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Celestial News: Have some fun with the harvest moon

Jimmy Westlake
For Steamboat Pilot & Today
The Harvest Moon. (Courtesy photo)

This year, summer officially ends and autumn begins at 12:21 p.m. Sept. 22, when the sun crosses the celestial equator, heading south. The last full moon of summer happens two days earlier on Sept. 20.

According to the Old Farmer’s Almanac, the last full moon of summer traditionally is known as the Full Corn Moon; however, the full moon closest to the autumnal equinox is also known as the Full Harvest Moon. Sometimes, this distinction is given to the last full moon of summer and sometimes to the first full moon of autumn, depending on which one is closer to the equinox. This year, our Full Corn Moon is also the Full Harvest Moon. You’ll see that big Harvest Moon peek over our eastern mountains at dusk Sept. 20, just as the last rays of the sun are fading in the western sky.

Watch for something unusual for several days surrounding the Harvest Moon. The Moon rises, on average, about 50 minutes later on 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 right after sunset 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, instead of rising a few minutes later on successive nights, can actually rise a few minutes earlier. Now, 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 does appear abnormally large in size. How big does it look to you? As big as a basketball or maybe a pumpkin?

The Moon’s exaggerated 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 up overhead. It just looks bigger.

Don’t take my word for it. Prove it to yourself. On Sept. 20, when you first see that big full moon rising in the eastern sky, hold out your pinky finger at arm’s length and observe that you 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 aren’t all in agreement 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 folks at sea observe the same illusion, where there is nothing on the flat 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.

For information about astronomy-related events in Steamboat Springs, including public star parties at CMC’s Ball Observatory, contact physics and astronomy instructor Paul McCudden, at pmccudden@coloradomtn.edu or 970-870-4537 or visit the SKY Club web page at coloradomtn.edu/skyclub.

Jimmy Westlake is the former full-time professor of physical sciences at Colorado Mountain College Steamboat Springs and former director of the Rollins Planetarium at Young Harris College in Georgia and the St. Charles Parish Planetarium in Louisiana. His “Celestial News” column appears monthly in the Steamboat Pilot & Today. Visit his website at JWestlake.com.


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