Full Worm Moon wriggles up, shows its head tonight

The nearly full moon rises within the framework of Duluth, Minnesota's iconic Aerial Lift Bridge yesterday evening March 4. Credit: Bob King

The nearly full moon rises within the framework of Duluth, Minnesota’s iconic Aerial Lift Bridge yesterday evening March 4. March’s Full Worm Moon is named for the time when the ground softens and earthworms show their heads again. Credit: Bob King

With 14 below on the thermometer this morning I don’t think Full Worm Moon quite fits as a description for tonight’s full moon. How about the Sap Moon, named for the time when sap begins to run again in maple trees? No, a little too early for that. These traditional Native American moon names obviously applied to other more clement regions of the country where March really does mean the start of spring.

More like the Full Stubborn Moon in my town. Yet the signs of spring are unmistakable. Longer days, shorter nights, a steeper slant to the sunlight. Spring stars are pushing up in the east as well – scintillatious Arcturus is up by 9:30 in the northeast, followed by Virgo’s Spica an hour or so later.

Last month's just-past-full moon rises below Jupiter over the ice-covered shore of Lake Superior in Duluth. Watch for the Full Worm Moon tonight. Click image to find the moonrise time for your location. Credit: Bob King

Last month’s just-past-full moon rises below Jupiter over the ice-covered shore of Lake Superior in Duluth. Watch for the Full Worm Moon tonight. Click image to find the moonrise time for your location. Credit: Bob King

We enjoy watching these seasonal changes. They help us get out of bed and renew that enjoyable sense of anticipation at what’s around the next corner. Constant motion. We’ve been told everything’s in continual movement. If you could shrink down to size of a bacterium you’d never be able to sit still. At that level, you’d feel the pinging of millions of molecules called Brownian motion. Getting up and walking to another room you’d be pelted by a hail of molecular snowballs.

Lucky then that humans are big enough to escape such atomic beatings. But the incredible variety of motions still can throw us off. The stars rise and set, reflecting the rotation of our planet, constellations shift westward with the seasons as Earth orbits the Sun, the moon and planets march east, contrary to the motion of the stars.

The Earth's rotation dominates the moon's apparent motion across the sky, causing it rise in the east and set in the west. Meanwhile, the moon itself is moving much more slowly toward the east or back down toward the horizon. Credit: Bob King

The Earth’s rotation dominates the moon’s apparent motion across the sky, causing it rise in the east and set in the west. The moon is simultaneously moving much more slowly toward the east or back down toward the horizon. Credit: Bob King

When you watch the moon rise tonight, be aware that as the Earth’s rotation brings it into view, the moon’s orbital movement wants to drag it back down to the horizon. Reminds me of trying to get a teenager out of bed. The Earth wins of course because the moon is far enough away that its apparent motion to the east amounts to only 1/2° or one moon diameter per hour. Meanwhile, Earth’s west-to-east rotation uproots our satellite and whips it to the west 30 times faster. Sorry moon, you lose.

It’s all illusion of course. The moon’s doing most of the moving. Its nightly rising in the east and setting in the west are nothing more than a reflection of our planet’s spin. Fake motion. By the way, those little 1/2 degrees add up – every 24 hours the moon moves more than a fist to the east.

Phobos, the larger of Mars' two moons, orbits just 3,700 miles above the planet's surface, so close it takes just 7 hours 39 minutes to complete its orbit. Phobos is only about 12 miles across. Credit: NASA/JPL-Caltech

Phobos, the larger of Mars’ two moons, orbits just 3,700 miles above the planet’s surface, so close it takes just 7 hours 39 minutes to complete its orbit. Phobos is only about 12 miles across. Credit: NASA/JPL-Caltech

While the moon may appear to lose to Earth, this isn’t true with Mars’ moon Phobos, which lies so close to the planet that it takes just 7 1/2 hours to orbit it. Moving eastward at 45° per hour (90 times faster than the moon), Phobos rises in the west and sets in the east after only 4 hours. Four hours later’ it’s back again at the western horizon ready for another trip across the night sky. Mars’ rotation rate of 24 hours 37 minutes just can’t keep up with this insanely fast moon.

