Astronomy and the Start of Autumn

The wonders of the fall sky and astronomy, including the equinox and blue moon and seven visible planets.

| August/September 1993

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    As fall comes, the monarch butterflies begin their annual migration.

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Once the Dog Days are over, the rest of August and September offers us the year's finest days to be outdoors and alive. This is particularly enhanced by the beautiful, somewhat bittersweet feeling that summer is gradually being lost. Once I feel the first late-August cold front truly break the siege of summer, see the swallows begin to gather for departure, and notice the sunsets get rapidly earlier, I know that summer's golden days are numbered and that autumn's pageant of beauty and change is near at hand.

Seasons and Calendar: Equinox and Blue Moon  

It's during the autumn equinox that the Sun crosses the equator, making day and night everywhere of equal length. Some cultures viewed this time period as so important that they actually began and based their calendar year at or during the equinox. Such was the case of both the ancient Spartans and Macedonians—the latter's calendar becoming of widespread importance when Alexander the Great whirled out of Macedonia and conquered the "known world." The Jewish New Year, Rosh Hashanah ("head of the year"), begins with the sunset nearest the new moon t hat falls closest to the autumn equinox. As most farmers can tell you, the harvest moon is said to be the full moon closest to the autumn equinox. However, this year the full moon of August 31 will hold its own special distinction: it will be a spectacular "blue moon:"



These days a blue moon is somewhat of a calendrical curiosity since the definition alters with time. Presently the blue moon is said to be the second full moon when there are two full moons in a calendar month. But there is also a phenomenon by which the Moon (or Sun) can actually appear distinctly blue in color (or sometimes green or "brass-colored"). This occurs when particles in clouds are all about the same (unusually small) size—often due to the effects of major forest fires or dusty volcanic eruptions. These particles scatter out the longer wavelengths of light, leaving alone the shorter wavelengths, which are bluer—or blue and green—in color. It was the pall of particles from Canadian forest fires, for example, that produced the amazing blue moon and Sun seen in New England and parts of the mid-Atlantic states on September 24, 1950. (I'll discuss this remarkable event in greater detail in future issues.)

Americans will be able to seek out the blue moon of August, as the Moon will be full on both August 2 and 31. Note: The eastern hemisphere will experience our August 31 moon on September 1 (their time) and their blue moon (the second of their month) on September 30. We can just fit two full moons into a 30-day-long month because the Moon's "synodic period" (the time from one phase to the next recurrence of that phase) is approximately 29 1 / 2 days. Of course, that explains why you'll never see the phenomenon in February—even on a leap year. But how often does a year have a month with the supposedly rare "once in a blue moon" blue moon? The interval between one such blue moon and the next is less than three years. Writer Guy Ottewell points out that about only one year in 19 has two months with a blue moon. The next will be 1999, when both January and March have two full moons. 

Astronomy: See Seven Planets  

The most exciting sky event likely to occur this season is the Perseid meteor storm.

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