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The Leonids (/ˈliː.ənɪdz/ LEE-ə-nids) are a prolific meteor shower associated with the comet Tempel-Tuttle. The Leonids get their name from the location of their radiant in the constellation Leo: the meteors appear to radiate from that point in the sky. They tend to peak in November.

Earth moves through the meteoroid stream of particles left from the passages of a comet. The stream comprises solid particles, known as meteoroids, ejected by the comet as its frozen gases evaporate under the heat of the Sun when it is close enough – typically closer than Jupiter's orbit. The Leonids are a fast moving stream which come close to or cross the path of the Earth and impact the Earth at 72 km/s.[1] Leonids in particular are well known for having bright meteors or fireballs which may be 9 mm across and have 85 g of mass and punch into the atmosphere with the kinetic energy of a car hitting at 60 mph. An annual Leonid shower may deposit 12 or 13 tons of particles across the entire planet. Sometimes these trails of meteoroids cause meteor showers and sometimes meteor storms.

The meteoroids left by the comet are organized in trails in orbits similar to though different from that of the comet. They are differentially disturbed by the planets, in particular Jupiter[2] and to a lesser extent by radiation pressure from the sun, the Poynting–Robertson effect, and the Yarkovsky effect.[3] Old trails are spatially not dense and compose the meteor shower with a few meteors per minute. In the case of the Leonids that tends to peak around November 17, but some are spread through several days on either side and the specific peak changing every year.[4] Conversely, young trails are spatially very dense and the cause of meteor storms when the Earth enters one. Usual counts during a storm exceed 1000 meteors per hour,[5] to be compared to the annual background (1 to 2 meteors per hour) and the shower background (a few per hour).


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