While I’d be the first to purchase a cheap ticket to Mars just to see crazy Phobos, I rather like the unhurried and deliberate moon that rises over Earth. Time to watch moonlight transform the landscape and help us slow down a little after a busy day.

What? Two Full Moons this weekend?

A bright waxing gibbous moon dons a colorful corona as passing altocumulus clouds diffract its light earlier this week. Click photo to find the time of moonrise where you live. Credit: Bob King

Lovers of moonlight, this is your weekend. The moon’s essentially full two nights in a row. Full phase doesn’t occur until 6:27 a.m. (CST) tomorrow morning or midway between the two nights. That means tonight’s moon appears in the east about 12 hours before full, while an equally filled-out moon will shine just 12 hours after full Saturday night.

Only a sharp-eyed observer would notice a half-day’s difference in the moon’s phase, so for most of us, the moon will look full both nights. What’s more, it happens to be in one of the coolest places in the sky, smack in the middle of the Hyades star cluster not far from Taurus’ brightest star Aldebaran.

Tonight’s moon will appear in the middle of the V-shaped Hyades star cluster. At a distance of 153 light years, it’s the closest star cluster to the solar system. Stellarium

The moon’s brilliance will make the star cluster difficult to see with the naked eye, but binoculars will offer up a fabulous view of our satellite ensconced in Hyades like an ornament amid twinkling holiday lights.

You can also watch the moon glide eastward and approach Aldebaran during the hours before midnight. At 11 p.m. (CST), they’ll be just 1.4° apart.

Aldebaran is more than twice as close to Earth as the Hyades; it hovers in the foreground and is unrelated to the cluster. Looking up tonight, our gaze will plumb all three dimensions of the sky in a single glance – from the relatively closeness of the moon to the middle distance of Aldebaran and further to the more distant Hyades.

What’s the Full Strawberry Moon doing in Ophiuchus?

A pinkish-orange full moon rises alongside the lighthouse at the end of the Duluth Ship Canal in Duluth, Minn. Click to find moonrise times for your town. Credit: Bob King

Ophiuchus. You may wonder what the moon’s doing in a constellation astrologers don’t consider a part of the zodiac. Aren’t there 12 zodiac constellations – one for each month of the year? Well, astrologically yes, but not astronomically. And it’s all because Belgium astronomer Eugene Delporte was tasked nearly a century ago with making sense of the haphazard constellation boundaries in use at the time. More about that in a moment.

Tomorrow night June 12 the Strawberry Moon will rise in the east around sunset. Although your calendar (like mine) may show June 13 as the date of full moon, much of North and South America will see it full the night of the 12th. The moment of maximum fullness occurs at 11:11 p.m. CDT.

Strawberry’s a fitting a name for the June moon as the berries are ripe for picking this time of year.  As if partaking of the berries’ juicy redness, hazy summer skies often color the moon a deep orange or even red at moonrise.

The full moon will shine from Ophiuchus tomorrow night. Thanks to crisp, modern constellation boundaries, the moon spends about one night a month in the constellation. The sun crosses Ophiuchus in the late fall and lingers there for several weeks. Should it be the 13th sign of the zodiac? What do you think? Created with Chris Marriott’s SkyMap software

Ophiuchus (oh-fee-YOO-cuss) the Serpent Bearer represents a man with the snake Serpens (a separate constellation) coiled around his shoulders. To the ancient Greeks he was the god of medicine. The snake represented healing because of its seemingly magical ability to shed its skin, giving it the appearance of being reborn.

Before 1930, the June moon would have cruised from Scorpius to Sagittarius in its monthly trip through the zodiac constellations. Now it spends a night every month in Ophiuchus. In 1925, Eugene Delporte of the Royal Observatory of Brussels proposed the need for clear, universally-accepted constellation boundaries to the International Astronomical Union (IAU).

