A colorful corona of light encircled the moon this past Saturday night. The multiple rings are caused by light diffracted by minute water droplets or ice crystals within the passing clouds. Photo: Bob King

I’ll be chasing dogs this morning up Lake Superior’s north shore as mushers and their teams compete in the annual John Beargrease Sleddog Marathon. Before I head out I thought you’d like to know that the International Space Station is back in the dawn sky beginning tomorrow morning. You can watch for it over the next couple weeks as it cuts a bright path across the sky before sunrise. The times below are for the N. Minn – N. Wis. region and are Central standard. For times for your town, please click HERE and type in your zipcode.

* Tuesday morning February 2 starting at 6:55 a.m. Moderately-bright pass beginning low in the southern sky and moving toward the east.
* Thursday at 6:06 a.m. Similar to above.
* Friday at 6:28 a.m. Higher pass across the south-southeast and much brighter!
* Saturday at 6:51 a.m. As good as it gets! The ISS will cut straight across the top of sky moving southwest to northeast and be brighter than Jupiter.
* Sunday at 5:41 a.m. moving from south to northeast. Bright pass.

The space shuttle Endeavor is set to launch this coming Sunday and will later rendevous with the space station. This will make for some cool "chases" across the sky. We’ll keep you up to date on when to look.

Jupiter is the single bright star easily seen in the southwestern sky about 35 minutes to an hour after sundown. I caught it here among power lines and withered tansies. Photos: Bob King

Last night I noticed that at around 5:45 p.m. (about 35 minutes after sunset) Mars and Jupiter were at the same altitude but in opposite corners of the sky. Jupiter’s fast sinking in the west while Mars ascends in the east. I found it fun to compare the planets’ colors and brightnesses.

Turn around and look opposite of Jupiter in the northeastern sky to find Mars during twilight. This photo was made at almost the same time as the one of Jupiter above.

The biggest planet highlight in the coming week is still … you guessed it, the beacon of Mars.

I spent yesterday afternoon shoveling my garage roof from more than a foot of snow that had accumulated since the Christmas Eve storm. Usually I’m on top of snow removal but had a bad cold at the time and just couldn’t muster the energy. I would have waited for spring to melt it away but the garage door started to jam against the ceiling joists after the last snowfall. It dawned on me that the weight of the snow had pushed the the roof and its supporting structures down just enough into the door opening to cause the jam.

It was hard work removing the compacted layers of old snow but I completed half the roof. The joists rebounded just enough to allow the door to move freely. It occured to me while I was chopping away up there that examples of rebound abound in nature. One is right on our doorstep here in Duluth: Lake Superior.

Satellite photo of Lake Superior taken yesterday just before 11 a.m. Like a tilting bathtub, water is slowly moving from the east end of the lake (right) to the west due to the crust rebounding since the glaciers departed some 10,000 years ago. Credit: NOAA/Space Science and Engineering Center, U. of Wis-Madison

In a recently published book by Dick Ojakangas titled Roadside Geology of Minnesota, I learned that the tremendous weight of the glaciers that once overlayed the lake depressed the crust. Since they began to recede some 10,000 years ago, the crust has been slowly rising back up to its pre-glacial level. But because the ice was thicker and lasted longer in the northeastern part of the lake basin, that part is experiencing a stronger rebound that the southwestern end of the lake. This makes the lake tilt toward the southwest causing water to rise at the Duluth end at the rate of a foot a century. In 500 years the grounds of the Duluth Entertainment Convention Center and new hockey arena will be sitting in five feet of water.

The crater Moltke is 4.3 miles across and a great example of a simple, bowl-shaped crater. The bumpy apron of material around the crater is impact ejecta — deposits of rock that were excavated during the impact. Credit: Apollo 10 / NASA

Anyone who’s ever looked at the moon through a telescope is familiar with craters. Almost all of them were formed by meteorites, comets and asteroids slamming into the moon’s surface at a speed of around 12 miles per second. At that speed, an impactor will blast out a crater 10 to 20 times larger than itself. Craters under 10 miles across look like simple bowls while larger craters will show additional features like terraced walls and central uplifts including mountain peaks. These peaks are some of the most dramatic features you can see through a small telescope. When the sun is just rising over the crater and its inner mountain peak catches the first rays of sunshine, you’re guaranteed to let out a wow.

