Earthlings will think their eyes are playing tricks on them Wednesday, starting from sunset: If you step outside, you'll see a form rising from the east that looks like the full moon, complete with craters forming a human-like face - but it will be massive. This is called the Solstice Moon Illusion. It arrives only two days before the beginning of summer, say space experts, and the sun and full moon are like children 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. The best time to view it is around moonrise, when the moon is peeking through trees and houses or over hills. Sky watchers have known for thousands of years that low-hanging moons look unnaturally big. At first, astronomers thought the atmosphere must be magnifying the moon near the horizon, but cameras proved this was not true: Moons on film are the same size, whatever the elevation. Scientists are still not sure how it happens, but they know that when you look at the moon, rays of moonlight converge and form an image about 0.15 mm. wide on the retina in the back of your eye. High moons and low moons make the same-sized spot, yet the brain insists one is bigger than the other. A similar illusion was discovered in 1913 by Mario Ponzo, for whom it was then named. He drew two identical bars across a pair of converging lines, like railroad tracks. The upper bar seems wider because it spans a greater apparent distance between the rails. Some researchers believe that the Moon Illusion is Ponzo's Illusion, with trees and houses playing the role of Ponzo's converging lines. Foreground objects trick your brain into thinking the moon is bigger than it really is. However, pilots flying at high altitudes sometimes experience the Moon Illusion without any objects in the foreground. This may be due to the shape of the sky, which is perceived as a flattened dome with the horizon far away. When the moon is near the horizon, your brain - trained by watching birds, clouds and aircraft - miscalculates the moon's true distance and size.