The full moon rises in Ophiuchus tomorrow evening June 12. Stellarium

Boundaries were vague at the time and depended upon the star atlas you were using. Understandably, this created confusion for astronomers about what was where. Take variable stars – stars whose light isn’t constant but changes over time. Variables are named after the constellation in which they’re found, so for instance, R Leonis is in Leo and U Geminorum in Gemini. As new variables were discovered, astronomers needed to agree to which constellation they belonged. That was only the half of it. What about new comets, novae and other new discoveries? Agreeing on terms is the foundation of communication.

A section of Johann Bode’s early 19th century star atlas featuring the constellations Cygnus and Lyra. If you look closely you’ll see the curvy borders around constellations in fashion until 1930.

The IAU thought it a great idea and put Delporte in charge. He drew up boundaries along existing vertical lines of right ascension and horizontal lines of declination, similar to latitude and longitude on Earth but applied to the sky.

Delporte’s proposal was approved in 1928 and written up in 1930, and that’s why the moon will linger in the Serpent Bearer tonight. Happy gazing!

I’ll have ice with my full moon, please

The nearly full moon rises over Lake Superior near Duluth last night March 15, 2014. Anglers fish for lake trout (left). Credit: Bob King

I took off for Lake Superior yesterday evening and walked out as far as I dared on its frozen surface. The ice was thick, maybe a foot or more, but the rumbling and booming coming from deep below gave me pause. The surface felt as firm and substantial as an airport tarmac, but it subtly flexed and heaved as if somehow alive.

As the sky darkened, moonlight left a trail of light on Lake Superior’s ice last night. Credit: Bob King

Above it all rose the moon shining pale orange against the dusky band of Earth’s shadow. This part of the planet would soon be plunged into night but not without a bright beacon to help us find our way in the dark.

The frozen “fingertip” of Lake Superior photographed yesterday March 15, 2014 by NASA’s Terra satellite. “X” marks the approximate location where the moon photos were taken. Credit: NASA

Glowing pale orange, the moon looked anything but icy, but we know that ice is hidden like buried treasure deep within craters in both its polar regions.

NASA’s orbiting Lunar Prospector, which could detect water up to 1.5 feet below the surface, found water signatures in permanently shadowed craters at both poles.

Lunar mosaic of ~1500 Clementine images of the south polar region of the moon where more area remains in permanent shadow than any other area of the moon. Temperatures in these craters remains steady around -280 F. Credit: NASA

The moon’s orbit is tilted 5.14 degrees to Earth’s and its axis just 1.54 degrees for a total possible inclination of about 6.7 degrees, a tilt small enough that sunlight only skirts the uppermost rims of craters in the north and south polar regions.

The interiors of some craters, particularly in the south polar region, are sunk so deeply in the lunar crust (up to 7.5 miles or 12 km), that no sunbeam has ever broken the darkness.

An image of debris ejected from Cabeus crater in the moon’s south polar region and into the sunlight about 20 seconds after the impact of NASA’s LCROSS probe on Oct. 9, 2009. Scientists detected a variety of compounds in the plume including pure water ice. The inset shows a close-up with the direction of the sun and the Earth. Click for more on the LCROSS mission. Credit: Science/AAAS

Water from ice-rich meteoroids and comets that have smacked the poles over the aeons has hidden out here for billions of years trapped in rocks or existing as solid ice in some cases. Water released by impacts elsewhere on the moon would quickly vaporize away in the sunlight except for here and there molecules that migrated to the polar regions. Bombardment of oxygen-rich rocks by protons (hydrogen plus oxygen = H2O) in the solar winds creates small amounts of water as well.

While not wet by any stretch, our satellite’s no stranger to the substance our lives depend upon.  As twilight deepened and the lake shuddered I sensed that ancient icy connection across a space of 240,000 miles.

Valentine’s Day auroras and a big full moon to boot

Valentine’s Day – and particularly Valentine’s Night – will be special this year. Not only is the moon full, but auroras are in the forecast. Illustration: Bob King

High speed solar blasts that departed the sun on Feb. 11 may combine to deliver a sweet auroral bouquet Friday night. NOAA space weather forecasters predict a 25% chance of minor storms Thursday night, but that rises to 40% Friday night with a 20% chance for a major storm. We’re not talking the high Arctic here – this is the prediction for middle latitudes where we wear less sealskin and more hoodies.

