While the moon is actually never full, this evening, Wednesday June 18th, it will seem abnormally huge to most people. Don’t worry, it’s only your mind playing tricks on you. This optical illusion, known as the Ponzo illusion, makes it seem as if the moon is bigger when it’s near the horizon. The effect is exaggerated during the “full” moon.
Unfortunately, human eyesight can’t be trusted when dealing with extremely large distances. The moon is not any larger overhead than it is near the horizon. Mario Ponzo first determined that our minds sometimes gauge the size of an object based on the background behind it. Because we perceive the sky as an extended dome, the moon appears to be very distant when on the horizon. On the other hand, things like clouds and airplanes, which are viewed directly overhead appear (and really are) much closer. So naturally. we tend to think that the moon is closer when viewed overhead too.
But the idea of the moon being farther away because it is on the horizon rather than overhead is only an illusion, as the moon is beyond the sky and hundreds of thousands of miles away, far enough to remain unchanging. Since during a full moon its size doesn’t appear to decrease, and yet it is located on the horizon, we are tricked into thinking it must be bigger. And that is in fact what we see.