for National Geographic News
The Perseid meteor shower peaks this week, and in the Northern Hemisphere the Perseids may be the best—or at least most convenient—meteor-watching event of the year (see Perseids pictures). So what's it all about?
What're Perseids? What's a Meteor Shower?
Like all meteor showers, the Perseids are set in motion when a comet leaves behind a trail of rocky debris, or meteoroids, as the icy orbiter circles the sun (comet facts).
Each year, Earth passes through the debris of comet Swift-Tuttle. The meteoroids get incinerated in our atmosphere, and the heated air makes the showy streaks we see as meteors, or shooting stars.
Because Swift-Tuttle's shooting stars appear to streak outward from a point near the constellation Perseus, we call them the Perseids.
Perseids' Double Discovery
Within a few days of each other, U.S. astronomers Lewis Swift and Horace Parnell Tuttle separately discovered comet Swift-Tuttle during the U.S. Civil War in 1862.
The Perseid parent's next appearance was in 1992. With its roughly 130-year orbit, comet Swift-Tuttle is expected again in 2125.
(Related: "Comet Swarm Delivered Earth's Oceans?")
Meteor? Meteorite? Meteoroid?
When we see the Perseids streaking through the sky, we call them meteors. But the terms can get tricky.
Meteoroids are pieces of rock and ice in outer space. It's when the meteoroids enter Earth's atmosphere and burn up—as many Perseids did last night and will continue to this week—that they become meteors.
If a meteor survives and ends up on the ground, it's rechristened a meteorite. But don't bother starting a Perseid-meteorite search party.
"Since Perseids are ice with a little dust mixed in, they never make it to the ground," said Bill Cooke, a meteoroid expert for NASA in Huntsville, Alabama.
The greatest concentration of Perseid meteors will hit Earth's atmosphere this afternoon, when they should be largely invisible in North America. But tonight should be just as good a show as last night (Perseids picture from 2008).
To watch, pack a blanket, bug spray, and snacks, then lie on your back away from city lights, with a view of as much of the sky as possible. The best viewing hours should be whenever skies are clear and whenever the moon isn't present. For example, the U.S. East Coast should have moonless skies between about 10:45 p.m. and 1 a.m. (check your local moonrise and moonset times).
Look for the shooting stars to streak out from the northeast to points across the sky, especially at and after midnight (see animated diagram below).
Beyond the Perseids: Next Big Meteor Shower
The next big meteor shower will be the Geminids, which begin in early December and peak close to the middle of that month.
With typical peak rates of about a hundred shooting stars an hour, the Geminids are always spectacular—lighting conditions, moon glare, and cloud cover notwithstanding.
According to Cooke, the Geminids are "the best shower of the year, but you've got to brave cold winter nights."
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