The full moon rises in Leo the Lion not far from its brightest star Regulus Friday night. Click map to find what time the moon rises for your town. Stellarium

Definitely one of the happier auroral forecasts I’ve seen in a while. Friday night’s a big night with lots of Valentine’s fun happening including the Full Snow Moon. Watch for the moon to rise around sunset, cross the south meridian around midnight and set at sunrise the next morning. Might I suggest a walk in the moonlight after dinner with your sweeheart?

While we’d normally be thrilled to have a big moon in the sky, it will put a ding in any auroras that might show. I’ll keep you updated.

The International Space Station will also be making passes of a less passionate sort this week and next. Below are times for the Duluth, Minn. region. Click HERE and HERE for times for your town.

* Tonight Feb. 13 beginning at 7:32 p.m. across the northern sky. Disappears in Earth’s shadow below the North Star a couple minutes later. In binoculars, watch as the ISS fades and turns orange and then red as the sun sets on the ship 250 miles high.
* Fri. Feb. 14 at 6:43 p.m. Bright pass across the north.
* Sat. Feb. 15 at 7:32 p.m. across the north. Disappears below the North Star again.
* Sun. Feb. at 6:43 p.m. Bright pass across the north.

Full Frosty Moon bumps into Pleiades and Hyades tonight

Tonight’s Full Frosty Moon will rise around sunset in the northeastern sky. November’s full moon is also known as the Full Beaver Moon. Credit: Bob King

Every full moon’s an occasion to get outside for a moonlit stroll. No binoculars or telescope needed. It’s surprising how much you can see by moonlight once your eyes get accustomed to its dark luster. Full moonlight, especially when the moon rides high in the sky as it does this month, makes me think of what daytime might look like on a planet orbiting a dim red dwarf star. Think how the sense of vision would have evolved in creatures on such a planet. To make the most of weak illumination, owl-like eyes could come in handy.

November’s full moon goes by two names – Full Beaver Moon and Full Frosty Moon. The first refers to the time to set beaver traps before the small waterways and swamps froze; the second to what covers the lawn after a clear night this month. Should you be fortunate enough to have clear skies this evening, you can watch the moon rise in the northeastern sky in the constellation Taurus. As always, the full moon rises around sunset directly opposite the sun. Click HERE to find the moonrise time for your city.

The full moon sits smack in the middle between the Pleiades and Hyades clusters tonight. Can you see them through the glare? Created with Stellarium

We all know how glaringly bright the moon is when you stare at it. Most stars in its vicinity are swamped by glare and invisible, but tonight, if you reach your hand up and block the brilliant disk, you might be able to make out two nearby star clusters, the Pleiades (Seven Sisters) and Hyades. First magnitude Aldebaran in the Hyades should be easy to see. While the bright orangish star looks like it belongs to the V-shaped group, it’s really in the foreground 65 light years away, 88 light years closer than the Hyades.

The Pleiades might be easier to spot, but if you’re having trouble with either, binoculars will make finding them a breeze. Well off to the moon’s upper left you’ll see the bright, twinkling star Capella in the pentagon-shaped constellation Auriga the Charioteer. Starting with Capella, can you trace the constellation’s outline?

Oblique view of the 8-mile-wide (13 km) crater Dugan J photographed by LRO. Dugan J is a simple, bowl-shaped crater. Debris from the rim has fallen downslope to fill the crater bottom. See closeup photo below. Credit: NASA/GSFC/Arizona State University

NASA’s Lunar Reconnaissance Orbiter page (LRO) is where I go when I need to visit the moon up close. I’ve included a couple recent postings so you can relish the details. Orbiting at just 31 miles (50 km) high, LRO can distinguish features as small as two feet (0.5 meter) across. When mission control took closeups of the Apollo landing sites, the spacecraft was lowered even further to 13 miles (21 km). Need more “lunar cowbell”? Check out the zoomable gallery.

