Fall Into Autumn

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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. (See "Enjoy a Perseid Shower" on page 21 for discussion of this potentially stupendous event.)

Diagrams for dusk and dawn show scenes about 45 minutes after sunset or before sunrise, as viewed from 40' north latitude (approximately correct for the United States or southern Canada). "Age" of the moon is amount of time since new moon.—Adapted from Sky Calendar, Abrams Planetarium, Michigan State University, East Lansing Michigan 48824.

Besides the possible "shooting star" deluge and the interesting full moons, there are plenty of planets visible this August and September. Saturn, at its brightest, rises in the east-southeast and is visible all night long. Bright Venus and always-low Mercury are at their highest in the east before dawn (watch 45 minutes before sunup) in early August. Jupiter and Mars have a close conjunction (meeting) low in the west-southwest at dusk on September 4 to 6, and form a lovely but low grouping with the star Spica, and a lunar crescent at dusk on September 17. Have veteran observers help you glimpse the far and dim Uranus and Neptune having their last conjunctions with each other until the middle of the 22nd century.

The most cloud-free period of the year is late August to late October.

Enjoy a Perseid Shower

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