One of the ways a crater’s central peak forms is through rebound of the crust. An asteroid impact depresses part of the lunar crust like the snow did to my garage roof except in a far more cataclysmic fashion. After rock is vaporized and blasted out to form a new crater, the crust rebounds into an elevated floor with one or more mountain peaks. A good example of a central peak crater is 52-mile wide Tycho (at right).

Craters larger than about 110 miles in diameter have ring-shaped uplifted zones in addition to peaks. The very largest craters are called impact basins and consist of muliple rings of ruptured crust. Lava from beneath the crust often wells up through the fractured rock and floods the impact’s center creating a bullseye-shaped pattern. The familiar dark spots that form the face of the man in the moon are great examples of impact basins.

This image, taken by NASA’s Clementine spacecraft, consists of color-coded topography overlaid on a shaded relief map of the Moon. Purples and blues are low, and orange and reds are high. Younger craters are superimposed on the basin. Credit: Clementine Science Group, Lunar and Planetary Institute

One of the largest impact basins in the solar system can be found just 238,000 miles away on the moon. Called the South Pole-Aitken basin, it measures a whopping 1500 miles across and 13 miles deep in some places. It’s located among countless craters on the lunar farside. Below are two more examples of large impact basins, one on Mars and the other on Jupiter’s moon Callisto.

The Hellas basin in Mars southern hemisphere is 1,300 miles across and up to six miles deep.The dark ring is some of the material thrown out from the impact. Credit: NASA

Valhalla on Callisto has more rings than any other impact basin known. They extend some 2500 miles across the moon which is just 3000 miles across. Credit: NASA

The next time you find yourself on the rebound, consider that you have plenty of company out there.

The rising moon’s reflection is pinched by a line of ice-covered
boulders near the Duluth Ship Canal last night. Photo: Bob King

Awash in moonlight. Flooded by moonlight. Blown away by the moon. I couldn’t believe how brilliant the landscape was last night. Aided by reflection from a fresh cover of snow, even the air glowed. What a tremendous night to be alive. I romped with the dog on the icy road and took "daylight" landscapes by moonlight.

Laurie Viets took this nicely composed photo of the moon over the Brule River State Forest in northern Wisconsin during evening twilight. What a great place to be on a moonlit night in winter.

It’s often difficult to see color when you’re outside at night unless the moon is near full. I had no problem seeing the blue-colored tarp on the woodpile, my orangey deerskin gloves and the blues and reds on a box of kitchen matches. Early this morning I arose to look at a recurrent nova in Scorpius called U Scorpii. Like a kid throwing a tantrum, it undergoes periodic eruptions, the last one in 1999 and the current one just this week. At 6 a.m. the moon and Mars were well down in the western sky so moonlight shone from the side instead of above. A bright, narrow swath of light shot through the now-shadowed front yard as if someone had parked in the backyard and left their headlights on. That’s exactly how it looked when I walked out the door.

Anglers were out early this morning fishing from their ice shacks in the harbor along Duluth’s Park Point. While many of us were asleep, they were treated to a gorgeous moonset. Lyle Anderson of Duluth took this picture about 7:20 a.m. Details: telephoto lens, 1/100-second exposure at ISO 1600 at f/8.

I wasn’t the only one out. Lots of people noticed the moon and remarked on how bright it was. Some of you took photos, which I’m happy to share today. Thank you very much for sending them! Tonight the moon will still be brilliant but not quite like last night. When you’re out looking at it you’ll notice the star Regulus in Leo just to its left.

