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.
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.
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.
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.
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.