Jimmy Westlake: The Harvest Moon
October 12, 2008
“Shine on, shine on, Harvest Moon!” That old, classic tune refers to an astronomical event called the Harvest Moon. What is the Harvest Moon, and what’s so special about it?
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 traditionally is called the Harvest Moon. This year’s Harvest Moon occurs Tuesday night, and if you are the least bit observant, you’ll notice something unusual for several days surrounding this full moon. Normally, the moon rises about an 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 moon rises just as the sun sets and provides a little extra light as darkness falls. Farmers, especially, 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 can actually rise earlier the next night! It all has to do with the tilt of the Earth’s axis and the inclination of the moon’s orbit.
Some folks are convinced that 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 big Harvest 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. Don’t believe me? Good! Prove it to yourself. On Monday or Tuesday night this week, when you first see that big old moon rising in the east, hold up your pinky finger at arm’s length and see that you really can totally 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 – the results will be the same!
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, 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 week, 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. He is an avid astronomer whose photographs and articles have been published on the websites of CNN.com, NASA’s “Astronomy Picture of the Day” website, Spaceweather.com, Space.com, Discover.com, MSNBC.com, NationalGeographic.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines. His “Celestial News” article appears weekly in the Steamboat Pilot newspaper. His “Cosmic Moment” radio spots can be heard on local radio station KFMU. Also, check out Jimmy’s website at http://www.jwestlake.com.