This is a question that has been debated for several thousand years. One popular myth, dating all the way back to Aristotle in the 4th century BC and which still endures somewhat today, is that it is simply a case of magnification caused by the Earth's atmosphere. While a "magnification" effect is taking place, it actually is going the other way and is more of a compression. Atmospheric refraction causes the Moon to appear very slightly smaller in the vertical axis when it is near the horizon vs. when it is high in the sky. This refraction, combined with the fact that the Moon is about 4000 miles further away when it is on the horizon, causes it to appear 1.5% smaller if you were to measure very precisely its apparent size on the horizon vs. higher in the sky.
So if it's not magnification from the Earth's atmosphere, what is going on here? In short, the Moon appearing bigger near the horizon is nothing more than an optical illusion. It's really as simple as that. You can verify this fact by taking a pair of calipers or even just a ruler and measure the diameter of the Moon on the horizon; later that night, when it is higher in the sky, measure it again. (Be sure and hold the measuring device at the same length away from your eyes each time to get accurate results.) If you do it precisely enough, you'll find that it actually will measure about the same size both times, despite appearing nearly twice as big to your brain when it is on the horizon.
Exactly what is going on in our brains to cause this optical illusion is still somewhat up for debate, but it seems to center around size consistency, where our brains are trying to grapple with the size of an object vs. how far away it thinks it is. For instance, when you see someone very far away from you and their head looks incredibly tiny, your brain doesn't for an instant think that the person and their head is actually tiny. It adjusts your perception based on what else is in your vision to gather that third dimension of depth.
Something of this same effect is thought to be happening with the Moon, only this time your brain is getting tricked into thinking that the Moon is further away when it is on the horizon, making it appear bigger to you. This is known as a Ponzo Illusion, named after Italian psychologist Mario Ponzo.
Mario Ponzo first demonstrated the "Ponzo Illusion" in 1913 (left). In this experiment, Ponzo drew two converging vertical lines on a piece of paper. He then drew two horizontal lines crossing these lines, one at the top, and one at the bottom. These two horizontal lines are the same length, but it appears that the one at the top is longer because it appears further away. This is due to our brains interpreting the two converging lines as parallel lines that only appear to be converging because they are getting further away. Thus, if both horizontal lines are making the same length "imprint" on our eyes, but one is further away, then the one that is further away must be much larger, so our brains perceive it as larger than it actually is.
So, in the end, it's a matter of our brain's getting tricked in terms of the distance from the Moon to us when it's on the horizon vs. high in the sky. When it's on the horizon, our brain has reference points to compare and judge the distance based on that, and similarly adjust the apparent sized based on that perceived distance. When it is high in the sky, there is nothing useful to compare it to so the apparent size changes based on how far away our brain thinks it is at that point, namely thinking that it is closer to us high in the sky and further away on the horizon.
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- This same effect of something appearing larger on the horizon can also be observed with the Sun and constellations as well.
- The Moon always has the same side pointing towards the Earth due to its rotation speed matching its orbit around the Earth exactly. This isn't by chance and it wasn't always this way. When the Moon first formed, its rotation speed was very different than it is now. Over time, the Earth's gravitational field gradually slowed the Moon's rotation until the orbital period and the rotational speed stabilized. This effect isn't unique to our moon, but also to many of the moon's orbiting various planets in our solar system.
- The Moon also has a somewhat similar affect on the Earth, slowing the Earth's rotation about 1.5 milliseconds every century. With that energy the Moon is stealing, it is gradually getting further and further away from the Earth, about 3.8 centimeters further away every year. This might not sound like much, but consider that, when the Moon was first formed, it was orbiting at about 14,000 miles away from the Earth. Today, it's over 280,000 miles from Earth. Assuming the Earth had an abundance of water around the time period when the Moon was only about 14,000 miles away, the tidal effects must have been drastic as the Moon orbited.
- Contrary to popular belief, the Moon isn't round, but rather is slightly egg shaped. What we see is one of the small ends, which faces the Earth.
- Tides aren't just caused by the Moon, but are also to a lesser extent caused by the Sun's gravitational field as well.