Make A Date For The Large Moon Illusion

It's a big moon a-rising this Wednesday, June 18th...but it will only be an illusion. Science@NASA has the lowdown:

There's no better time to see it. The full Moon of June 18th is a "solstice moon", coming only two days before the beginning of northern summer. This is significant because the sun and full Moon are like kids on a see-saw; when one is high, the other is low. This week's high solstice sun gives us a low, horizon-hugging Moon and a strong Moon Illusion.

We've all seen that the moon on the horizon seems unusually large. I had always accepted the standard explanation that the light from the moon travels through a greater part of the atmosphere when it's on the horizon, and this naturally distorts the apparent size of the moon. Now I find that this ain't necessarily so. As the picture below shows, according to the camera lens, the moon's size really stays constant throughout it's march across the sky.


There are several explanations for why the moon appears large, but apparently debate still looms. For example, some say the fact that the moon is low to the horizon, foreground objects provide a size reference that we normally don't have when the moon is overhead, fooling us into thinking the moon is a different size. On the other hand, pilots who don't have this reference also experience the moon illusion.

For more, see here and here.