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 they only happen around once a year. So what’s going on? Your brain is trying to find out 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 kinds of 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 if it’s projected on a dome, similar to a planetarium. Actually, big moon, small moon, or half-moon are just illusions of the eye and brain.
Why does the moon look so big?
The moon looks bigger when it’s on the horizon because of something called 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, mainly because its elliptical orbit takes it closer or farther from the earth over time.
During a night, though, it’s a tiny change. And despite what our brains try to tell us, the moon is about 2% smaller when it’s on the horizon compared to 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 a little 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,” and it has fascinated humans for a very long time. The moon looks bigger on the horizon, where it’s farther away. The human brain changes the sizes of things to add extra contrast. Ebbinghaus illusion state that brains are used to objects moving towards the horizon becoming smaller like clouds. So when our brain sees the moon at the horizon appearing the same size in our retina, it compensates for perspective and assumes it.
So if we see something like a person get smaller, 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. When the Moon is near the horizon, it must be farther away, 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 all the far stuff like stars and the moon onto an imaginary dome above 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, just like the Earth, has a day side and a night side: a part that’s facing the sun and a part that’s not. So when we see a tiny sliver of the bright moon, we’re seeing a small amount of the moon that’s facing the sun. At the same time, the rest of the side that’s 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. As it moves around the planet, the day-time the side lit up from the sun faces different directions.
During its month-long trip in orbit around the Earth, the moon’s bright part appears to grow and then shrink from right to left! The different shapes that we see of 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 bit of a sliver, kind of like 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 just a tiny sliver of light, and it’s almost full! The moon is at its brightest at the full moon.
That’s when the fully-lit side of the moon is facing 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 moon is full, it starts to shrink back down to a gibbous moon. But this time, the left side is shining bright instead of the right! And after the gibbous phase, the moon will look like a half-circle again when it reaches the quarter moon phase.
And then, after a few days, it shrinks down to a bit of 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.
Read more: Why Moon Is White Colour?