The Leonids are named after the area of sky from which they seem to originate - the sickle-shaped constellation of Leo (the Lion). The famous meteor shower appears each year between 15 and 19 November, but usually the numbers are few - perhaps 20 or 30 per hour at peak times. However, every 33 years or so the shower strengthens dramatically, with thousands of glowing meteor trails illuminating the early morning skies.

Illustration courtesy of Sky & Telescope

The most recent peak in the number of Leonids took place in 1999, when the shower produced more than 3000 meteors per hour.  Most spectacular of all were the events of 1833 and 1966, when the sky was awash with the incandescent streamers from more than 100 000 shooting stars per hour.

Most meteors are caused by cosmic dust burning up as it enters the Earth's upper atmosphere. The dust originates from comets, which for most of their elliptical orbits remain in deep freeze, far from the Sun. When they approach the Sun, however, their icy surfaces are warmed and start to vapourise, generating powerful jets of gas and dust which spurt into space. The dust which is ejected eventually spreads out around the comet's orbit.

The periodic bursts of activity that are observed in the Leonids are tied to the motion of comet P/55 Tempel-Tuttle, which returns to the vicinity of the Earth and the Sun every 33 years. Like a gigantic chimney travelling through space, the comet belches out a dense cloud of gas and dust each time that its icy nucleus vapourises in the heat of the Sun. When the Earth ploughs headlong through this cloud, frictional heating incinerates any debris that enters the atmosphere, producing the annual Leonid light show.

	Figure 2 - Right. 17 November 1966, New Mexico State University Observatory, A. Scott Murrell
Figure 1 - Left. The great Leonid shower storm of 1833 (Reproduction of a wood-cut engraving by Adolf Vollmy based upon an original painting by the Swiss artist Karl Jauslin)

The reason for the Leonids unpredictable behaviour is that, although the main stream of debris from comet P/55 Tempel-Tuttle trails for millions of kilometres behind the comet, it is not very wide, perhaps 35 000 km across. Within this narrow stream, the dust ejected during each of the comet's close approaches to the Sun forms a series of separate ribbons. Their characteristics vary considerably. Generally, the most recent dust streamers are thin and dense, while the older material, which has had time to spread out, forms wider, less densely populated bands.

The location of the stream also changes with time as the gravity of the planets, especially Jupiter, exerts an influence. Sometimes the Earth ploughs right into a dense stream of debris, causing a storm of bright meteors. Sometimes it misses almost all of the tightly confined dust trail, resulting in very few meteors that can be seen.

The Leonids are renowned for producing bright fireballs, which outshine every star and planet. Their long trails are often tinged with blue and green, while their vapour trains may linger in the sky for five minutes or more. Although the incoming particles are small, ranging from specks of dust to the size of small pebbles, the Leonids glow brightly because they are the fastest of all the meteors. A typical Leonid meteor, arriving at a speed of 71 km s^-1, will start to glow at an altitude of about 155 km and leave a long trail before it is extinguished.

The reason for this high-speed encounter is that, like their parent comet, the particles travel around the Sun in a direction almost directly opposite to the orbital motion of the Earth. The result is a head on collision with a high relative velocity between the planet and the comet's dust trail.

The unpredictable Leonids