Friday, March 18, 2011
Tomorrow night (March 19th) will be a perfect night for a moonlit stroll. Not just any moonlit stroll but a Supermoonlit stroll. Now, we've all see the illusion where the moon looks huge in the night sky, and it is just that, an illusion.The Moon isn't actually bigger or smaller as it travels across the sky. Why you see the Moon looking huge when it is low on the horizon is an optical illusion owing to the circuitry in your brain. There is no clear consensus on why. It could be an Ebbinghaus Illusion - where identically sized objects appear to be different sizes when placed in different surroundings (click link for picture) - the objects of known size to your brain that are near the horizon (trees, houses, etc.) provide a false frame of reference for the size of the Moon. When the Moon is high in the sky you have no such references and so see the Moon as smaller. Another possibility is how our brains interpret foreground objects for reference in terms of distance and size. Foreground objects of known size appear to be far away and so when something large, like the Moon, appears behind it and even farther away but still appears big then we interpret is as being larger than it is.
In the case of tomorrow night's Moon, it will actually be larger in the sky. Our Moon has an elliptical orbit around the Earth with one side of the ellipse, perigee, about 31,000 miles (50,000 km) closer than the other side, apogee. The moon will be in the perigee part of the orbit which will take it closer to Earth than it has been in two decades, since March 1993. This means that the moon will appear about 14% bigger and 30% brighter than when the it is in apogee (other side of the ellipse). Admittedly, that isn't all that big of a difference. It can be tricky to even tell the change in size, especially if you have no scale for reference, but the best time to look will be when the moon is near the horizon. This will be when the illusion of a bigger moon mixes with the reality of a closer moon to produce an amazing view.
Oh, and no, there will not be any cataclysmic, apocalyptic results of a close moon. Only slightly stronger tides.
Here's some more info:
(image from universetoday article)