Will the upcoming harvest moon look as big as a pumpkin or small enough to be eclipsed by your pinky finger? Witness the "moon illusion" for yourself this coming weekend.

Jimmy Westlake/Courtesy

Will the upcoming harvest moon look as big as a pumpkin or small enough to be eclipsed by your pinky finger? Witness the "moon illusion" for yourself this coming weekend.

Jimmy Westlake: Shine on, harvest moon

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Jimmy Westlake

Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.

Find more columns by Westlake here.

Get ready for a big ol' harvest moon this weekend. There's a lot of folklore out there concerning the unusual harvest moon phenomenon. Let's see if we can separate fact from fiction.

The cycle of lunar phases brings us a full moon once every 29 1/2 days, or roughly once each month, and each full moon has its own name, steeped in folklore and Native American tradition. October's full moon is traditionally called the harvest moon. This year's harvest moon occurs Saturday night, and if you are the least bit observant, you'll notice something unusual for several days surrounding this full moon.

Usually, the moon 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 is rising just as the sun is setting and provides a little extra light as darkness falls. Farmers in particular welcomed the extended hours of light right at the peak time of harvesting the fields, hence the popular name for this month's full moon. 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 inclination of the moon's orbit.

Some folks are convinced 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 do you think it looks? As big as a tennis ball or a basketball? Believe it or not, you can cover that giant harvest moon with the tip of your pinky finger held at arm's length. The moon's bloated appearance when seen near the horizon is a famous optical illusion called the "moon illusion." The moon is really no larger when seen near the horizon than it is when seen overhead.

Don't believe me? Prove it to yourself. On Friday, Saturday or Sunday evening this weekend, when you first see that big full moon rising over the mountains in the east, hold up your pinky finger at arm's length and see that you really can totally eclipse the moon with your little appendage. Later in the evening, when the moon has risen higher in the sky, perform the same experiment. Seeing is believing - the results will be the same. Something about holding up your pinky finger at arm's length makes that big full moon shrink down to size.

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 against trees, mountains, houses 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 to compare it to and it looks tiny. 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 harvest moon this weekend, turn around, bend over and look at it upside down from between your legs. Lo and behold, it looks normal size again.

Shine on, harvest moon.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published all around the world. Check out Jimmy's Web site at www.jwestlake.com.

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