Up in the Air

Up in the Air: June is quite a big month for a full moon

June 16, 2013 

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University.

The June sky is full of interesting celestial objects. The low-hanging fruit for our exploration are Venus and Mercury, who do a do-si-do low in the western sky after sunset. As the sky darkens, Saturn is highest at prime time, presenting a good view even in binoculars or small telescopes. For early birds, Mars is very low in the dawn sky, maybe still too low to see.

Perhaps more interesting is this month’s full moon, which occurs on the 22nd, a few days after it passes close to Saturn as a waxing gibbous moon. Normally the full moon is of little interest since it looks boring: Its bore-holes are top-lit. Let me explain.

The interesting areas of the moon are its craters, holes bored into the moon when it was bombarded by solar system debris billions of years ago. Actually, not so much bored as exploded since the craters are created by the explosion of the impactor as their energy of motion was turned into heat on impact. The explosions, which are centrally symmetrical events, create circular craters, no matter the impactor’s approach angle.

The full moon is not so interesting because the craters facing us are illuminated by the sun behind us. A hole lighted from the top has no shadow and it’s the shadows that give craters definition. So, a full moon not only washes out the dark sky, it’s not even worth looking at itself.

So why might it be interesting this month? Because of where it is – at perigee, the closest point to Earth in the moon’s elliptical orbit. Since Earth tides are highest at full (and new) moon, and the moon is closest to us at full phase this month, we will get extra-large (high and low) tides. Because the moon is so near, it will also appear as the largest of full moons for the year.

What will not happen on June 22, though, is an increase in births. This common myth intrigued me some years ago and I plotted the total U.S. births vs. lunar phase for some 55 million births. There was no correlation and there was none either when plotted vs. “anomalistic phase,” the position with respect to perigee.

But, we should not expect any correlation since there is no biophysics that can be affected by the minuscule gravitational force of the moon on the human body. The myth will live on, though, probably created by weary delivery room nurses who notice the full moon because, rising at sunset and setting at sunrise, it is the most likely moon to be seen at night. And, you remember what you see, not what you don’t.

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu. More on this month’s column: www.upintheair.info.

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