You probably know already that if you care to stay up tonight for an hour beyond the witching hour, and then gaze into the eastern sky, you may be rewarded by a fireworks display of shooting stars. But then this is one area where the astronomers, normally such faultless forecasters, can sometimes be caught out. The Leonids, and indeed any of the other meteor showers, are unpredictable.
Shooting stars, or meteors, are a kind of space pollution; tiny specks of interplanetary dust that become visible by frictional incandescence as they encounter the rarefied air of the Earth's upper atmosphere. Occasional meteors may be seen on any clear, moonless night, but at certain times of the year their number increases very noticeably when Earth, in the course of its passage around the sun, intersects a concentration of the meteoric dust.
The result is what we call a "meteor shower".
Most of the tiny specks of dust that end up as meteors are scattered widely around the galaxy, but thick concentrations of such material are often associated with a comet, and "showers" occur when the Earth passes through the dustiest part of a comet's tail, when it crosses the orbital path of the comet.
In the case of the Leonids the body in question is Comet Temple-Tuttle, discovered in December 1865 by a Mr Temple, and rediscovered independently a month later by his colleague, Mr Tuttle.
Like others of its kind it orbits the sun on an eccentric, elongated path, and approaches our particular corner of the universe every 33 years. The Earth is in the vicinity of its orbital path at around this time every year.
The really spectacular Leonid showers occur near the time when Temple-Tuttle is at perihelion, at its nearest point to the sun on its 33-year odyssey. They were unmissable in 1799, 1833 and 1866, but disappointing on other such occasions as, for example, in 1933.
The last perihelion was in 1966, and on November 17th that year observers in the United States saw a veritable storm of shooting stars, raining down - harmlessly, of course - at a rate of 40 every second.
In 1999, although the Earth will reach Temple-Tuttle's orbit 622 days after the comet has passed by, the distribution of its dusty debris may provide another notable display. One encouraging sign is that the 1998 shower was not similar to that of 1965, the year before the storm of 1966.
Most astronomers are not expecting a comparable display in 1999, but a spectacular show cannot be ruled out. If it happens, it will peak at about 2 a.m. tomorrow.