The Sciences

September Slowdown

Constellations seem to move more slowly in the fall.

By Bob BermanSep 1, 1994 5:00 AM


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If past years are any guide, I expect to be stuffed to semiconsciousness at the family’s Labor Day barbecue. Let’s see, with my biomass doubling each month, I will outweigh the planet in . . . how long?

No use. My brain shrugs off such effort and turns instead to the constellations now emerging from the deepening twilight. Odd--they haven’t changed from last week. Has my end-of-summer lethargy spread to the entire universe? Each September it seems that way, as a curious phenomenon afflicts the heavens. The stars now seem to move in slow motion! It’s a wonderful event for those who find inertia exciting.

Of course, the stars haven’t really slowed down. They still shift positions nightly. Why do they shift at all? It’s because our clocks are designed to be in sync with the sun, not the stars. A sidereal day--a day by the stars--is the time needed for a star to return to the same spot in the sky. That takes 23 hours and 56 minutes--one complete rotation of our planet. But a solar day is 24 hours long. By our clocks, then, the stars seem to jump like trained fleas on successive nights: they rise and set about four minutes earlier. This quickly adds up to a two-hour leap per month. In just a few weeks, the patterns now hovering above the western horizon will have set, and the east will display celestial scenery not visible before.

And the curious autumn slowdown? The culprit is fall’s earlier sunsets. Each evening the parade of seasonal constellations takes its usual little four-minute jump westward at nightfall, but nightfall comes sooner as well. So stars that would normally have disappeared below the horizon by sunset remain visible because the sky darkens earlier. The stars materialize at dusk in virtually the same spot as the night before--their nightly displacement is disguised. It’s a dramatic contrast to April, when Orion and his friends tumble into the west and disappear like gangbusters because the springtime produces the opposite effect: the later sunsets and extended daylight gobble up the stars before night sets in.

So you have all the time in the world for the beautiful constellation Sagittarius, poised at the galaxy’s center in the south, and the year’s best view of the Milky Way, splitting the heavens like a bold artist’s stroke.

The September lethargy seems to afflict even the solar system. The south’s brightest star is Saturn. You’ve got all fall to point your telescope at its beautiful rings, which are tilting ever closer to the edgewise pencil line they’ll adopt next year.

Meanwhile, Venus hovers at its brightest, low to the left of the sunset. Its broiling surface boasts the most sluggish rotation in the known universe--some 250 times slower than Earth’s. Don’t miss the planet’s dazzling conjunction on September 8 with the moon--which itself is slowing down as it spirals away from us at the rate of an inch a year.

For the cognoscenti there’s also Andromeda, with its silky galaxy, almost straight overhead, the M of Cassiopeia high in the north, and Perseus in the northeast. Within Perseus lies a glitzy double cluster of stars, visible with a telescope; each is about 70 light-years wide. All these are displayed with the frozen repose of showcase gems.

It’s as if the sky deliberately keeps pace with Earth as it prepares to succumb to its winter drowsiness. Just as I will succumb, after my Labor Day feeding frenzy.

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