As if Cinco de Mayo wasn't enough, now there's a new reason to celebrate this Saturday!
On Saturday, May 5th, two events will coincide. The moon will be full. It will also be at its perigee, or closest approach to Earth. The effect of all this will be a fantastically bright moon, some 16% brighter than a normal full moon.
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In addition to the extra light and powerful tides, an unusual psychological effect will be easy to experience. When the moon rises on Saturday, take a look at it. It will appear impossibly huge. This always happens with a moon near the horizon, but the effect will be even stronger with the moon so close.
The effect is well known, but poorly understood. A common theory is that when the moon is high in the sky, there's nothing to compare it to to get a sense of size. When it is near the horizon, we can visually compare it against trees, buildings, and so on. The comparison tricks our brain into thinking that the moon is much larger than it actually is.
To dispel the effect, look at the moon upside down. It will appear normal sized.