黃思樺 Lisa 沈羿呈 Regina 陳怡君 Jocelyn Astronomy JRL 黃思樺 Lisa 沈羿呈 Regina 陳怡君 Jocelyn 黃信彰Jason 蘇桂田Jube

Meteors and meteor showers Those spending enough time under the night sky eventually will see a "shooting star," a streak of light that flashes across the sky in less than a second. This is a meteor, a glowing trail caused by the incineration of a piece of celestial debris entering our atmosphere. Many meteors are quick flashes, but some last long enough for us to track their brief course across the sky. Under a dark sky, any observer can expect to see between two and seven meteors each hour any night of the year. These are sporadic meteors; their source bodies — meteoroids — are part of the dusty background of the inner solar system. A bright Geminid fireball lights up the sky above Organ Pipe Cactus National Monument, Arizona. In this wide-angle view, Orion the Hunter (top right) hovers above Sirius (bottom right), the brightest star in the sky. Frank Zullo

Quadrantids Generally visible between December 28 and January 6, the Quadrantids have a sharp activity peak around January 3. Typical rates vary between 40 and 100 per hour; about 5 percent leave trains. When the shower was first recognized as annual in 1839, the radiant occurred in a constellation no longer recognized — Quadrans Muralis (Wall Quadrant). It's now divided between Hercules, Boötes, and Draco. The cold nights of northern winters and typically faint meteors keep this shower from being truly popular. For many northern locations, the Quadrantid radiant is above the horizon all night long, but observing is best after local midnight.

Lyrids The Lyrids appear from April 16 to 25 and peak (at 10 to 15 per hour) around April 21; the radiant lies between Hercules and Lyra. Chinese observations of this display date back to 687 B.C., making the Lyrids the earliest recorded meteor shower. Astronomers recognized the Lyrids as an annual shower in 1839 and connected it it to its parent comet, C\1861 G1, in 1867. Lyrid meteors are bright and rather fast (30 miles [48 km] per second), and about 15 percent leave persistent trains.

Southern Delta Aquarids This is the most active of a diffuse group of streams and, as the name suggests, is best seen in the Southern Hemisphere. These meteors may be seen between July 12 and August 19 and peak (15 to 20 per hour) near July 28. The meteors are medium speed (27 miles [43 km] per second); they tend to be faint, and few leave trains.

Perseids The best known of all meteor showers, the Perseids never fail to put on a good show and — thanks to the shower's late-summer peak — are usually widely observed. The earliest record of this event comes from China in A.D. 36. Generally visible from July 17 to August 24, meteor speed (37 miles [60 km] per second), brightness, and a high proportion of trains (45 percent) distinguish the Perseids from other showers active at this time. It became the first meteor shower linked to a comet (109P/Swift-Tuttle) in 1865.

Orionids This is the sister stream of the Eta Aquarids, also arising from the debris of Halley's Comet. Discovered in 1864, the Orionids were not linked to Halley until 1911. Orionid meteors can be found between October 2 and November 7, with a peak of about 25 per hour around October 21. Orionid meteors are among the fastest (42 miles [67 km] per second); they generally are faint, and about 20 percent leave trains that persist one or two seconds.

Leonids Leonid meteors generally arrive between November 14 and 21, with a peak hourly rate on November 17 of between 10 and 15 meteors per hour; about half of these meteors leave trains that can persist for several minutes. Because Earth runs into the orbiting particles almost directly head-on, Leonid meteors travel faster than those of any other shower — 45 miles (71 km) per second. The shower's most notable feature is its habit of producing periodic, dramatic meteor storms as Earth intercepts streams of dense material ejected at previous returns of Comet Tempel-Tuttle.

Geminids The Geminids are active between December 7 and 17 and peak near December 13, with typical hourly meteor rates around 80 but occasionally more than 100. Because the Geminids intersect Earth's orbit near the side directly opposite the Sun, this shower is one of the few that are good before midnight. The parent body of the Geminids is a curious object designated 3200 Phaethon. What makes Phaethon interesting is that it appears to be an asteroid instead of a comet. Planetary scientists suggest that many of the asteroids whose orbits cross Earth's may be, in fact, worn-out comets.