Leonids

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The most famous depiction of the 1833 meteor storm actually produced in 1889 for the Adventist book Bible Readings for the Home Circle based on a first-person account of the 1833 storm by a minister, Joseph Harvey Waggoner on his way from Florida to New Orleans.

Time-lapse photograph of the 1966 Leonid meteor shower.

The Leonids 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 stream from that point in the sky.

1 Meteor Shower
2 History
3 See also
4 External links

[edit] Meteor Shower

The meteor shower is visible every year around November 17, plus or minus a week, when the Earth moves through the meteoroid stream of particles left from the passages of the comet. The stream comprises solid particles, known as meteoroids, ejected by the comet as its frozen gases evaporate under the heat of the Sun which begins to warm the comet as it comes within the orbit of Jupiter. A typical particle is no bigger than fine dust. The main source of light of a meteor is caused by the meteoroid ramming air ahead of it as it slams into the Earth's atmosphere at tremendous speed (71 km/s in the case of the Leonid meteoroids). The tremendous pressures cause the dust spec to suffer compression stress that fragments and atomizes the dust [1] and the resulting spray of microscopic debris collides with individual atoms of the atmosphere which then cool by glowing (not by friction as commonly thought). Larger particles leave a stream of smaller particles and compress larger amounts of air and form a bolide or fireball, which can leave a glowing trail in the atmosphere. Leonids in particular are well known for having such bright meteors.

The passage of the comet leaves these meteoroid particles distributed in similar orbits to the comet itself and the average peak of activity, when the Earth passes the middle of the loose clouds of particles, is November 17. In most years the rate at which meteors are likely to be seen is not great - one meteor every two or more minutes. However, in addition to the random distribution of particles some can be shepherded by gravitational and radiation influences and form some clumping of orbits of the meteoroids. These are not trains of particles - one lined up behind the other. These are places where the orbits of particles nearly meet though at other parts of their orbits they may be far apart. When such a place intersects with the passage of the Earth a meteor storm results and counts can far exceed both background levels of meteors (of about one per hour) and of even the strongest regular meteor shower (of about one per minute.)

[edit] History

Woodcut print depicts the shower as seen at Niagara Falls, New York. Mechanics' Magazine said this illustration was made by an editor named Pickering "who witnessed the scene."

The Leonids are famous because their meteor showers, or storms, can be, and have been in a few cases, among the most spectacular. Because of the superlative storm of 1833 and the recent developments in scientific thought the Leonids have had a major effect on the development of the scientific study of meteors which had previously been thought to be atmospheric phenomena. The meteor storm of 1833 was of truly superlative strength. One estimate is over one hundred thousand meteors an hour[2], but another, done as the storm abated, estimated in excess of two hundred thousand meteors an hour[3] over the entire region of North America east of the Rocky Mountains. It was marked by the Native Americans, slaves and owners, and many others. It was, in a word, unignorable. Many thought it was the end of the world.

Even outside the 1833 event, storms in peak years can feature thousands of meteors per hour; notable events were observed in 1698, 1799, 1832, 1833, 1866, 1966, 1999, 2001 and 2002. A graph published in Sky and Telescope adapted from Comet 55P/Tempel-Tuttle and the Leonid Meteors(1996, see p.6) shows relative positions of the Earth and Comet has only a vague relationship for meteor showers and storms (basically that the particles are behind and outside the path of the comet but exact paths for powerful storms are very near paths of nearly no activity.) However the work of David Asher, Armagh Observatory and Robert H. McNaught, Siding Spring Observatory, following on from research by Kondrat'eva, Reznikov and colleagues at Kazan, is generally considered the breakthrough in modern analysis of meteor storms, starting with the 1999 Leonids establishing the prime importance of meteoroid streams left behind the comet which, possibly several revolutions around the sun later, could intersect with the Earth. Whereas previously it was hazardous to guess if there would be a storm or little activity the predictions of Asher and McNaught timed bursts in activity down to five minutes though the relative brightness of the meteors took a little longer to establish. Examples of streams accounting for spikes in activity include 2004 and passages of the Comet Tempel-Tuttle in 1333 and 1733 because the Earth passed through both paths, as altered by gravitational and radiative forces over the time in between [4] while spikes in activity in 2001 were due in part to the passage of the comet in 1833.[5] The 1833 storm itself was not due to the recent passage of the comet, but from a direct hit with the recent 1800 passage trail left behind [6] and the 1966 storm was from the 1899 passage of the comet.[7]

However, a close encounter with Jupiter is expected to perturb the comet's path, and many streams, making storms of historic magnitude unlikely for many decades.