The moon through a small telescope last night was completely round and without shadow. During full phase the sun shines squarely at the moon rather than off to one side or the other so shadows virtually disappear giving the moon a "pasty" look. Details: f/14 at 1/125-second, ISO 200. Photo: Bob King

In other lunar news, Florida newspapers and msnbc are reporting that the Obama admininstration will extend International Space Station (ISS) operations but cut NASA’s fledgling program to return astronauts to the moon. NASA instead will be directed to work with private companies to develop a commercial "taxi" service to bring astronauts to and from the ISS. The administration also wants the agency to concentrate on developing a heavy-lift rocket that would take astronauts beyond Earth perhaps by the early 2020s. Exactly where isn’t outlined but possibilities include an asteroid, a Martian moon or lunar orbit. We’ll know the details this coming Monday. For more of the story, please click HERE.

Losing the moon program is disappointing news indeed, but we can still look up tonight and find joy and inspiration in this brightest light of the night.

We can’t forget Mars! It was in conjunction with the moon and accompanied it all night (upper left).
Photos: Bob King

The night sky looks like day when you make a time exposure by bright moonlight. The star Sirius is at upper left. Details: 20mm lens at f/2.8, ISO 400 and 7-second exposure. 

Mars and the full moon will make for one eye-catching duo tonight. The two will rise around sunset in the northeastern sky. Created with Stellarium

This is amazing. Not only is Mars making its closest approach to Earth in more than two years this week, but tonight’s full moon will be the closest full moon of the year. At the same time, the two luminaries will be in conjunction and closely paired up throughout the night. It’s like rolling the Yahtzee dice and getting four sixes. This rather rare combination of events will unfold beneath clear skies — at least in the Duluth region in the upper Midwest.

The moon’s distance varies during the approximately one month it takes to go from one full moon to the next. Closest approach is called perigee; farthest is apogee. Illustration: Bob King

Because of the moon’s elliptical orbit, its distance from Earth varies between about 221,457 and 252,712 miles with an average distance of 238,856 miles. Tonight, or I should say tomorrow morning at 12:17 a.m. Central time, the moon will be at its fullest and the closest to us for the month. The shiny orb will be 30 percent brighter and about 14 percent bigger than a distant full moon.

Not only is the moon larger by about 1/10 its diameter at perigee vs. apogee, but it’s about 30 percent brighter, too. Credit and copyright: Anthony Ayiomamitis

Since both the moon and Mars come up around sundown, why not see if you can shoot a picture of them? Even point-and-shoot digital cameras do pretty well in twilight. The key is to shoot early before the sky gets dark. For those living in the Duluth-Superior area, the moon will rise at 4:50 p.m. this evening with Mars off to the left and above. Sunset is 5:10 p.m. Anytime from 5 until about 5:45 should be good for picture taking. There will be a short interval in that span when Mars is easily visible but the moon won’t be overly bright in your picture.

To find sunset and moonrise times for your town, click on the favorites link in the column at right.

You can go in close with a telephoto or shoot a scene. It’s your creative choice. If you get an image you like, please send me a copy at rking@duluthnews.com. I’d love to share your photos with our readers.

A lunar halo wheels next to neighboring Mars (left) in the sky last night around 10:30. Details: 15mm lens at f/2.8, ISO 250 and 10-second exposure. Photo: Bob King

Tiny ice crystals fell from the sky last light like salt from a shaker. I had to constantly keep brushing off the grains from my camera lens while taking pictures of the pretty halo around the moon. It was one of those clear but not clear nights with a thin veil of cirrostratus clouds that softened the light of the moon and stars.

A model of the solar system showing the outer planets (Uranus at left), a comet’s orbit, the asteroid belt and the inner planets huddled near the sun. Credit: ESO

Since we’ve been focusing on Mars and Earth this week let’s pause to consider the scale of the solar sytem. You typically see diagrams and illustrations of the solar system like the one above that depicts miniature planets in ractrack-like orbits around the sun. Models like these help us understand the basic layout of our planetary neigborhood but lack a key ingredient: scale. Because everything is compressed into a single picture several inches across, something must be sacrificed. To visualize the stark reality of how tiny the planets are and how vast the solar system truly is, we have to select a scale that frames it all into bigger picture. One of the my favorite ways of doing this is to start with the sun and scale its 864,000-mile diameter girth down to the size of a regulation softball which measures just under four inches in diameter (3.8"). At this scale planet Earth is about the size of a grain of sand (0.8mm) and 34 feet from the sun. The largest planet Jupiter is not quite a half-inch across and 177 feet from the sun. Dwarf planet Pluto is only a tenth of a millimeter and a quarter mile away.