Zooming into Dugan J on the LRO site almost feels like you’re standing on the rim looking in! Credit: NASA/GSFC/Arizona State University

Here comes the Harvest Moon – Find out what makes it special

The Harvest Moon rises over Lake Superior in Duluth, Minn. When you’re out enjoying this year’s full moon on Wednesday and Thursday nights, watch for the dark band you see in the photo. That’s the Earth’s shadow. It’s visible for about 15-20 after sunset and topped by the pink-tinged Belt of Venus, where the atmosphere is still reflecting reddened sunlight. Credit: Bob King

Every month’s full moon tells us a little about the season. That’s why the Cold Moon happens in February and May’s moon is named for flowers. The Harvest Moon refers to the late summer-early fall harvest time, which many of us partake of in a small way with our gardens. My tomato harvest just wrapped up, but there’s still a zucchini or two on the way.

You’ve probably noticed how squished the moon looks when it first comes up. Stronger refraction from the thicker, denser air closer to the horizon “lifts” or refracts the bottom part of the moon upward more than the top, “squeezing” it into an egg shape. Once the moon’s higher up, air density is more uniform, refraction effects less and the moon looks round. Credit: Bob King

The Harvest Moon is the full moon that occurs closest to the autumnal equinox, the first day of fall in the northern hemisphere. This year that happens overnight on Sept. 18-19. Since full moon dates bounce around a bit, the Harvest Moon can happen anywhere from mid-September to mid-October, making it the only full moon to carry the same name in two different months.

The Harvest Moon harks back to our agrarian past when farmers used its welcome light to continue harvesting their crops past sundown. WIth electricity, moonlight was important part of farming. Once the crop was ripe, you needed to gather it up as soon as possible, and that could mean working into the night.

The travels east (to the left seen in the northern hemisphere) as it orbits Earth at the rate of one moon diameter an hour. Illustration: Bob King

It’s the Harvest Moon’s angle to the horizon that makes it unique and useful. Because the angle of the full moon’s path to the horizon is very shallow in September and October, the time difference between successive moonrises is only about 20-30 minutes instead of the usual 50-60. With moonrise happening on the heels of sunset, the Harvest Moon’s return at practically the same hour gave farmers nearly continuous light from sunset to sunrise to work the crops.

Night to night the moon moves about 12 degrees along its orbit or a little more than one fist held at arm’s length to the east. You can see this for yourself by referencing the moon’s location with respect to a bright star in its neighborhood.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in September, the moon’s path is nearly parallel to the horizon with successive moonrise just 25-30 minutes apart. In spring it tilts steeply southward, delaying successive moonrises by over an hour. Moonrise times are shown for the Duluth, Minn. region. Illustration: Bob King

Around the time of September’s full moon, a significant amount of that 12 degrees is toward the north direction which causes the moon’s path at the horizon to flatten out. A shallow path means the moon’s eastern movement only puts it a little bit further below the horizon for several nights around full moon. Earth only has to rotate for 20 to 30 minutes to carry the moon into view. Six months ago at the spring equinox, the full moon’s path was much more steeply tilted to the horizon and night to night rising times more than an hour apart.

A Harvest Moon rises over a desert landscape near Bishop, Calif. Credit: Andrew Kirk

That’s why I call spring the time-when-the-moon-gets-out-of-the-sky-in-a-hurry as opposed to the current full moon that never sleeps. While I’ve been known to gather my garden goodies by moonlight when frost threatens, most of us can take advantage of the full moon’s slanted path to enjoy night after night of watching a big moon rise without to stay up late. Consider it a harvest of moonlight and a veritable feast for the eyes.

Since full moon occurs during the early morning hours of the 19th across the Americas, the moon will look almost identically full on both Sept. 18 and 19. Two nights of the Harvest Moon – can it get any better? Click HERE to find out when the moon rises for your town. If you take a photo, please send me a copy at rking@duluthnews.com. I’ll publish a little gallery on the 20th. Thanks!

Tonight’s Full Moon could hit your eye like a big pizza pie

The nearly full moon is another “ball” in play during a soccer game last night in Duluth, Minn. Credit: Bob King

The full August moon, called the Sturgeon or Red Moon, will roll its cyclopean eye around the heavens tonight, beaming from the east around the time of sunset and lighting your path until dawn. Stare it back in the face while you’re out for an evening stroll.

