The moon illusion |

The moon illusion

After two weeks of threading its way between the stars Virgo, Libra and Scorpius, the moon shines full Monday night. To the naked eye, however, the moon will appear full tonight and Tuesday night. July’s full moon is the first of the summer and traditionally is known as the “Buck Moon.” This name comes to us from the Native Americans, who noted that July is the time of year that buck deer begin growing their new, velvet-covered antlers.

This year’s Buck Moon appears against the stars of eastern Sagittarius. The familiar teapot-shaped asterism of Sagittarius will appear just to the west of the brilliant full moon.

The Buck Moon always occurs when the moon is near its lowest point along the ecliptic — as far south as it can go. This means the moon can’t rise very high in the sky before it begins setting, barely clearing the mountains to the south. Seen low in the sky, the Buck Moon shines through a thick layer of atmosphere and often has an orange or reddish color. Perhaps this is why the Buck Moon also is known as the Blood Moon.

When seen near the horizon, the rising full moon can appear abnormally large. How big do you think it looks? As big as a tennis ball or a basketball? Believe it or not, you can cover that huge full moon with the tip of your pinky finger held at arm’s length. The moon’s enormous appearance, when seen near the horizon, is a famous optical illusion called the “moon illusion.” It really is no larger when seen near the horizon than it is when seen overhead. But it’s OK if you don’t believe me. Prove it to yourself.

On Sunday or Monday, when you first see that big moon rising in the southeast, hold up your pinky finger at arm’s length and see that you really can eclipse the moon with that tiny appendage. Later in the evening, when the moon has risen higher in the sky, perform the same experiment. Seeing is believing!

Psychologists don’t agree on why the rising full moon looks so abnormally large. One explanation is that when seen low on the horizon, the moon’s size can be judged unconsciously with trees, mountains and other foreground objects, and it looks large by comparison. But when seen overhead, the moon appears in a big, dark, empty sky with nothing by which to judge its size. This explanation, however, does not explain why sailors at sea observe the same illusion where there is nothing on the distant horizon for comparison.

Here’s another lunar illusion mystery: when you see that big full moon this week, turn around, bend over and look at it upside down from between your legs. Lo and behold. … It looks its normal size again. No fooling!

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