One thing that any astronomer loves is a nice dark and clear night. When the power goes out, it’s even better – less light pollution!Let’s assume that it’s just after dusk, and the moon is just starting to break through the horizon. The moon looks huge doesn’t it! Go back inside, that’s some scary stuff.. the moon being so close to Earth and all. Wait a few hours and let’s go back outside, tilt your head up and look for the moon, assuming that there’s still no clouds in the sky. What happened? The moon is so tiny and far away? Did Earth’s gravitational field throw it away with centripetal force? Does the moon naturally come close to Earth and move back every night? The answer to both questions, is no.
The moon stays at an average distance of 384,400 kilometers from the surface of the Earth, measured at equatorial sea level. It comes as close as 356,700km to 406,300km – a distance that is not noticeable to the naked eye. Given the actual size and distance of the moon, if humans were to be able to consciously tell the difference in that distance while standing on the Earth, it would be the same as being able to tell the size increase of a beach ball because a single bacteria landed on the surface.
So what causes the moon to appear larger when it’s near the horizon? You do. Your brain is not so smrt.. I mean.. smart. Your brain determines the size of an object based on similarly sized objects and compares the two. This is how your brain tells you “That object is bigger than the other object” and vice versa. Since you probably have not walked on the surface of the moon, or orbited it in a space craft, your brain doesn’t know how big the moon actually is, so it guesses. That guess changes based on your surroundings to things sizes that you do know, such as buildings, trees and landscape features. When your brain sees the moon on the horizon of a cityscape, it believes it’s just as big as the building is, so it amps up the size of it in the sky. Later on i the night when the moon is way up top away from the horizon, it’s small because your brain has to take a guess of it’s size, based on the stars and clouds nearby.
This is called an Ebbinghaus Illusion – a term coined by Psychologist Tichener in 1901, named after Psychologists Hermann Ebbinghaus. Take a look at the following image:
See that orange dot in the middle? Your brain probably thinks that the orange dot surrounded by the larger circles appears smaller than the orange dot surrounded by the smaller circles. It’s actually the same size.
Want to see this for yourself, while making yourself look like a crazy person? Well that’s just what I enjoy doing – making you appear like you just were hit on the head by a falling anvil, and you didn’t pull out your umbrella in time.
Pull the ol’ switcheroo on your brain and look at the moon on the horizon through your own legs. Bend over and moon the moon, then look at the moon’s reaction. Upside down trees and buildings don’t register in your brain as valid objects for size comparison and perception, so the moon will appear the correct size to your eyes; the same size it would when it’s high in the night sky. Or you could take a photo of it, but that nearly isn’t as fun as making yourself look crazy..
It’s for science.. and science stops at no bounds!