I’ll never forget an evening long ago when my good friend Rick looked up at the moon and started singing “when the moon hits your eye like a big pizza pie, that’s amore” from the classic Dean Martin tune “That’s Amore”: Since he tends to be on the serious side, I had to chuckle when he burst out in song.

And who can deny that the broad white cratered plains look like so much mozzarella sprinkled around the dark, sausage-like forms of the lunar seas?

Large areas of the U.S. are experiencing typical hazy summer skies from dust, humidity and forest fires this August, making the moon look paler than usual especially at moonrise.

Sorry, couldn’t resist. Dean Martin and Jerry Lewis sing “That’s Amore”

If you’re one of the lucky ones to have a deep blue sky tonight, you’ll see the moon crest the horizon on schedule. Many of us will have to wait a few minutes for the moon to first clear the haze. I suspect it will look ghostly orange for a time until it rises high enough for its true brilliance to show. Click HERE and key in your location to find the time of moonrise.

Last night the hazy atmosphere served as an ideal filter for observing the nearly full moon through the telescope. Normally its glare is intense – you walk away from the scope blinded in one eye, practically stumbling. Not yesterday. The thick air made the moon easy on the eyes and tinted the lunar globe the color of September straw.

Full Buck Moon gets a free ride from Earth tonight

Don’t miss the rising of July’s Full Buck Moon tonight. Credit: Bob King

If you’re a full moon watcher, tonight’s your night. The Full Buck Moon rises around sunset in the eastern sky and will light your path till tomorrow’s sunrise. Yesterday evening while finishing up mowing the lawn after sunset, I turned a corner and glimpsed a light in the distance. Since I wasn’t expecting it, I was pleasantly surprised to see it was the moon.

The name refers to the time when those little antler nubs first poke out of a male deer’s head. Other names for July’s full moon are Thunder Moon and Hay Moon. To see tonight’s moonrise, click HERE to find when it happens for your location. Be sure to pick the second moonrise time shown, not the “preceding day” time.

While you’re out moongazing tonight, you’ll notice how squished the moon appears near the horizon (left) compared to when it’s higher up. Earth’s atmosphere acts like a prism to refract or “lift” the bottom half of the moon into the top, giving it a flattened appearance. Credit: Bob King

Rising’s a funny thing. The full moon appears to lift itself from the horizon and gradually travel in an arc from east to west, setting around the same time the sun rises tomorrow morning.

Earth’s rotation carries the full moon up from the horizon along Lake Superior in Duluth, Minn. In this moonrise sequence a photo was taken about once every 2 minutes. Credit: Bob King

Of course, the moon’s really getting a free ride thanks to our rotating Earth. As the planet turns, your city and its moongazing inhabitants are gradually turned to face the moon at some time in the evening. The moment its shiny orange disk crests the horizon we pronounce it moonrise.

Meanwhile, the Earth keeps rotating until the moon is directly (more or less) above your city; it finally sets when you’ve rotated far enough for the curvature of the Earth to block the moon from view.

But the moon’s a bit of a sneak with moves of its own, which it employs every night to counter Earth’s bully-like rotation. As you gaze at tonight’s moonrise, keep in mind that that big bright ball is fighting a losing battle.

The moon revolves around the Earth once every 27.3 days from west to east in the sky. That’s slow enough that you and I don’t usually notice, but every hour that passes, the moon moves its own diameter (1/2 degree) to the east traveling at an average speed of 2,300 mph (3,700 km/hr).

Sorry Charlie. That’s not fast enough to counter Earth’s much more rapid rotation, which as we saw earlier, moves the moon across the sky from east to west during the night.

