Jimmy Westlake: The Harvest Moon illusions
September 6, 2018
Summer ends and autumn begins this year at 7:54 p.m. Saturday, Sept. 22, when the sun crosses the celestial equator, heading south. Just two days later, the moon turns full.
The first full moon of autumn traditionally is called the Harvest Moon. You'll see that big Harvest Moon peek over our eastern mountains on Monday, Sept. 24, just as the last rays of the sun are dying in the western sky.
Watch for something unusual for several days surrounding this month's full moon. The moon normally rises about 50 minutes later each successive night, but at the time of the Harvest Moon, it rises only about 25 minutes later each night.
This means that a big, bright, full moon takes the sun's place and provides a little extra light for several nights in a row, just as darkness falls. Farmers especially welcomed the extended hours of light provided by this full moon, right at the peak time of harvesting the fields.
The Harvest Moon effect is even more pronounced as you head north. In fact, up around the latitude of Anchorage, Alaska, the Harvest Moon actually can rise earlier the second night. That's just weird.
It all has to do with the 23.5-degree tilt of the Earth on its axis and the 5-degree tilt of the moon's orbit, relative to the Earth's.
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Some folks claim 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 does it look to you? As big as a pumpkin?
Believe it or not, you can cover that giant full moon with the tip of your pinky finger held out 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 up overhead.
You can prove it to yourself.
On Monday, Sept. 24, when you first see that big full moon rising in the eastern sky, hold up your pinky finger at arm's length and observe that you really can totally eclipse the moon with that tiny appendage. Later that 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 all agree on why the rising full moon looks so abnormally large. One possible explanation is that when seen low on the horizon, the moon's size can be subconsciously judged against trees, mountains and other foreground objects, and it looks large by comparison. But, when seen overhead, the moon appears in a big empty sky, with nothing nearby to judge its size, and it looks tiny. This theory, however, does not explain why sailors observe the same illusion at sea, where there is nothing on the distant horizon for comparison.
Here's another looney lunar illusion. When you see that big Harvest Moon rising and looking as big as a pumpkin, turn around, bend over and look at it upside down from between your legs. Lo and behold — it magically shrinks and looks normal size again.
Jimmy Westlake retired from full time teaching at Colorado Mountain College's Steamboat campus in 2017, after nineteen years as their Professor of Physical Sciences. His “Celestial News” column appears monthly in the Steamboat Pilot & Today newspaper. Check out Jimmy's astrophotography website at http://www.jwestlake.com.