Science, Art, Litt, Science based Art & Science Communication
Size constancy and moon illusions
Q: Is there a scientific explanation for why people perceive a full moon to be larger than other moons?
Krishna: The moon stays approximately the same size, whether measured by apparent size or actual size.
The moon varies in its distance from the Earth so it does not always appear the same size.
The Moon has an elliptical orbit with a perigee (shortest distance to Earth) of 362,200 km and an apogee (longest distance to Earth) of 405,400 km.
So when it comes nearer, we see a big moon.
When a full Moon occurs at the closest point to Earth during its orbit, it appears larger and brighter, becoming what is commonly known as a 'supermoon'. A super moon is widely recorded as being much larger and far brighter than the Moon on any other night - and there's an easy explanation for it.
Supermoons are not unusual. They occur as a regular part of the Moon's orbit of Earth. There is no formal scientific definition of a supermoon, but it occurs when there is a full Moon and the Moon is also closest to the Earth. It is very rare that an orbiting body follows an exactly circular path. The Moon has a slight "eccentricity", meaning it travels in an elliptical path around the Earth - so it is sometimes nearer and sometimes further away.
The distance of the moon from the earth only changes by about 10%. This means that the observed diameter of the moon only changes by 10% throughout the month (1).
Perigee is the point at which the Moon is closest to the Earth. When the Moon is at this point, it appears around 10 to 14% bigger than a full Moon at the furthest point away, at apogee - when the Moon is sometimes called a 'micromoon'.
There are only usually three or four supermoons in a year.
On rare occasions, a lunar eclipse occurs on the same night as a supermoon. Because the Moon looks red, it is known as a 'super blood Moon'.
Even more rarely, this also coincides with a blue Moon, the name given to the second full Moon in a calendar Month.
Super moon ( Source: google)
Supermoons cast about 30% more light on the planet than when the Moon is at its dimmest. This is because as it is closer, more of the Sun's rays that reflect off the lunar surface make it to Earth.
Sometimes full Moons coincide with the Moon passing especially close to Earth. When this happens, it results in a particularly spectacular supermoon.
Moon looks big when it's rising or setting. It is called the Moon Illusion. It is a trick the brain plays on us.
Moon Illusion (Source: Google)
The Moon is actually the same width near the horizon as when it's high in the sky, but that's not what we perceive with our eyes. So it's an illusion rooted in the way our brains process visual information.
You can actually prove this illusion wrong!
Hold up your outstretched index finger next to the Moon. You'll find that your fingernail and the Moon are about the same size. Or try looking at the Moon through a paper tube, or bend over and look backward between your legs. When you view it like this, the Moon will be nowhere near as big as it had seemed.
Another way to check the size of the Moon is to take a photo when it's near the horizon, and another when it's high in the sky. If you keep your camera zoom settings the same, you'll find that the Moon is the same width, side to side, in both photos. (It may actually appear a little bit squashed in the vertical direction when it's near the horizon. This is the result of the atmosphere acting like a weak lens.)
We don't really know. Maybe it depends on the way our brains have evolved and work on the Earth!
The reason the moon looks bigger is purely in our minds. Our mind interprets the things we see in interesting ways.
In general, the proposed explanations have to do with a couple of key elements of how we visually perceive the world: how our brains perceive the size of objects that are nearer or farther away, and how far away we expect objects to be when they're close to the horizon. It seems that our brains don't know that the Moon's distance doesn't change that much no matter where it is in the sky on a given night.
There's also some thinking that objects in the foreground of your lunar view play a role. Perhaps trees, mountains, and buildings help to trick your brain into thinking the Moon is both closer and bigger than it is (2).
We only think the moon looks bigger on the horizon because of a psychological trick. When the moon is low, we can visually compare it to distant mountains and conclude that the moon is much bigger than a mountain. But when the moon is high, there is nothing to compare it to, so our minds perceive it to be smaller. The exact mechanism is still debated by scientists.
There's an effect discovered a century ago called the Ponzo illusion that describes how this works. In the illusion, you have a scene where two lines are converging, like railroad tracks stretching away into the distance. On top of these lines are drawn two horizontal bars of equal length. Surprisingly, the horizontal bars appear to be different sizes, because your brain's hard-wired sense of how distance works forces you to perceive it this way. This effect is related to how forced perspective works in paintings.
Another theory about the moon illusion says that when the moon is near the horizon we perceive the moon's distance to be farther away from us than when it is high in the sky. But since the moon is actually the same size, our minds make it look bigger when it is near the horizon to compensate for the increased distance.
An alternative explanation holds that the moon illusion is caused by the way our eyes focus on distant and close objects. When we focus on the horizon moon, we focus on the moon at a great distance. The overhead moon lacks visual cues that tell us how far away the moon is, so we focus on the moon as if it was a short distance away.
Shape constancy and size constancy also play a role
Size Constancy
Take a look at the picture below. All poles are the same size, but appear to be different. Your mind accounts for the distance and decides that the back pole is farther away that the front pole , so your brain adjusts the size to make up for the increased distance. This phenomenon is called size constancy.
Size constancy is happening all the time. If you look down the street and see a sports car about 50 feet away, and behind it, about 100 feet away is a big SUV, you know that the SUV is bigger, even though it produces a smaller image on your eye.
However, there is no right answer to the Q, "Why Moon illusion occurs?". But the one thing people agree on is that the moon does not physically change its size or distance from Earth at any point during the moon's orbit; it's an optical illusion. It's all in our heads.
Footnotes:
1. https://www.wtamu.edu/~cbaird/sq/2012/12/15/why-does-the-moon-get-b....
2. https://science.nasa.gov/solar-system/moon/the-moon-illusion-why-do...
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