As Earth’s rotation makes the moon (and stars, sun, etc.) move from east to west, the moon is quietly – and slowly – moving counter to that direction to the tune of its own diameter per hour. Credit: cseligman.com

So Earth wins this game of cosmic arm wrestling. But does it? All those 1/2 degree increments accumulate over the hours. By tomorrow night, they’ll amount to 12 degrees or more than a fist held at arm’s length against the sky. Since the moon will be a fist further east, another hour of Earth-rotation will be needed to carry it back up to the horizon. That’s why the moon rises about an hour later every night.

So in the end, our favorite satellite can celebrate at least minor victory, don’t you think?

Tonight’s supermoon invites us to experience the music of the spheres

The full moon rises over Lake Superior in Duluth, Minn. Watch for a bigger-than-normal full moon to rise tonight in Duluth at 9:13 p.m. Credit: Bob King

Don’t forget to watch the big, full moon rise tonight. To find out when it pops up in your neighborhood, click HERE and select a city. You’ll be shown the moonrise time for the preceding day and the current day. Not handy enough for you? Download one of many moonrise/sunrise apps available for iPhone and Android. Hardcore moon photographers will find the full-featured The Photographer’s Ephemeris app essential.

Surface tension causes raindrops to bead up on the surface of a hosta leaf. Credit: Bob King

While taking out the garbage this morning, I noticed beautiful beads of water lined up in the grooves of my wife’s hosta plants. Water forms little spheres because of surface tension. Simply, water molecules adhere better to themselves than to the waxy surface of the leaf. Like so many pill bugs, they “curl” into little balls.

My daughter Maria demonstrates a feat of great strength by holding a favorite sphere. Photo illustration: Bob King

Earth, moon and planets are spherical for an entirely different reason. Up to a certain size, objects can be any shape imaginable, but beyond about 240 miles (385 km) across, self-gravity is strong enough to pull an object’s material towards its center until it collapses into a sphere.

All the stars, planets and some asteroids are large enough to have literally crushed themselves into balls.

Looking for a cubical and hexagonal planets? Forget it. Their pointy corners would soon be crunched down and absorbed.

The universe delights in creating spheres. Gravity pulls every part of object’s surface toward the center, creating a spherical shape. We recognize gravity’s powerful role whenever an architect designs a new building. The bigger the building, the stronger its foundation must be or the weight of building will crush the foundation and it will topple. If you had a car, bathtub or head bigger than about 240 miles (385 km), they’d crunch down into spheres too.

Our solar system provides an excellent example of the borderline between the spherical and the irregular. Mimas (MEE-mus), one of Saturn’s 34 moons, is very nearly 240 miles in diameter and composed mostly of ice. It’s the smallest spherical object in the solar system.

Mimas (left), moon of Saturn, and Proteus, moon of Neptune. Credit: NASA

Proteus (PROH-tee-us) is Neptune’s second largest moon at 250 miles across, and it’s irregular in shape. They’re so close in size and composition that you’d think Proteus would be spherical. It isn’t. Proteus is much further from the sun and hence colder than Mimas. Colder means harder, so while Proteus tried, it was too firm to become a sphere. A little larger, a little more material for gravity to work on and it would look much like Mimas. As you might guess, differences in an object’s composition (internal strength of materials) and temperature can shift the “spherical limit” a little one way or another.

Good examples of decidedly out-of-round are the Martian moon Phobos and asteroid Eros. There’s simply not enough material in these objects to give gravity the upper hand. Irregularity rules.

The asteroid Eros, photographed up close by the NEAR Shoemaker space probe, and Mars’ moon Phobos are both too small to be spherical. Credit: NASA

From softballs to stars, the sphere is probably the most common shape in the universe but try to find a perfect one. Everything is spinning – from the ponderous 243-day rotation of Venus to the zippy 42.7 second spin of the near-Earth asteroid 2008 HJ. Spinning causes a planet’s equator to bulge outward, giving it a tad extra girth.

Consider the Earth. Its equatorial diameter is 7,926 miles, while the distance between the two poles is 7,900. The moon rotates much more slowly – once every 27 days – with a difference of just 2.5 miles (4 km) between polar and equatorial diameters.

So when you see the moon rise tonight, think of gravity fashioning spheres from the flotsam and jetsam of the universe with the ease of a child rolling a ball of play dough between her hands.