Sky Lights

The sky's ups and downs—not to mention rights and lefts—are full of meaning

By Bob Berman|Monday, August 02, 2004
RELATED TAGS: COSMOLOGY

Outer Space has no latitude or longitude, no north or south, no vertical or horizontal. And yet direction—or sometimes the lack of it—is crucial to understanding our place in the universe.

WHAT’S UP IN THE AUGUST SKY

Good news for early risers: Venus dazzles in the predawn sky, and the Perseid meteors peak after midnight. Meanwhile, evening planets Jupiter and Mars slide toward invisibility.

AUGUST 5   Neptune lies at opposition, closest to Earth, but is still too faint for the naked eye.
AUGUST 11–12 The Perseid meteors streak the sky, with little interference from the moon.
  AUGUST 13 Saturn returns to view and meets up with the crescent moon before sunrise.
AUGUST 17 Jupiter pairs with the moon, a swan song low in the sky at twilight.
AUGUST 17 Venus reaches its greatest elongation from the sun and dominates the predawn east.
  AUGUST 27 Uranus comes closest to Earth. Binoculars show it as a green dot in Aquarius.

Take the Milky Way, the starry band of light that encircles the sky. Observers in the 19th century discovered an odd pattern: No spiral nebulas could be found within its boundaries, even though they were common in other parts of the heavens. So prominent was this absence that scientists called the region around the Milky Way the Zone of Avoidance. During the 1910s, American astronomer Heber Curtis and others deduced that dusty gas in our galaxy blots out the light of more distant objects when we look lengthwise through our flattened galaxy. In directions roughly perpendicular to the plane of the Milky Way, such as toward the Big Dipper, we look through much less obscuring material and get an open window to the larger universe beyond. Curtis concluded that spiral nebulas must lie well outside the bounds of our galaxy. We now know that those whirlpools are other galaxies, many as majestic as our own.

While Curtis was pondering the Zone of Avoidance, his rival Harlow Shapley was contemplating another type of directionality: the uneven distribution of globular clusters, spherical swarms of up to a million stars. He noted that the 130 known globular clusters were sprinkled more on one side of the sky than on the other. (One of the brightest of these clusters, the glorious M13 in Hercules, appears in the west on August evenings, easily seen through binoculars.) When he measured the distances to these clusters he found that they were arranged in a halo around a remote part of our galaxy. Shapley recognized that the midpoint of the halo must be our galaxy’s nucleus. Our solar system, long thought to reside at the heart of the Milky Way, turned out to lie 26,000 light-years away, more than halfway to the rim.

More recently, directionality—or rather, its unexpected absence—helped solve a long-standing cosmic riddle. Since the early 1970s, scientists had debated the nature of gamma-ray bursts, enigmatic flashes of energetic radiation that pop off hundreds of times a year. The prevailing theories held that the objects doing the bursting must lie inside our galaxy. But in 1991, data from the orbiting Gamma Ray Observatory showed that the bursts appeared equally in all parts of the sky, not just along the band of the Milky Way. That meant they must come from an external source. Further study showed that gamma-ray bursts are the most distant and powerful explosions in the universe.

On the largest scale, the whole universe seems to display a preferred orientation. All of space is filled with a faint crackle of microwaves, believed to be energy left over from the Big Bang. In the 1970s, cosmologists determined that the microwaves appear hotter than average toward the constellation Leo and cooler than average at the opposite side of the sky. The surprise implication: We are plunging—literally—in the direction of Leo at a rate of 375 miles per second, drawn in by the gravitational pull of vast clusters and superclusters of galaxies. As we race through space, microwaves from the forward part of the sky gain some energy, just as raindrops hit more vigorously against the windshield of a moving car.

Backyard astronomers can witness a small-scale version of this process on August 11, when the Perseid meteor shower reaches its peak. At this time each year, Earth passes through a cloud of fragments from comet Swift-Tuttle. They arrive from the constellation Perseus in the northeast, almost straight ahead of us in space, so they hit at a blistering 37 miles per second and leave bright streaks. By contrast, the Geminid meteors on December 13 travel at a right angle to Earth’s motion, producing relatively cool, leisurely trails as they disintegrate.

Even when there is no shower, you will always see twice as many meteors after midnight than before simply because you are then on the side of Earth that faces forward in our motion around the sun. It matters where you look!

 

As the solar system wheels through the Milky Way, our perspective on the universe will slowly shift. In 100 million years or so, the Andromeda galaxy—now the nearest and brightest spiral—will disappear from our skies, blocked from view by clouds of gas and dust in the bulge at the Milky Way’s center.
 

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