The moon’s orbit around the Earth is not a perfect circle. The supermoon is about 17% larger than an average full moon and about 30% brighter but only happens yearly. So what’s going on? Your brain is trying to determine how the moon is situated in physical space. It’s constantly mapping out where things are concerning ourselves so we can move through the world. To do that, it uses different visual cues, like vanishing points and shadows.
Scientists believe that our brains have evolved under the assumption that light usually comes from above us. Our brains perceive the night sky as projected on a dome, similar to a planetarium. A big moon, small moon, or half-moon are only illusions of the eye and brain.
Why does the moon look so big?
The moon looks bigger on the horizon because of the Moon illusion, which has puzzled people for thousands of years. Indeed, the moon does get larger and smaller in angular size. It can change by up to 13% in diameter because its elliptical orbit takes it closer or farther from the earth over time.
During a night, though, it’s a tiny change. Despite what our brains tell us, the moon is about 2% smaller when it’s on the horizon than when it’s high in the sky. When the Moon is directly overhead, the Earth’s radius boosts us a little closer, making it seem larger.
Scientific explanation: Have you ever looked at the Moon when it’s shallow in the sky? It has that reddish-orange color, and it seems so much bigger. The color is a natural effect caused by the Earth’s atmosphere. The light has to pass through more air near the horizon, and air scatters away the blue half of the spectrum. The same thing happens with the sun at sunrise and sunset. But the hugeness of the Moon is a complete illusion.
It’s called the “Moon Illusion,” which has fascinated humans for a long time. The moon looks more prominent on the horizon, farther away. The human brain changes the sizes of things to add extra contrast. Ebbinghaus’s illusion state that brains are used to objects moving towards the horizon, becoming smaller clouds. So when our brain sees the moon at the horizon appearing the same size as our retina, it compensates for perspective and assumes it.
So if we see something, a person gets smaller, and our brains interpret it as though they are moving farther away. The flip-side is that if something looks like it’s getting farther away, but doesn’t change the size, then it must be growing. It could happen with the Moon. It must be farther away when the Moon is near the horizon, but it’s always the same size. So your brain thinks it’s bigger, and it changes your perception accordingly.
The clouds above us look bigger than the clouds on the horizon because they are closer to us, while the moon’s size stays the same. We judge the size of objects differently based on their background. It is known as the Ponzo illusion. When something looks further away, then it appears larger to us. Therefore our brain makes us believe that the moon must be bigger on the horizon than high in the sky.
The problem is that very few people think the horizon moon looks farther away. Most people say it looks closer. The sky-dome model assumes a curved sky above flat ground. It suggests that maybe our brains map everything like stars and the moon onto an imaginary dome above our heads. Technically, everyone has their observable universe around them.
Why does the moon change size?
Why does the moon appear to change shape during a month? The moon doesn’t change shape. But our view of it does change. Why? The moon has a day side and a night side: a part facing the sun and a part not. So when we see a tiny sliver of the bright moon, we see a small amount of the moon facing the sun. At the same time, the rest of the side facing us is dark.
The size and shape of that tiny sliver of lit-up moon change because the moon orbits or moves around Earth. It takes the moon about one month to make one complete orbit around the Earth. In the daytime, the side lit up by the sun faces different directions as it moves around the planet.
During its month-long trip around the Earth, the moon’s bright part appears to grow and then shrink from right to left! The different shapes we see on the lit-up part of the moon are called phases. There are eight phases of the moon.
- The first phase is the new moon. This is when the nighttime side of the moon. Its dark side is facing us. We can’t see it during the new moon phase!
- The next phase is called the crescent phase. The moon looks like a sliver, the tip of your fingernail! After the crescent comes, the quarter moon looks like a half-circle!
- The next phase is called gibbous. A gibbous moon is when it’s missing a tiny sliver of light, and it’s almost full! The moon is at its brightest at the full moon.
That’s when the moon’s fully-lit side faces us, so we see a big, bright circle. After the full moon, the lit-up part of the moon appears to shrink. The sunny side of the moon will shrink from right to left too!
So, after the full moon, it shrinks back to a gibbous moon. But this time, the left side is shining bright instead of the right! After the gibbous phase, the moon will look like a half-circle when it reaches the quarter moon phase.
After a few days, it shrinks to a sliver again, the second crescent moon. After that, the tiny sliver shrinks down until the lit-up side of the moon can’t be seen at all. It takes a month for the moon to go from the new moon through all phases and back to the new moon.
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Chris; Gillis, Jeffrey J.; Bussey, Ben. “The constitution and structure of the lunar interior.” Reviews in Mineralogy and Geochemistry.
Lang, Kenneth. The Cambridge Guide to the Solar System (2nd ed.). Cambridge University Press.
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Williams, David. “Moon Fact Sheet.” NASA/National Space Science Data Center.
Smith, David E.; Zuber, Maria. “Topography of the Moon from the Clementine lidar.” Journal of Geophysical Research.