Jimmy Westlake: The harvest moon illusion
September 24, 2012
The first full moon of autumn traditionally is called the "harvest moon." Watch for that big harvest moon to rise over the eastern mountains just as the sun sinks in the west this Saturday.
You might notice something unusual for several days surrounding this full moon. The moon usually rises about one hour later each night, but at the time of the harvest moon, it rises only about 25 minutes later each night for several nights in a row. This means that a big, bright full moon takes the sun's place and provides a little extra light just as darkness falls for several consecutive evenings. Farmers in particular welcomed the extended hours of light provided by this full moon right at the peak time of harvesting their fields.
The effect is even more pronounced the farther north you go. In fact, up around the latitude of Anchorage, Alaska, the harvest moon actually can rise earlier the second night. It all has to do with the 23.5-degree tilt of the Earth's axis and the 5-degree tilt of the moon's orbit.
Some folks say the harvest moon looks much bigger than other full moons. When seen near the horizon, the rising full moon can appear abnormally large in size. How big does it look to you? As big as a basketball? Believe it or not, you can cover that giant 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 is really no larger when seen near the horizon than it is when seen overhead.
Prove it to yourself. On Saturday, when you first see that big moon rising in the eastern sky, hold up your pinky finger at arm's length and see that you really can totally eclipse the moon with your smallest 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 large. One explanation is that when seen low on the horizon, the moon's size can be judged in comparison to trees, mountains and other foreground objects, but when seen overhead, the moon appears in a big, dark, empty sky with nothing nearby to judge its size. This explanation, however, does not explain why sailors at sea observe the same illusion when there is nothing on the distant horizon for comparison.
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Here's another lunar illusion mystery: When you see that big harvest moon rising, turn around, bend over and look at it upside down from between your legs. Lo and behold — it looks normal size again.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Visit hiswebsite at http://www.jwestlake.com.
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