Try to picture how these planets would be layed out in your neighborhood. If our softball sun blazed in your living room, Mercury would be in the next room, Earth in the middle of your front yard and speck-sized Pluto about four blocks away. Except for Saturn and Jupiter, which are the size of peanuts, all the other planets would be so tiny you’d hardly see them unless you knew exactly where to look.

An artist’s impression of a portion of the Milky Way galaxy with the sun’s orbit shown. The galactic center is at upper left. I’ve added the sun to the illustration for clarity. Credit: European Southern Observatory (ESO)

If we continue outward on our imaginary distance scale, the nearest star Alpha Centauri would be 1740 miles away. Wow, that’s all the way to Phoenix from where I sit at home in Duluth, Minn.! How about Sirius the Dog Star that twinkles so brightly these winter nights? At 3522 miles away that’s almost the flying distance to Belfast, Northern Ireland.

It didn’t take long to leave the neighborhood but I’m still itching for a major road trip. How about we swing by the center of the Milky Way? Plan on taking on taking a few months off from work because to make the 11.3 million mile journey by spacecraft you’ll need at least that long to get there, take a few snapshots and then return just in time to discover you no longer have a job.

Imagining the scale of space is fun exercise and helps to get a better feel for the true size of the universe. If you’d like to try you own hand at making a solar sytem model, you’ll have a great time at the Exploratorium’s Build a Solar System website. Just fill in the diameter you want the sun to be, click on the CALCULATE button and you’re off! The site suggests that the entire solar system can be built on a roll of toilet paper if you scale the sun to .4 inches. Now there’s a scale I never imagined.

This view shows the sky as you look toward the east around 8:30-9 p.m. this evening. Mars is in the dim constellation Cancer the Crab and about one outstretched fist to the right of the bright Sickle of Leo the Lion. Created with Stellarium

Look to the east to see the Red Planet blazing brightly. Tonight Mars will be the closest it’s been to Earth since 2008. Every 2 years and 2 months Mars reaches opposition, when both it and the Earth are lined up on the same side of the sun. From our vantage point Mars is directly opposite the sun in the sky — when the sun goes down in the west, Mars pops up in the east. Since both planets are right alongside one another, we’re also at our closest separation for this particular opposition.

Here are all the Mars-Earth lineups (oppositions) from the most recent closest approach in 2003 to the next closest approach in 2018. The present opposition isn’t the best, but all Mars oppositions give us the chance to enjoy a bright planet and see its surface features better than at any other time. Illustration: Bob King

All planets orbit around the sun in elliptical or oval-shaped orbits. Some like Earth’s are very close to being circles. Mars’ orbit is more eccentric or squashed compared to Earth’s which means Mars’ distance from the sun varies much more than our planet’s does. If an opposition happens at the same time Mars is farthest from the sun, the two planets are only relatively close. Mars will be bright and show a nice array of features on its tiny globe, but it’s nowhere near as brilliant or large compared to an opposition when the planet is closest to the sun. That last happened in August 2003 when Mars outshone Jupiter and was only 34.7 million miles from Earth. This week’s closest approach is nearly twice that distance or 61.5 million miles.

