Not another supermoon

SkyCaramba weekly astronomy blog for the week ending October 22, 2011

On October 26, the moon will be almost as close to the earth as it was back on March 19. Back then, some people worried about the moon being so close and being on the opposite side of the earth as the sun. They felt gravitational forces would cause earthquakes or other natural disasters. 

The idea that things in the sky influence events on Earth is nothing new. Astrologers have tried for thousands of years to explain the rise and fall of an empire or why someone’s life turned out the way it did by relating it to where the planets were at various times. Astronomers don’t bother with that. 

There’s a long published almanac that has claimed that when the moon is farthest north (northern lunistice) there’s a greater chance of earthquakes in the northern hemisphere. Likewise, it has claimed there’s a greater likelihood of earthquakes in the southern hemisphere during southern lunistice. 

People in the American Midwest remember an earthquake that never happened December 3, 1990, the day after a full moon. Iben Browning, a climatologist, had predicted a 50% increase in the chance of an earthquake within two days of that day because of the way the sun, earth, and moon would align. Since the chance of an earthquake happening on any particular date is very small, a 50% increase in the chance shouldn’t have been a big deal. News media, however, misreported that Browning had predicted an overall 50% chance of an earthquake! 

The almanac and Browning, at least, started with a valid notion. The sun and moon do exert a measurable gravitational force on the earth. The earth’s orbit around the sun is a somewhat wavy ellipse because of the moon’s tug. And people who spend enough time along the coasts of the world’s oceans know that tides are higher during certain moon phases. So it seems reasonable that things on the earth ought to be a lot more affected by solar and lunar gravity. 

But let’s not forget the earth has gravity too. It has more than enough to resist the external tugs. If it didn’t, it couldn’t hold itself together or hold on to us and our belongings very well. Tides don’t happen because the moon directly pulls on the oceans’ waters the way a magnet pulls on a compass needle. They happen because the water is being sloshed around like in a cup that’s moved back and forth. The solid parts of the earth, although moving against each other to cause earthquakes, are a lot more like the cup. 

Next week, the center of the moon will be 347,100 kilometers (221,900 miles) from the center of the earth. Compare that to March 19 when the distance was 356,600 kilometers (221,600 miles). The two bodies will be almost as close. But it’s happening during a last quarter moon, so nobody’s generating attention talking about another “supermoon”. That’s okay. Look up mid-week and enjoy the biggest last quarter moon you’ll see in 2011.