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As Earth wheels about the sun in its vast, nearly 200-million-mile diameter orbit, from time to time it encounters dusty debris that has been boiled and blown from the heads of comets as they periodically approach and then recede from their gravitational master, the sun.
When such debris strikes Earth’s atmosphere, it does so at speeds in excess of 30 miles per second, or over 100,000 mph. The friction between such a high-speed particle and the atmosphere causes the particle to heat up, glow and then burn out as it falls toward Earth. The overall effect is visible briefly from the surface of Earth as a “falling star,” a “shooting star” or, more formally, as a meteor.
Most of the meteors that strike Earth’s atmosphere are random events and occur every night of the year at a rate of about five to six meteors per hour. Such sporadic meteors originate from cometary material that, over the eons, has dissipated throughout the inner regions of the solar system.
There are, however, streams of cometary debris which have not yet had a chance to disperse and are thus characterized by higher particle densities over much smaller volumes of space. When Earth passes through one of these meteoroid streams, the rate of observed meteors increases noticeably. Such an increase in the rate of observed meteors is referred to by astronomers as a meteor shower. Because these meteor showers strike the earth’s atmosphere in parallel paths, to an Earth-based observer they appear to diverge from a “radiant” point in the sky, much as parallel rails on a train track appear to diverge as they approach us from the horizon.
The position of this radiant among the constellations determines the name of the meteor shower. Thus the Orionids of October appear to radiate from Orion’s “club” stars, the November Leonids from the Sickle of Leo the Lion, and so on.
Meteor shower displays can vary considerably from the thousands of meteors per hour which were observed for the Leonids in 2001 to barely a dozen an hour for some of the lesser displays, which astronomers sometimes refer to whimsically as meteor “sprinkles.”
One of the oldest and most storied of the meteor showers, the Perseids, visits the Colorado sky during the second week of August. The Perseid meteor stream is thought to be associated with a comet known to astronomers as Swift-Tuttle and boasts recorded observations dating as far back in time as 36 BC.
As one might imagine, legends and folklore concerning the Perseids abound. For example, the prominent early Christian Saint Lawrence was tortured and killed by the Roman Emperor Valerian on Aug. 10, 258. Two days later, a prominent display of the Perseids was explained in some Christian quarters as being the fiery tears of the slain martyr. Over the centuries the “Tears of Saint Lawrence” have always dutifully appeared on or near the feast day of this great saint of the church.
Typically, at the time of maximum activity for the Perseids any observer may expect to see about 50 to 60 meteors per hour radiating from the “head,” or northernmost region, of the constellation of Perseus the Greek Hero. In recent years, however, this shower has exhibited brief bursts of activity in which Perseid meteor counts have, for 20 to 30 minute periods, soared to rates of over 200 meters per hour.
This year promises an excellent year for the Perseids. The moon will set at about 1 a.m. on the peak night of Aug. 11-12, and meteor experts are predicting Perseid rates as high as 160 meteors per hour. If the skies are clear on those nights, look to the northeast from a dark-sky observing site, preferably after midnight, and at the very least you should see a meteor a minute from this most prominent and consistent of the annual meteor showers.
Elsewhere in the sky: The planets Jupiter, Mercury and Venus hug the western horizon nearly all month long. At the start of August, the three planets lie along a straight line from Jupiter to the upper left through Mercury to Venus at the lower right. The planetary trio is joined by the crescent moon on Aug. 4 and 5. As the month progresses, Mercury slides back toward the solar glare, while Venus continues to rise farther out of the twilight. On the evening of Aug. 27, Venus and Jupiter will exhibit an extremely close pairing or conjunction, while Mercury will be located below and to the left of the duo for about a half hour after sunset.
The planets Mars and Saturn are located in the constellation of Scorpius and form an attractive trio with the bright ruddy Scorpius star Antares. Mars will pass directly between Antares and Saturn on the evening of Aug. 23. An interesting naked eye exercise is to compare the colors of Mars, Antares, and Saturn for a few nights around the 23rd.