These excellent photos of Mars were taken earlier this month and show a wealth of detail. Most obvious is the north polar cap (the south direction is at top in these pictures). Syrtis Major, the tornado-shaped patch in the left image, and Sinus Sabeus and Sinus Meridiani (the "drumstick" shape near the top of the right image) are both visible in 4-inch and larger telescopes this month and next. Credit: Damian Peach

Actual opposition happens this Friday, but Mars will be nearly as bright as Sirius and close enough over the next month that those with modest-sized telescopes can spot the north polar cap and larger surface markings with clarity. Go out every night you can and use the highest power you can that still gives a sharp image. The more you look, the better you’ll become at seeing. All the planets repay continued observation with delicious if fleeting details. One of the biggest telescopic changes you’ll see on Mars is the shrinking of the polar cap as spring progresses in the planet’s northern hemisphere. A series of sketches would be a wonderful way to record this.

The other hemisphere of Mars is not as feature-packed as the one above but will still show interesting details to the keen eye especially the cark "collar" around the polar cap. In the photo at right, the white patch on the left side of the planet is a water ice cloud that formed during the Martian afternoon. Mars has shown lots of cloud activity the past couple weeks. Credit: Damian Peach

Naked eye and binocular sky watchers can use Sirius to watch Mars’ slowly fade over the next few months. And while Sirius twinkles a lot, Mars remains a steady, unblinking eye because its disk is large enough to resist the pushing and shoving of atmospheric turbulence. On Friday we have a special treat: the nearly full moon will park itself just below the planet for a double-whammy of radiance.

The next opposition in 2012 will be slightly farther than this one; Mars finally starts closing in again during the 2014 approach. That makes tonight the closest the planet will be to Earth for next four years. We welcome the warm orange face of Mars on these coldest nights of the year!

A cast to beat the Oscar crowd shines across the east and south during late January around 9-10 o’clock. You can use the moon as your guide tonight to find them all. See the same map below to identify the celebs. Maps created with Stellarium

I look forward to a clear sky tonight because 9 of the 23 first magnitude stars will be joined by Mars and the moon for a gathering of some of the sky’s most scintillating celebrities. The Greek astronomer Hipparchus invented the original magnitude scale to classify stars by their apparent brightness. The brightest were first magnitude while the dimmest were sixth magnitude. A first magnitude star is about 2.5 times brighter than a second, a second is 2.5 times brighter than a third and so forth.

The scheme was later expanded upon to include magnitudes of much fainter and brighter stars as well as subdivisions into fractions of a magnitude. Now the brightest stars have negative magnitudes. For example, Spica in Virgo is magnitude 1, Vega is 0 and Sirius -1.5 in increasing order of brightness. The faintest stars visible in the Hubble Space telescope are magnitude 30 while the sun’s brightness is -26!

Here they all are again, this time with names and brightnesses shown to the tenth of a magnitude. With its large negative number, the moon clearly outshines them all!

If we make magnitude 1.5 the cutoff for our definition of first magnitude and brighter stars, there are just 22 of them visible across the entire sky. That’s not many, yet nine of these are all gathered in the sky tonight and throughout the remainder of winter. Throw in the gibbous moon and planet Mars and one additional star, Castor, that’s right on the cusp of first magnitude-hood at 1.58, and you’ve got a lot of sparkle up there. 

One star I really don’t pay much attention to is Adhara in the triangle of stars below Sirius that forms part of the constellation Canis Major the Dog. That’s probably because it’s low in the sky from the northern U.S. and affected by the greater thickness of air we have to peer through to see it. Adhara registers exactly magnitude 1.5, only a tad fainter than Regulus in Leo the Lion. Tonight I will make sure to give it a proper welcome into the brotherhood of first magnitude stars.

I hope it’s clear at your home tonight so you can enjoy the scene.

Density makes a difference. By mid-winter, the temperature is usually cold enough that storms bring the light and fluffy variety of snow that’s relatively easy to shovel and snowblow. This weekend’s weather brought the glop. The kind where each shovelful drops with a splat. Easy-shoveling snow has more air in it and a much lower density than the slushy kind, which is one step away from liquid water.

