Why is the moon at the beginning of every day bigger than the moon at the top?

Why does the moon appear so big when it is just rising or about to set? This is the moon illusion, a trick the brain plays on us. Photos prove that the Moon is the same width near the horizon as it is high in the sky, but that's not what we see with the naked eye. So this is an illusion rooted in the way the brain processes visual information. Although we have observed it for thousands of years, there is still no satisfactory scientific explanation.

Go out on a full moon night and find a great vantage point to watch it rise. The moon will make any stargazer take their breath away and say "Wow!" in awe. When we look at the Moon near the horizon, it often appears very large—whether it's rising over a distant mountain ridge or sea, or hanging behind a cityscape, looming behind a dense grove of trees. The truth is: it's all only in your mind. Indeed, the Moon's appearance of being larger is a real optical illusion, rather than being affected by our atmosphere or some other physical aspect. There are many ways you can verify this yourself.

How to confirm the moon illusion?

Put your index finger next to the moon. You will find that the moon is roughly the same size as your fingernail. Or try looking at the moon through a rolled-up paper tube; or bend down and look back through the gap between your legs. Either way, you'll find that the moon isn't as big as it looks.

Another, more reliable way to check the size of the moon is to take a picture of it. Keeping the camera's focus setting unchanged, take a photo when the moon is close to the horizon and when it is high in the sky. Comparing the moons in these two photos, you will find that the two moons are touching each other and have exactly the same width. (The moon in the picture close to the horizon may look a little flat vertically, because the atmosphere acts like a weak lens.)

Photographers can use a telephoto lens to take photos close to the ground. The moon is framed by nearby buildings, mountains, and trees to simulate the moon illusion. So when you see a gorgeous photo of a huge moon, don't forget that this is a distant, near-Earth object captured with a zoomed-in lens. That is, the moon appears larger in these photos only after magnification.

The 2017 "Super Full Moon" rising over Washington. A super full moon occurs when the moon reaches its orbit closest to the earth (perigee) and coincides with a full moon. Image source: NASA/Bill Ingalls

The moon does look more golden when it is closer to the horizon

It is worth noting that there is indeed a difference when the moon is closer to the horizon. The Moon tends to have a more golden, even orange, hue when it hangs low in the sky than when it hangs high in the sky. This is because moonlight has to travel a longer distance to pass through the atmosphere and reach people's eyes. As the path length of moonlight increases, the shorter, bluer wavelengths of light are scattered away, leaving more of the longer, redder wavelengths. (Dust and pollutants also deepen the reddish color.)

Why do we have the moon illusion?

Be prepared – we really don’t know why. Well, actually you can’t say that. It depends on how you think about it: this news may be unsatisfying, or it may be awe-inspiring to the wonders of the brain. But the truth is, even though people have been observing the moon illusion for thousands of years, we still lack a solid scientific explanation.

Overall, possible explanations relate to several key factors in how we visually perceive the world: How does our brain perceive the size of objects that are closer or further away from us? When an object approaches the horizon, how far away do we think it actually is? Our brains don't seem to know that no matter where the moon is in the night sky, the distance between the earth and the moon actually doesn't change much.

Some people also speculate that the object in front of the moon disrupts people's vision and creates an illusion. Maybe those nearby trees, mountains, and buildings trick our brains into making it seem like the moon is actually closer and larger. An effect known as the Ponzo illusion, discovered a century ago, provides a related explanation. The Ponzo illusion describes a scene in which two straight lines extend into the distance like railroad tracks, and there are two other horizontal lines of equal length at different distances. Surprisingly, the two horizontal lines do not appear to be the same length. This is because the way your brain processes distance forces you to see this. This is the same as forced perspective in painting.

But this explanation is still not perfect. NASA astronauts in orbit can also see the moon illusion, and there are no foreground objects in their line of sight to interfere with distance judgments. So we still have to work hard to find out why. Why not be beautiful?

Although the reason for the moon illusion has not been completely explained, we still have to say that whether it is truth or an illusion, a huge full moon hanging high in the sky is indeed a wonderful sight. So until someone can figure out what our brains are doing, we'll just have to enjoy the moon illusion and the ever-changing, hauntingly hazy scenery it brings.