Moon Illusion: Why Does the Moon Look So Big Tonight? | The Old Farmer's Almanac

The Moon Illusion: Why Does the Moon Look So Big Tonight?

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The moon sometimes appears enormous, and this is because of the Ponzo Illusion.

Learn Why the Moon Appears Larger on the Horizon

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Who hasn’t turned down a road to confront a low, horizon-hugging Moon that seems absolutely enormous? Why does the Moon look so big? This is called the “Moon Illusion”! Bob Berman explains.

The Moon Illusion

The Moon looks especially large shortly after it rises while still touching the horizon. But it’s just the result of a trick your brain is playing.

Many people assume that this common effect is caused by our atmosphere magnifying the image, but the explanation for this optical illusion is far simpler:

When the Moon is high overhead, it is dwarfed by the vast hemisphere of the heavens and appears to our eyes as a small disk in the sky. 

By contrast, when the Moon is low, it is viewed in relation to earthly objects, such as chimneys or trees, whose size and shape provide scale. Your brain compares the size of the Moon to the trees, buildings, or other reference points, and suddenly, the Moon looks massive! 

Yup, it’s that simple!

Next time you notice a gigantic Moon on the horizon, look again when the Moon is higher up. With nothing to compare the Moon to, it will look a lot smaller.

How does the Moon illusion differ from a “supermoon”? Read up on the supermoon so as not to confuse the two!

A “Moon dog” can also occur when the moon is low on the horizon; learn more about the phenomenon of a Moon dog.

Why Does the Moon Appear Orange When Low in the Sky?

When the Moon appears extra-large near the horizon, you may also notice that it seems more orange or red. This is where Earth’s atmosphere comes into play.

When the Moon is low in the sky, it is farther away from you than directly overhead. Because of this, the light being reflected off of a horizon-hugging Moon has to travel farther—and through more particles of air—to reach your eyes.

By the time we perceive this light, the shorter wavelengths of light (the “blue” ones) have been scattered by the air, leaving only the longer wavelengths (the “red” ones) to reach our eyes. Thus, to us, the bluish hues are filtered out, and the Moon takes on an orange tinge!

In contrast, when the Moon is directly overhead, its light doesn’t need to travel through as many air particles, and the blue wavelengths can reach our eyes.

The Moon Illusion: How to See Through It

Don’t believe your eyes? Here’s how to reduce the Moon from enormous to ordinary!

  1. Find a paper tube, like the kind that holds paper towels.
  2. Close one eye and look through the tube at the enlarged Moon. It will appear normal.
  3. Now close the eye in the tube and open your other eye. The Moon appears huge again.
  4. Observe the Moon with the tube when it’s high and again when it’s low in the sky. The Moon will appear to be the same size both times.

When will the Moon rise? See the Almanac’s Moonrise/set calculator and just type in your zip code!

And here’s a handy trick to know “When Will the Moon Rise Today.” 

About The Author

Bob Berman

Bob Berman, astronomer editor for The Old Farmer’s Almanac, covers everything under the Sun (and Moon)! Bob is the world’s most widely read astronomer and has written ten popular books. Read More from Bob Berman