When the Full Moon Isn't Round

Full Moon in Darkness

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Is the full Moon round? Let’s look at the August full Moon, which happens this Saturday night.

The exact time of fullness is 2:35 PM EDT or 11:35 AM PDT.  

So, if you notice it at 9:30 PM Saturday night, the Moon already had between 7 and 10 hours to move past full and become a bit “out of round.”

Do you care? Well, perfect circles are actually pretty rare in nature. To see a truly perfect disk, look again a month from now, the night of the Harvest Moon, September 27. Then the moon will be full around nightfall. It’ll appear as a flawless circle. The line-up of the Sun, Earth, and Moon will be so perfect, the moon will go into Earth’s shadow.

Imagine: The Harvest Moon in total eclipse!

In truth, the Moon is just four miles “out of round” in its 2,160 mile diameter. One part in 500. Absolutely imperceptible, visually. So at the hour of fullness it’s a flawless disk, to the eye.

The Circle: Nature’s Perfect Shape

Through the ages the circle was considered nature’s “perfect shape.” All parts of a ring lie at the same distance from the center, with no sharp angles, so the figure seemed infinite. That belief still lingers with us in the exchange of wedding rings.

In the distant depths of the night, there are far more spheres than any other geometric shape.

  • Stars are round. They don’t look like the five-pointed “stars” our teachers stuck on our papers.
  • Yet meteorites—those 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 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, you could roll it into a little ball between your palms. A ball was always the tiniest you could make it. Then it had the smallest surface area, and needed the least amount of paint when it dried.

Only objects with too little gravity can’t become perfect balls. Celestial bodies below a certain mass, like asteroids and meteors, are irregular.

But stars? Ours is a universe made of great balls of fire.

Yet, if you love the circle, the full moon won’t quite cut it, this weekend.

~ By  Bob Berman

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on stargazing and astronomy. Wondering which bright objects you’re seeing in the night sky? Want to learn about a breathtaking sight coming up? Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, we’ll cover everything under the Sun (and Moon)!

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Round in the universe

Certain things are lovely to be meditated upon. You are a poet, Bob Berman. And you are right. Thanks!

I love the Play-Doh analogy -

I love the Play-Doh analogy - although the disc shape seems to be how lots of the balls arrange themselves in a galaxy.

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