Wednesday, July 27, 2005
Grain by grain, our Earth grows
John Goss
John Goss is chairman of the Mid-East Region of the Astronomical League and a former president of the Roanoke Valley Astronomical Society.
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Blog
The Earth is growing. Our tiny blue planet sweeps up nearly 140 million tons of space debris every year. From where does it all come?
An unimaginable number of stone-sized, rocky asteroids orbit the sun. Some of them eventually are captured by Earth's gravity and survive a fall through our atmosphere. We call these rocks meteorites. But, as can be seen on August nights, there is another source that continually fattens our planet. In the extreme outer reaches of the solar system lies the realm of comets. Once in awhile, a cometary body swings close to the sun, ejecting gas and dust from its ancient nucleus, and sometimes forms a spectacular 10 million mile long tail. Years later, if its orbital path intersects that of the Earth's, some of the remnants of the tail enter our planet's upper atmosphere.
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| Click the image for a map of the Roanoke Valley's August sky |
A portion of this material is composed of sand-sized grains that streak through our tenuous atmosphere some 70 miles high. Fortunately for us, the frictional heat they generate quickly disintegrates them long before they reach the ground. Thus, a meteor shower is born.
In the very early hours of Aug. 12 and 13, the Earth will be in the middle of a monthlong passage through a debris field shed many years ago from the tail of Comet Swift-Tuttle. The result? Southwest Virginia stargazers enjoy the Perseid meteor shower.
Viewing the Perseids is easy, especially this year when the near first-quarter moon sets just before midnight on the 11th and 12th. After moonset, find an area that's away from city lights - perhaps at an overlook along the Blue Ridge Parkway. Bring bug spray and sit in a comfortable chair preferably, but not necessarily, facing the northeast and wait for the meteors to come to you.
If you're lucky, you'll spot 30 or more in less than an hour. The Perseids emanate from an area in the constellation Perseus, which is just below the more familiar "W" of Cassiopeia. Meteors found in this part of the sky are relatively short because they are traveling approximately toward you - don't worry, they completely vaporize 50 miles up!
Longer trails can be found farther from the W. Can you see any "trains" that meteors leave in their tracks? Binoculars will be virtually useless because, moving at 40 miles per second, these grains of comet sand will be gone before you can exclaim, "Oh, my!"
More than 4,000 times farther away than speeding meteors lies our moon. Early evening stargazers will see a pretty sight Aug. 7 when the moon shimmers next to bright Venus. In the early evening of Aug. 11, it passes just north of the bright star Spica. Use binoculars to discern the moon's orbital motion.
Venus shines low in the western sky 45 minutes after sunset. All month long, it appears not to change its position each evening as twilight ends. Jupiter, on the other hand, appears to slowly creep toward the brighter Venus. On Aug. 31, they sit close to each other, making an attractive pair in the deepening twilight.
Meteor watchers will have a hard time ignoring the bright red object rising in the east before midnight. Mars is getting closer and brighter every night.
On warm August nights, see if you can spot a few Perseid meteors. Thanks to Comet Swift-Tuttle, you're witnessing Earth's weight gain, grain by grain.
