On clear, dark nights, it's often possible to see shooting stars — brief, brilliant streaks of light that race across the sky. Of course, these luminous streaks aren't actually stars. They're the fiery trails of fast-moving bits of space rock — meteors — that burn up when they hit Earth's atmosphere.
Sometimes, large numbers of meteors appear to come from the same part of the sky, a phenomenon known as a meteor shower. Spectacular meteor showers can yield more than one shooting star a minute; a shower that took place in November of 1833 produced 100,000 meteors every hour.
There's a meteor shower almost every month, though some offer better skywatching than others. The most spectacular ones tend to be the Perseids, which occur each year in mid-August, and the Leonids each November.
What causes meteor showers?
Most meteor showers occur when Earth's orbit around the sun takes it through the debris that trails behind comets, icy bodies that orbit the sun. The Perseid meteor shower, for example, is caused by debris from Comet Swift-Tuttle, while the Leonids in November are associated with Comet Tempel-Tuttle.
"Comets shed debris, dust and ice, when they move around the sun," says Bill Cooke, a meteor expert at NASA's Marshall Space Flight Center in Huntsville, Alabama. "We get a meteor shower every time the Earth runs into that debris."
A few meteor showers are caused not by comets but by large space rocks known as asteroids that trail small particles that hit Earth's atmosphere. The Geminid meteor shower in December is caused by the asteroid 3200 Phaethon; the Quadrantids in late December and January are caused by the asteroid 2003 EH1.
Though they're caused by debris within our solar system, meteor showers are named after the constellations of stars from which they appear to come. The Leonids seem to radiate from the constellation Leo, the Perseids from the constellation Perseus, the Geminids from Gemini and so on.
How big are meteors?
Meteors associated with meteor showers are typically tiny, from smaller than a single grain of sand to the size of a small pebble. Because they're so little, most burn up entirely in Earth's atmosphere and never hit the ground.
A few meteors, however, are so big that they don't fully burn up in the atmosphere but fall to the ground, at which point they become meteorites.
Meteorites can leave impact craters and sometimes widespread destruction. Astronomers believe Chesapeake Bay was formed by a gigantic meteorite impact about 35 million years ago.
Which meteor showers are most spectacular?
The brightness of meteors depends on their size and speed. The faster and the bigger they are, the more spectacular the streaks of light they create.
The brightest meteor shower of the year is usually the Perseids, created by very fast-moving meteors. Fifty to 100 meteors an hour can often be seen at the shower's peak.
Other major meteor showers are the Orionids in October; the Leonids; the Geminids; the Quadrantids in December and January; the Lyrids in April; the Eta Aquariids in May; and the Delta Aquariids in July.
The Leonids are among the fastest meteors, with speeds of about 44 miles per second. Meteors from slower showers like the Alpha Capricornids move about 15 miles per second.
What is the best way to see a meteor shower?
The best viewing comes on nights when meteor showers are at their peak. The American Meteor Society publishes annual meteor shower calendars to help observers plan.
Find a dark place, away from sources of light, and simply lie back and look straight up into the sky. No special equipment is needed, nor is there any need to look at any particular direction. The shooting stars should be visible in every part of the sky.
Meteors can be hard to see on nights when the moon is full or nearly full. "The light from a full moon washes out the faint meteors and you only see the bright ones," Cooke says. "So instead of seeing, say, a Perseid every minute, it is going to knock that right down to maybe a Perseid every four or five minutes."
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