The newly formed sun lights up its dusty birth cloud in this artist’s depiction of what the early solar system may have looked like. The planets formed in the disk around it through accretion of dust and ice particles. Credit: NASA/JPL-Caltech

Denser stuff also sinks beneath lighter stuff like a rock tossed in water. Take the planets for instance. You’ll find all the lighter, less dense matter on top in the form of an atmosphere. Down on the surface a crust of lighter, less dense rock sits atop denser rock which encloses a core made of very dense materials like iron and nickel.

Piecing together clues gleaned from meteorites and rocks from the moon and Mars, we’ve learned the solar system formed just over 4.5 billon years ago from a cloud of gas and dust composed of ice particles, rock grains and gases spewed by earlier generations of stars. Most of the material was near the center of this spinning cloud and eventually condensed through gravity to form the sun. All the material beyond the sun accreted to form the planets.

The "seeds" of the planets or planetesimals formed when bits of dust and ice within the original birth cloud called the solar nebula stuck together to create larger rocks. These embryos would go on to form the planets as we know them today. Credit: NASA/JPL-Caltech

As with so much in nature, most big things start small. The little dust grains touched and stuck together to make small clods and pebbles which grew over time to sizeable planetary embryos up to several miles across. Over the next 50-100 million years these "planetesimals" grew larger and larger through collisions, mergers and gravitational attraction to form the planets.

Because planet formation involved so many collisions and bombardments, the planets were probably all molten for a time from the tremendous heat generated by millions of impacts. Heavier materials would have to sunk down into their cores, leaving the fluffier stuff on top. Today on Earth the lighter crustal plates, composed of granites, "float" on top of denser mantle rock. Heat still left over from the ancient days of impact, gravitational contraction of the planet and the heat given off by the decay of radioactive elements not only keep the core hot but drive currents in the mantle that cause the plates to shift. Plate movements create new mountain chains, deep oceanic trenches, faults and earthquakes like the recent one in Haiti.

This week’s planetary highlight is Mars. Over the past year, Mars and Earth have been gradually approaching each other as we orbit around the sun. On the 27th, Mars will be at its closest since 2008. We’ll have much more on Mars on Wednesday so until then … clear skies!

(Cutaway of Earth above from the Lawrence Livermore National Laboratory)

Framed by brilliant Mars and Sirius, Procyon and the constellaton Canis Minor are easy to see across the east-southeast around 8 o’clock. The entire groups is visible throughout the winter. Created with Stellarium

Mars and the brightest star Sirius are now well placed in the eastern sky around 8 o’clock. This coming week Mars will be the closest it’s been to Earth in over two years and almost match Sirius in brightness. Can you tell the difference between them? On the magnitude scale used by astronomers, Mars is -1.2 magnitude and Sirius -1.4, just a tad brighter.

Framed by these two stellar luminaries, Procyon (PRO-see-on) in Canis Minor the Little Dog is easily passed over. Its name means "before the dog" which refers to its rising before the brilliant dog star Sirius — 42 minutes earlier if you’re watching from Duluth, Minn. Sirius is the alpha male in the much larger constellation Canis Major while Procyon belongs to Canis Minor, a stick-shaped pattern of just two stars. Appropriate I suppose for a small dog.

Procyon and Canis Minor get a leg up on the big dog Sirius, who resides at Orion’s feet. The faint constellation of Monoceros the Unicorn lies between the two canines. Created with Stellarium

Both Sirius and Procyon have small, compact companion stars called white dwarfs in orbit about them. We looked at these exotic, superdense stars when we got acquainted with the star Omicron in Eridanus the River two weeks ago.  A white dwarf is the end of the road for stars like the sun, Sirius and Procyon. Five billion years from now, the sun will have blown away its outer atmosphere to reveal what it’s been cooking up all those years: a tiny Earth-sized star so dense a thimbleful of it weighs more than a ton.

