Comets are insubstantial bodies, loosely knit assemblages of frozen gas and dust that loop around the Sun on highly elongated orbits. They return to the inner Solar System at intervals ranging from a few years to many thousands of years, becoming visible to us on Earth as ghostly, glowing apparitions for a few weeks or months before receding back into obscurity. Our Galaxy has two small companion galaxies called the Magellanic Clouds. To the naked eye they appear like detached portions of the When far from the Sun, a comet shines only by reflecting sunlight. At that stage it is small -- usually no more than a few kilometres across -- and faint. Approaching the Sun, the comet warms up, turning its icy surface into gas. Under the influence of the Sun's radiance the gases of the comet begin to fluorescence, in similar fashion to the gas in a neon tube, thereby considerably increasing the comet's brightness. Gas and dust released from the warming comet produce a halo or coma 100,000 km or so in diameter. At the centre of the coma is the nucleus, the only solid part of the comet, consisting of a "dirty snowball" of ice, dust and perhaps some rock. In a large comet the nucleus may be a few tens of kilometres across, but most are only a kilometre or so wide. Well over a thousand million comet nuclei would be needed to equal the mass of the Earth. Not all comets develop tails, but many do. One part of the tail consists of gas blown away from th comet's head by the solar wind of atomic articles streaming from the Sun. The other part of the tail is made up of dust particles liberated from the head by the evaporating gases. Comet tails always point away from the Sun. A comet's tail can extend for 100 million km or so, further than the distance from the Earth to the Sun, as did the gas and dust tails of Comet Hale-Bopp in 1997. Yet, for all its glorious appearance, the tail is less dense than a laboratory vacuum -- stars shine through it undimmed. The tail of a comet gives it the appearance of speeding across the sky, but actually its movement against the stars is barely noticeable during the course of a night. Two dozen or more comets may be visible through a telescope each year, and several can come within the range of binoculars, but only occasionally does one become bright enough to be prominent to the naked eye. Each year's batch of comets is a mixture of known specimens returning to the Sun and completely new discoveries. About a thousand comets have well-known orbits, and more are being discovered all the time. Dedicated amateur astronomers sweep the skies to discover new comets; each new comet is given its discoverer's name. Comets with the longest orbital periods -- more than a few centuries -- are though to come from an unseen swarm of thousands of millions of comets, the Oort Cloud, that envelops the Solar System at its dim outer edges, about a light year from the Sun. The gravitational influence of passing stars nudges comets from this cloud into new orbits that bring them towards the Sun. An inner comet cloud, termed the Kuiper Belt, lies just beyond the orbit of Pluto. Most so-called periodic comets, which orbit the Sun in under 200 years, are thought to come from the Kuiper Belt rather than the Oort Cloud. Our Galaxy has two small companion galaxies called the Magellanic Clouds. To the naked eye they appear like detached portions of the The comet of shortest known period is Encke's Comet, which orbits the Sun every 3.3 years. It is so old that it has lost most of its gas and dust, and is too faint to see with the naked eye. Most famous of all is Halley's Comet, named after the English astronomer Edmond Halley, who calculated its orbit in 1705. Halley's Comet returns every 76 years or so, and last appeared in 1985-6. Its orbit takes it from 88 million km from the Sun (between the orbits of Mercury and Venus) out to 5300 million km (beyond Neptune). Dust lost from a comet disperses into space. The Earth and other planets are continually sweeping up cometary dust. When a particle of cometary dust comes whizzing into the atmosphere, it burns up by friction at a height of about 100 km, producing a streak of light known as a shooting star or meteor. The whole event is over quite literally in a flash, usually lasting less than a second. On any clear night, a few meteors are visible each hour as particles of dust dash at random to their deaths in the atmosphere. Such random meteors are termed sporadic. Occasionally, though, the Earth crosses the orbit of a comet and encounters a dense swarm of dust. This gives rise to a so-called meteor shower, in which meteors maybe visible coming from one direction in the sky at the rate of dozens per hour. The area of sky from which the meteors seem to come is known as the radiant. However, this is not the best point to look at, since the meteors are approaching head-on from this direction and so their trails are shortest; the longest trails will be seen up to 90° from the radiant. A meteor shower is named for the constellation in which the radiant lies. For instance, the Perseids, an abundant shower of bright meteors which the Earth encounters each August, seem to radiate from Perseus; the Geminids from Gemini; and so on. One historical oddity concerns the Quadrantids, which come from an area in Boötes that was once part of the now-defunct constellation of Quadrans Muralis, the wall quadrant. Our Galaxy has two small companion galaxies called the Magellanic Clouds. To the naked eye they appear like detached portions of the The strength of a meteor shower is measured by its zenithal hourly rate (ZHR), which is the number of meteors that an individual observer would see if the radiant were directly overhead in a dark sky. Since the radiant is seldom, if ever, at the zenith, the number of meteors actually seen per hour will be less than the theoretical ZHR. In addition, bright moonlight and sky glow from artificial lights will wash out the fainter meteors, again reducing the observed ZHR. Amateur astronomers make valuable observations of meteor showers, counting the number of meteors visible with the naked eye and estimating their brightness. Comfort is essential for a meteor watch, which may last several hours: wrap up warm and recline in a deckchair or on a sunbed. Typical meteors are of magnitude 2 or 3, but some are brighter than the brightest stars and the occasional spectacular example, termed a fireball, can cast shadows. Some meteors seem to split up as they fall, and some leave trains of glowing gas which take several seconds to fade. The table shows the main meteor showers visible each year. The ZHR is only a guide, and can vary considerably from year to year. One extreme case is the Leonids, normally a modest shower, which bursts into life at 33-year intervals when its parent comet, Tempel-Tuttle, returns to perihelion. An intense storm of Leonids was seen over the United States in 1966 with as many as 100,000 in an hour, like celestial snowflakes, and astronomers in Europe saw rates of about 2000 an hour in 1999.