What are the Leonids?
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Every year in mid-November, Earth passes close to an area of the debris trail left from the passing of periodic comet Tempel-Tuttle. The trail is littered with very small sand grain sized pebbles and dust that have been eroded off the comet by the solar wind and radiation. As Earth collides with this trail of dust every year, we usually can observe a meteor shower with some 15 to 20 meteors visible per hour.

Comet Tempel-Tuttle orbits the sun once every 33 years. When the comet is close to the sun, it comes close to Earth's orbit replenishing the vicinity with fresh meteoric debris. It is just after this comet passes close to Earth's orbit, as it did in 1999, that we have a higher probability of a meteor storm. Fortunately, the comet crosses Earth's path when the Earth is on the other side of the sun so there is no chance the comet slamming into Earth.

As pieces of this cometary debris enter Earth's atmosphere at an extremely high rate of speed, they sometimes cause the atmosphere to glow along their path leaving a visible ionized meteor trail. As Leonid meteors appear to emanate from, or point back to, the constellation Leo, it is known as the annual Leonid Meteor Shower.

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Leonid
n.狮子座流星群
Leonid Meteor Shower
狮子座流星雨