Why is the full moon so big when it rises?

The most spectacular views of the full moon each month are right when it rises over the horizon. The same thing often happens when it sets. During this time, the moon will often be much larger in the sky than any other time of night.

Why is that? There are two popular explanations:

1. During sunrise or sunset, the moon is much closer, hence the larger size. This is wrong. While the moon is lower in the sky from your point of view, it is no closer to the Earth than any other time of day or night. The moon is actually further away from you at this point than when it is directly overhead.

2. The long path through the atmosphere magnifies the moon’s image. When the moon first rises, the image has to travel through a log of atmosphere before it hits your eyes, due to how low it is in the sky. That atmosphere serves as a lens to change the size of the image before it hits your eyes. But guess what? Any atmospheric distortion would actually make the moon look smaller, not bigger. This explanation is wrong.

So, why is the full moon so much larger when it rises or sets? By the way, the same phenomenon happens to the sun as it rises and sets. We know that the moon does not get bigger and smaller as the night goes on. (It’s not a balloon.) It turns out, and this is the most interesting part, the image of the rising or setting moon is not any bigger than it is the rest of the night. To be clear, not only is the moon the exact same size all night long, the image is the same size too. Your brain is just interpreting it wrong!

This probably does not seem believable yet. How can what appears to be a giant moon really be the same image we see later on when the moon looks so small? You will likely need more convincing so here are some tests you can use to convince you that the image you see does not change size at all.

• Take a picture of the rising full moon that looks so big. The image will not show what you think you see. The giant moon you saw will look like a normal-sized small moon in your picture….the same size as if you took a picture later on at night.

• Use both of your hands and turn them into a fake telescope. Look through your fake hand-telescope so you can see the giant moon but nothing else. The moon is no longer giant. It is normal-sized.

• If you are still not convinced there is one more test. But it is an odd one. Bend over and look at the moon with your head between your legs. The moon that was “giant” just seconds ago is now normal-sized. You have to be wondering why viewing the moon upside down and through your legs helps your brain fix the size of the moon. Truth is, scientists have not been able to fully explain why this works yet but in some way your brain no longer misinterprets the moon’s image. Maybe this is because your brain is spending all its energy wondering why you are looking at the moon upside down and between your legs!

This is not a physics problem. This entire phenomenon can only be explained as a psychological (brain) issue. Your brain is being tricked into believing that it is seeing a very large moon. As amazing and powerful as our brains are, they are not perfect. But why?

The best way to explain this is to actually view another example of it. Do a quick search for “Ponzo Illusion.” You have seen this before. If you look at two lines heading off into the horizon (like a set of train tracks) and then place two same-sized parallel lines across those train tracks, you would bet lots of money that the furthest away line is larger than the one closer to you. But they are the same. Look it up… Ponzo Illusion.

What is happening is that your brain is outsmarting itself. It knows that distant objects appear smaller so it tries to overcompensate the image as it processes it. It turns the small image into one much larger. When you think about it, this is an example of how smart our brain really is… but in this case, it is also so wrong. Illusion or not… it’s still okay to enjoy the massive looking full moon at sunrise. It’s quite beautiful.

Mike Szydlowski is a science teacher and zoo facilitator at Jefferson STEAM School.

Time for a pop quiz

1. Why does the full moon appear so much larger on the horizon?

2. What is wrong with the two most popular theories about this phenomenon?

3. If you purchase a photo of a giant rising full moon, why do you know it has been edited?

4. In what other ways is our brain tricked into seeing something that is not actually there or happening?

5. Use images on the internet or a drawing to create your own Ponzo Illusion

Last week's quiz answers

1. Why do animals mate?

The sole purpose of animal mating is to produce offspring so their species survives.

2. In which example are the offspring always identical to the mother? Why?

The whiptail lizards only have females. Their offspring are always identical clones of the mother because there is no exchange of genetic material.

3. What is evolution?

Evolution is the process where a species changes over time due to mutations over time that provide some benefit to the species.

4. How does a specific behavior evolve to become common in a species?

Evolution is not purposeful. Evolutionary change happens when a random mutation happens to be beneficial to an individual and then that advantage gives them a great chance at creating more offspring that also have that mutation.

5. Research any other animal mating behavior and explain it here.

Answers will vary.

This article originally appeared on Columbia Daily Tribune: Why is the full moon so big when it rises?