Earthly Shadows

By Bob Berman|Sunday, September 01, 1996
RELATED TAGS: SOLAR SYSTEM
Just days after the autumnal equinox, as if to celebrate the start of fall, the full moon wanders into the spot of sky exactly opposite the sun, ushering in the last total eclipse in the United States until the year 2000.

You would think that total eclipses of the moon would take place more frequently. Earth’s million-mile-long shadow stretches way beyond the quarter-million-mile-away moon.

At that distance, our tapering shadow is still twice the moon’s width, so it doesn’t take much luck for the moon to plunge into it, especially since the lunar orbit is inclined a mere 5 degrees from the sun- Earth plane, in which our shadow is cast. Even when it hits off-center (as it does this month) instead of cleanly, the moon manages a total eclipse.

Lunar eclipses are also visible to all who have the moon in their sky at the time--half of Earth’s population--unlike total solar eclipses, which are seen from just a narrow geographic band. You’d think lunar eclipses would be as common as bottle caps.

But no. Lunar eclipses are almost twice as rare as the solar variety. And scarcer still are those in which the eclipsed moon is visible above the horizon for the entire mainland United States, as it will be on September 26. We’ve had only four such eclipses in the past quarter century.

A lunar eclipse occurs at the antisolar point of the heavens-- precisely opposite the sun--so the moon must be illuminated at a 180-degree angle. In other words, it must be perfectly full. Normally the full moon is the sky gazer’s scourge, the least interesting phase because craters and other topographic features are whitewashed into oblivion. But an eclipse changes everything. Suddenly this pariah moon undergoes an hour-long series of peculiar patterns, displaying eclipse-induced slices unlike anything seen throughout the lunar month. Finally the moon buries itself fully in Earth’s shadow and turns--red!

No, not black--except on rare occasions. You and I may cast a dark shadow when we walk down the street, but Earth’s is coppery red. That’s because all our planet’s sunrises and sunsets refract their ruddy light into the shadow.

Sprawling across the dry lunar terrain, that red light enlivens our drab, ashen neighbor with the only color its surface ever displays. Furthermore, any extensive cloud cover or volcanic dust over parts of Earth will grant an artistic variety to the moon’s eclipsed appearance; it may look two-toned, irregular, or, every now and then, even an ominous black. (I’ve seen the moon become pitch black and impossible to find.) One never knows, and such uncertainty gives lunar eclipses a tantalizing unpredictability.

In contrast to solar eclipses, which happen at differing times depending on when the moon’s shadow sweeps across a location and blocks out the sun, lunar eclipses are seen by all observers simultaneously. Of course, your unique location on our planet’s curved surface determines how high the moon appears in your sky when it enters Earth’s shadow, and what your clock reads. Eastern states will find the event beginning at 9:12 p.m., with mid-eclipse at 11 and the moon high up. The farther west viewers are, the lower the moon will appear in the sky, and at a time closer to sunset. The West Coast sees the eclipse beginning in twilight, shortly after moonrise.

On that same night Saturn, too, stands opposite the sun and at its closest, brightest, and biggest of the year. This star just below the moon presents the final touch to September’s spectacle--a rare and wonderful nexus at the antisolar point of the heavens.
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