Super lua: apogeu e perigeu. South Florida Museum explica.
Have you heard about the “supermoon”?
I hadn’t either, until today. “Supermoon”, it seems, is a term recently coined by an astrologer who sees great potential for natural disasters tomorrow night, because we will have a full moon at perigee. (Sometimes, people don’t get their science quite right.)
OK. So you’ve heard of a full moon. As a refresher, we get a full moon when the sun, earth and moon are all in a line, with the earth in the middle. The side of the moon that we can see is full-face to the sun, giving us an un-crescented view. Tides tend to be higher during full moons, because both the sun and the moon are pulling at the earth and its water in opposite directions. The water “bulges” up in the direction of both gravitational pulls.
You may not be familiar with the term “perigee”. In a nutshell, the moon’s orbit around the earth isn’t perfectly round. It’s elliptical, an oval, like an egg. Sometimes, then, the moon is closer to us. Sometimes it is farther away. When it is closest, we say it is at perigee. When farthest, the word is apogee. Here’s a picture, wildly out of scale to give you an idea of what I mean…
The moon’s orbit precesses over time, which means that oval kind of swings around the earth like a hula hoop. I tried to show that with the slightly faded orbit line. I mention this because the “supermoon” happens when the full moon happens to coincide with the perigee. This coincidence only happens every few years, so it’s rare and worth noting.
We get a full moon every month, and the moon hits its perigee and apogee every month. (Month is approximate here. A lunar cycle is 27.3 days. But i’m a sucker for the old-style words. “Month” is derived from “moonth”, after all). When the “hula hoop” happens to be at the right part of its swing, we get the full moon at the same time as the perigee.
Does this mean we should expect earthquakes, volcanic eruptions and floods? No. The tides might be a bit higher than normal. Areas prone to tidal flooding might see water levels one to six inches higher. The solid earth is pulled by the tides, too. But study after study after study shows that there is no correlation between tidal forces and earthquakes and volcanic activity.
Nonetheless, the moon is at its closest (~221,000 miles), which will make this particular full moon look just a bit larger and brighter than is normal. Unless you go out to see every full moon and have an awesome photographic memory, you might not notice the difference. But it’s still worth going out to see a big, bright full moon.
Here in Bradenton, supermoonrise is scheduled for 7:55 pm on March 19th. Catch the moon while it is low on the horizon. Having trees and buildings in the scene creates an optical illusion that makes the moon look even larger. Put the worry of natural disasters out of your mind for a few minutes and enjoy a beautiful moon on what promises to be (for us Floridians) a clear Saturday night.
Inconstant Moon by John Walker