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Why summer moons appear bigger, brighter, and more colorful

Even though the moon's size remains constant throughout the year, our brain perceives it as larger when it's near the horizon. Here's why.

MOLINE, Ill — You are enjoying a warm, summer evening on the patio around sunset looking east when suddenly an enormous, colorful moon begins to rise on the horizon. At times early on, it looks like it stretches hundreds of miles across! As the night progresses, it appears to get smaller as it reaches higher into the sky. So, what gives? Why do moons during the summer months appear so much bigger, and brighter, and also appear to have more color? Let's dig in.

This phenomenon, known as the "moon illusion" has been well known for thousands of years. Cameras don't capture it, but our eyes do. It's an optical illusion! What makes the moon appear so low in the sky is the fact that the sun and the full moon are on opposite sides of the sky. During the summer, when the sun is high, the full moon is low. 

Credit: NASA

Same size, different appearance

When looking at the moon, rays of moonlight converge on each other and form an image about 0.15 mm wide in the back of your eye. Both high and low moons make the same-sized spot. So, why do they appear to be different? Cue Ponzo's Illusion. 

Credit: WQAD
Even though both yellow boxes are the same size and width, the upper box appears bigger.

Ponzo's Illusion

Researchers believe that the Moon Illusion is related to Ponzo's Illusion. Foreground objects trick your brain into thinking that some objects are bigger than they really are, like the moon for example. Also, remember that the sky isn't a flat dome, like we perceive it to be. 

You can experience this illusion right here in this story using the picture above! Focus on the two yellow boxes. At first glance, the yellow box at the top appears to be wider, bigger, etc. compared to the one at the bottom. In reality, when you connect to the two with the magenta-colored lines, they are indeed the same size! 

Something else to try

Try looking at the full moon directly and then through a narrow opening, like the cardboard tube from a toilet or paper towel roll. This will hide the foreground terrain. See if you can make the illusion vanish! 

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