Compare the sizes of the sun, Procyon and Sirius. Procyon is four times the diameter of the sun, Sirius is 3.5. The little dog star is not so little after all! (sizes shown are approximate) Illustration: Bob King

Procyon is a white star about 3 1/2 times larger than the sun and radiates seven times more brightly. At 11.5 light years away, it’s the 14th closest star system to Earth. Back in 1844 German astronomer Friedrich Bessel measured a slight wobble or shift in Procyon’s and Sirius’ positions he believed were caused by the gravitational tug of unseen companions. 18 years later, Sirius’ companion — Sirius B — was discovered by the keen-eyed telescope maker Alvan Clark. Procyon’s companion, Procyon B, is both closer to its parent star and fainter than Sirius B and had to wait until 1896 for discovery. Bessel’s method of measuring stellar wobbles is still used to this day to detect companion stars and extrasolar planets in orbit around their parent stars.

Procyon’s companion star Procyon B is half again as small as shown in this diagram. Despite the enormous difference in size, the dwarf contains more than one third of the mass of its parent star Procyon. Illustration: Bob King

While Procyon is some three million miles in diameter, its companion star is only about 10,700 miles across. I’ve tried to show the white dwarf’s size in relation to Procyon but at the scale that fits on this page, I couldn’t draw it any smaller. Picture a pinpoint next to Procyon and you’ll have an even better idea of its size.

Procyon and its little buddy circle around each other once every 40.8 years at a average distance similar to that separating the sun and Uranus (1.4 billion miles). The little dwarf is much, much fainter than its parent and would shine only a couple times more brightly than the full moon as seen from an imaginary planet closely orbiting Procyon. From Earth, 11th magnitude Procyon B is lost in Procyon’s brilliant glare and inaccessible to most amateur astronomers’ telescopes but we can picture there in our imagination the next clear night. In the far future, Procyon will shed it stellar cloak just as its companion once did and become a white dwarf, too.

Two peas in the pod if there ever could be.

The Big Dipper and Cassiopeia the Queen strike a balance in the northern sky around 10-10:30 p.m. over the coming week. Created with Stellarium

Wow, is it slippery out there today! I did manage to survive the walk to the mailbox to pick up the paper this morning. It’s freezing rain in Duluth. Better to grab your skates than get around in boots.

Yesterday we touched on Vega and the spring stars gaining ground in the morning sky at the expense of the winter constellations. Other stellar seasonal changes are afoot in the evening sky. One of them involves the Big Dipper and Cassiopeia.You may have noticed that the Dipper is already standing on its handle in the northeastern sky by 8 o’clock. If you draw a line through the two "pointer" stars at the end of the bucket, it will take you to Polaris, the North Star.

Cassiopeia made its grand entry in the northeastern sky late last summer, remained high up and easy to see throughout the fall and early winter and now in late January begins its descent into the west. Right around 10:30 tonight Cassiopeia will lie directly across from the Big Dipper on the other side of the North Star. Just follow the pointer stars to Polaris and keep going until you come to Cassiopeia’s zigzag. The two constellations remind me of a scale. For a time they’ll be exquisitely balanced before the Dipper gains the upper hand and ascends to greater heights while Cassiopeia drops back into obscurity. Every constellation has its day in the sun, or should I say night?

I subscribe to NASA’s Earth Observatory, a weekly service that e-mails links to interesting pictures of our planet taken by a variety of Earth-orbiting satellites. Here are a couple I thought you might enjoy.

A 5-by-6 mile long iceberg that calved from the Mertz Glacier along the coast of Antarctica photographed on January 1 this year by NASA’s Earth Observing-1 satellite. The iceberg has a rippled surface just like the glacier. Hi-resolution image. Credit: NASA/Jesse Allen

The Yukon River delta in Alaska photographed on January 11 by NASA’s Aqua satellite. The river and its tributaries recall the forms of an old, wizened tree or network of capillaries. Hi-resolution image showing the delta and surrounding area. Credit: NASA/Jeff Schmaltz

On January 15 the moon passed in front of the sun causing an annular solar eclipse. This photo, taken by the Aqua satellite over India and the Bay of Bengal, shows the dark shadow cast by the moon as it eclipsed the sun. A small amount of sunlight spilled from around the edge of the moon, enough to brighten the clouds. Hi-resolution image. Credit: NASA/Jeff Schmaltz