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September 28, 2001 Harvest moon is a wonder to scientists, too, Purdue experts sayWEST LAFAYETTE, Ind. – Before the frost is on the pumpkin, before the autumn leaves show their colors, the arrival of fall is announced by a bright orange celestial ball hanging low in the evening sky. The harvest moon is often the first beautiful sign of the new season – but why does the harvest moon appear so different from the way the moon looks the rest of the year? Scientists know some of the reasons, but other questions about the harvest moon are thought to be the oldest scientific questions still up for debate. By popular definition, the harvest moon is any yellow or orange full moon that appears in late September, October or early November. However, the technical definition of a harvest moon is the full moon nearest the autumnal equinox, or the arrival of fall, on around Sept. 22. It can arrive either before or after this date; this year's harvest moon arrives on Tuesday (10/2) at 1:47 p.m. local time. Robert Nowack, professor of earth and atmospheric sciences, says equinox means equal nights, the two days a year when there are exactly 12 hours of daylight and night. "At full moon, the sun and the moon are opposite each other. So just as the sun is setting, the moon is rising, and you have a full moon right at dusk, which is very bright," Nowack says. Years ago, people named various moons: April has the grass moon, and a full moon in late August or early September is a fruit moon. "The full moon after the harvest moon is the hunter's moon," Nowack says. But the harvest moon is the one of the 12 full moons that everyone knows, and the name is self-explanatory: This is the moon that provides light during the time of harvest. Richard Grant, professor of agronomy, says the harvest moon is important to farmers. "Today we have short- and long-season varieties of our crops, and the crops are not planted all at the same time, so the harvest time is spread over several weeks. But before cross-pollinated crops were introduced in the 1930s, all of the fields of a crop had essentially the same variety and had to be harvested at nearly the same time," Grant says. "And the quality of the crops is diminished by frost as well as moisture content from rain. So there wasn't much time to get the crops in once they were ready. Farmers depended on the light of the moon to have a good harvest." Today, most farmers have bright automobile-style headlights on their combines to work late into the night during harvest, but not all. "For some Anabaptist farmers, such as the Amish, the harvest moon can still be important," Grant says. Perhaps the most striking thing about the harvest moon is its color, but Grant says the moon itself doesn't change color. What changes is the air quality in the lower atmosphere. "The moon reflects the light of the sun, and so what we're seeing from the moon should be white to gray," Grant says. "As you look at the moon when it is near the horizon, the light has to pass through a lot of dust and dirt in the air, and that makes it look yellowish or even orange." Grant says the same phenomena can also be seen when a setting sun turns red as it approaches the horizon. "It's possible that harvest activities contribute to the colorizing effect of the atmosphere because during the harvest the grain is separated from the chaff and dust and dirt are thrown into the air," he says. Color isn't the only striking thing about the harvest moon – when it pops over the horizon, it appears twice as large as when a moon is high overhead, and this large size, together with the bright color, gives the moon an unnatural, otherworldly appearance. It is unnatural, in the sense that the moon doesn't really change its size during the night. The XXL-sized lunar image is actually an optical illusion known as the "moon illusion," but how that illusion happens is something scientists have yet to agree on. Ancient writings indicate that people in China and Egypt were thinking about the moon illusion thousands of years ago, which has caused some people to label the moon illusion as the oldest scientific dispute. One explanation of why the moon appears so large is that it is caused by the bending of light called refraction. Just as light bends as it passes through water, light bends as it passes through the atmosphere. Like a lens in a pair of glasses, the air bends the light and makes the moon appear larger. Because the autumnal air is colder and denser, especially near the surface on clear nights, the light bends more than it does in the summer. "If you have very cool conditions near the surface, then the light gets bent more, and the moon looks bigger as it rises. Near the horizon it looks huge in those cool evenings," Grant says. "The same thing happens with a setting sun." Others say that the moon appears larger because of a psychological illusion, caused by the size of objects nearer on the horizon, which scientists call "size-distance perception." Proponents of this theory claim that if you make a tunnel out of a cupped hand and look at a harvest moon through your hand, it will appear smaller. "It's probably a combination of both psychological and natural phenomena," Nowack says. A cousin to the harvest moon, the blue moon has a more convoluted history but a simpler explanation. A blue moon is commonly defined as the second full moon in a month. However, this definition began as an erroneous answer to a question put to Sky & Telescope magazine in 1946. This simple definition of a blue moon has become so popular that it is now widely accepted, even among scientists. Unfortunately, the moon almost never looks blue during these events. There are four full moons between Oct. 1 and Dec. 1, and only three months, so obviously there will be a blue moon in one of those months. However, because two of the full moons appear on Nov. 1 and Nov. 30 Greenwich Mean Time, the month the blue moon is in depends on where you are because of the vagaries caused by time zones and the International Date Line. From Chicago to Tokyo, the blue moon will appear in October. From Tokyo to Moscow, the blue moon will appear in December. And from Moscow back to Indianapolis, the blue moon will appear in November. "The timing of the next blue moon depends on where on the globe you happen to be," Nowack says. "In this case, because of the timing, a couple of hours really does determine which month has a blue moon." An earlier definition of what constitutes a blue moon created by The Farmer's Almanac defines a blue moon as the third full moon in a season that has four full moons (most seasons have three full moons). By this definition, the next blue moon will occur on Aug. 22, 2002. However, a true blue moon can occur at anytime, and has nothing to do with the calendar. The moon really appears blue when there is a large amount of dust very high in the earth's atmosphere, such as following a major volcanic eruption or a huge wildfire. The dust particles scatter more of the red spectrum of light than the blue, so the air becomes a blue filter and the moon appears blue when it is high overhead. Writer: Steve Tally, (765) 494-9809; tally@aes.purdue.edu Sources: Richard Grant, (765) 494-8048; rgrant@purdue.edu Robert Nowack, (765) 494-5978; nowack@purdue.edu Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; https://www.agriculture.purdue.edu/AgComm/public/agnews/ Related Web sites: Purdue News Service: (765) 494-2096; purduenews@purdue.edu
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