Leonids

Leonids, annual meteor shower that occurs in the month of November. Meteor showers are intense displays created by meteors, or pieces of rock that enter Earth’s atmosphere from space.

The Leonid shower occurs every year between about November 14 and 20, with the peak rate usually falling on November 17. The peak rate for this shower is usually about 10 to 15 meteors per hour, but it can increase to several thousand meteors per hour when Earth crosses Comet Tempel-Tuttle’s orbit while the comet is nearby. Comets leave trails of dust in their orbits, and when Earth passes through the comet’s orbit, the dust entering the atmosphere causes a meteor shower. The concentration of dust is usually much higher when the comet is nearby at the time that Earth crosses its orbit.

The Leonid meteors appear all over the sky, but they seem to radiate from one point in space. The meteors actually move in parallel paths, but the same effect that makes train tracks seem to converge on the horizon gives the impression that the meteors have a central radiant. The Leonids’ radiant is in the constellation Leo.

The Leonid meteor shower is mentioned in Arab and Chinese chronicles dating from about ad 900. The modern study of meteor showers was inspired by a Leonid storm in 1833. The storm was most visible in the sky above the eastern United States. Very bright meteors fell at a rate of thousands per hour for an extended period of time and caused a panic. Scientists put forth a number of theories to explain the shower.

American astronomer Denison Olmstead noticed that the meteors seemed to radiate from one point in space, suggesting that they were coming from far away. He also noticed that a similar shower had occurred the previous year in Europe. These observations led him to conclude that the storm was caused by a cloud of particles in space.

German astronomer Heinrich Olbers discovered many historical accounts of similar meteor storms, allowing him to predict a return of such an intense shower about every 33 years. There was another storm in 1866. In 1865 and 1866 French astronomer Ernst Tempel and American astronomer Horace Tuttle independently discovered the comet that became known as Comet Tempel-Tuttle. Several astronomers noticed a correspondence between the period of the comet and the cycle of November meteor storms, making the Leonids the second meteor shower to be associated with a comet. (The first comet and meteor shower pair was Comet Swift-Tuttle and the Perseid meteor shower.) The Leonid shower of 1998 had peak rates of several hundred per hour.