When the Full Moon Isn’t Round

(Photo by Dion Ogust)

It’s the full moon this Saturday night. But the exact time of fullness is 2:35 p.m. that afternoon. So if you’re observing at 9 p.m. Saturday night, the moon has had 6 1/2 hours to get “out of round” — and you’ll easily detect that it’s not a perfectly round disk.

To see a truly perfect circle, look again the night of the Harvest Moon, September 27. Then, the moon will be full around 10 p.m., our time. A complete circle. So perfect a line-up, the moon will go into Earth’s shadow. (Imagine: The Harvest Moon in total eclipse, at the same hour it’s at its closest approach of the entire year! What an eclipse next month!)

The moon is just four miles “out of round” in its 2,160 mile diameter. One part in 500. Absolutely imperceptible, visually. A flawless disk, to the eye.

Through the ages the circle was considered nature’s “perfect shape.” All parts of a ring are the same distance from the center, so the figure seems beginningless and infinite. The vestige of that belief is still with us in the exchange of wedding rings. In any case, the circle is nature’s favorite shape. In the distant depths of the night, there are far more spheres than any other shape. Yet when meteorites are discovered these chunks of stone and metal are never round. What goes on? Why are small objects irregular while big ones are balls?

The answer is simple. When a celestial body forms, it is either gaseous or molten and easily malleable. All of its atoms attract each other by simple gravity, so it pulls itself inward to the most compact figure possible — which is always a globe.

You discovered in childhood how a sphere has the smallest surface. When you played with clay, you could pattycake it into a thin piece with an enormous surface — or else you could roll it into a little ball between your palms. A ball was always the tiniest you could make it, with the least amount of surface that required the smallest amount of paint when it dried.

Only objects with too little gravity to do the job escape being balls. That’s why celestial bodies below a certain mass, like asteroids and meteors, are irregular.

The universe does allow loopholes. Speedy rotation forces large fluffy planets like Saturn, the bright star in the southwest after sunset these nights, to bulge exotically. Some stars like brilliant Vega overhead spin so quickly they look as though squeezed in a giant vise. But such exceptions aside, it’s a universe of spheres. Some of these balls are larger than Jupiter’s orbit. Others, like the Crab Nebula pulsar, could fit within the boundaries of Los Angeles. But the glowing jewels that make up the constellations — nearly all are great balls of fire. Perfect looking spheres.

But not the full moon this weekend.


Want to know more? To read Bob’s previous ‘Night Sky’ columns, see our Almanac Weekly website at HudsonValleyAlmanacWeekly.com.



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