Twin Cities, Minnesota Institute for Astrophysics (during fall and spring semesters): www.astro.umn.edu/outreach/pubnight

12/20/12 Contact: Deane Morrison, University Relations, (612) 624-2346, morri029@umn.edu

Find U of M astronomers and links to the world of astronomy at http://www.astro.umn.edu.

Tags: College of Science and Engineering

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