Jupiter is the king of the planets, and the most fascinating of all to study with small instruments. Humble binoculars reveal the planet's cream-coloured disk and four main moons, called the Galilean satellites after the Italian scientist Galileo Galilei who discovered them in 1609. Some people with exceptionally acute vision can see the Galilean satellites with the naked eye as faint starlike points either side of the planet. A small telescope brings into view some of the details of Jupiter's disk: dark belts of cloud parallel to the equator and an eye-shaped marking in the southern hemisphere known as the Great Red Spot, first observed in 1831 (a similar feature was seen as early as 1664). Careful study of these features reveals that Jupiter's rotation period varies with latitude, from 9 hours 50 minutes at the equator (the fastest rotation of any planet in the Solar System) to 9 hours 55 minutes at higher latitudes. In addition, the Great Red Spot drifts slightly in relation to its surroundings. These effects demonstrate that the visible surface of Jupiter is not solid. We are looking at clouds, constantly seething and swirling, changing in colour and shape. Jupiter never appears the same twice. That is its attraction. Jupiter orbits the Sun every 11.9 years at an average distance of 778 million km, over five times the distance of the Earth, and is well placed for observation every 13 months. Because Jupiter's cloud features are so impermanent and mobile, it is impossible to give more than a generalized description of the planet's appearance. Its disk is crossed by alternating bright and dark bands, termed zones and belts respectively. Frozen ammonia crystals form high, cold clouds in the bright zones, where gas ascends; the darker belts, where the gases descend, are lower and warmer (although 'warm' is only a relative term, for the temperature of the cloud tops is around -150°C). The colours of the belts can vary from yellow and brown to orange, red or even purple as a result of complex chemicals in the atmosphere of Jupiter, sulphur being one of the prominent colorants. High speed winds of up to 500 km per hour whip the edges of the zones and belts into turbulent eddies, giving them a scalloped appearance. The weather on Jupiter is unpredictable. Dark and light spots can suddenly erupt in the clouds, lasting for weeks or even decades before fading away. One of the main roles of amateur observers is to track these storms as they erupt and move around the planet. Of all the markings on Jupiter, the most famous, and by far the most permanent, is the Great Red Spot. It certainly is great: 14,000 km wide and as much as 40,000 km long, enough to swallow three Earths. But it is not always red: most often it is pinkish, and sometimes it can fade to a colourless grey. Its colour is believed to be due either to red phosphorus or sulphur. By good fortune, the spot was particularly prominent when the Voyager 1 and 2 spacecraft reached the planet in 1979. Even now its nature is not fully understood, but it appears to be an upwardly spiralling column of gas similar to a hurricane on Earth, its top spreading out about 8 km above the surrounding cloud deck. Jupiter's other, smaller spots, including a series of long-lived white ovals, are believed to be similar swirling storm systems. As if to emphasize the storminess of the planet, the Voyagers and the more recent Galileo probe photographed massive flashes of lightning on Jupiter's night side, far larger than in any thunderstorm on Earth. The key to Jupiter's meteorology is the fact that it gives off twice as much heat as it receives from the Sun. The planet was hot when it formed, and still retains some of that heat today. This internal store of heat drives the complex cloud systems of Jupiter, keeping the Great Red Spot and its smaller relatives alive for far longer than any storms can persist on Earth. Interestingly, Jupiter has virtually the same chemical composition as the Sun: mostly hydrogen and helium. There is thought to be a rocky core about twice the size of the Earth at Jupiter's centre, but no space probe could ever land on it. Beneath the wispy high-altitude clouds of frozen ammonia are complex chemicals that give the dark belts their colour. Deeper still, temperatures are similar to those on Earth and clouds of water vapour condense. About 1000 km below the visible cloud tops, temperatures and pressures have increased to the point where hydrogen is compressed into a liquid. The liquid hydrogen seas of Jupiter are about 20,000 km deep. Below, under the crushing pressure of 3 million Earth atmospheres, hydrogen is compressed into a superdense state with the properties of a metal; hence it is known as metallic hydrogen. Convection within the hot metallic hydrogen interior of Jupiter is thought to be responsible for the planet's intense magnetic field, 10 times stronger than the Earth's, extending out for 100 times Jupiter's radius into space. If the magnetic field around Jupiter were visible to the naked eye, it would appear over twice the size of the full Moon. One of the most remarkable events in the history of planetary observation took place in 1994 when Comet Shoemaker-Levy 9, which had previously been captured into orbit around Jupiter and broken into more than 20 fragments, crashed into the planet, leaving dark markings in its clouds that could be seen through small telescopes. Over the ensuing months, the dark patches spread out into a band around Jupiter that took over a year to fade away. Crater chains on Jupiter's moons Callisto and Ganymede suggest that they, too, have been struck by fragments of disintegrated comets in the past. Jupiter possesses a fascinating collection of satellites, like a mini Solar System. With the simplest optical aid the four largest, known as the Galilean satellites, can be seen performing a merry dance around Jupiter, changing position from night to night - sometimes out of sight behind the planet, sometimes transiting across its face and sometimes being eclipsed in its shadow. The closest to Jupiter of the Galielean satellites is Io, 3630 km in diameter (slightly larger than our own Moon), orbiting every 42½ hours. Io is the most volcanically active body in the Solar System. The Voyager 1 probe in 1979 photographed eight volcanoes erupting simultaneously on it. Hundreds of other volcanic vents were visible, although not actually erupting. Those volcanoes erupt not just molten rock (lava) as on Earth but also sulphur which solidifies to form the garish red, orange and yellow of Io's surface. Io is molten because it is caught in a gravitational tug of war between Jupiter and the other Galilean satellites; their opposing pulls release tidal energy that melts Io's interior. Io recycles its interior onto its surface, endlessly turning itself inside out. Some of the sulphur escapes and showers onto the innermost moon of Jupiter, Amalthea, giving it an orange coating. Amalthea is an irregularly shaped lump of rock only about 200 km in diameter, too faint to be seen in amateur telescopes. Within the orbit of Amalthea the Voyager probes discovered a faint ring of dust, extending inwards to a mere 30,000 km above Jupiter's cloud tops. This tenuous ring of Jupiter is believed to result from the break-up of one or more tiny moons, two of which, Adrastea and Metis, were discovered by the Voyagers, orbiting at the ring's outer edge. Moving outwards past Io we come to Europa, smallest of the Galilean satellites with a diameter of 3140 km. Europa is encased in a white shell of ice, veined with fine cracks probably produced by tidal forces. Underneath its fractured icy crust, Europa may have an ocean of liquid water. Next in line from Jupiter is Ganymede, the largest and brightest of the satellites; what's more, at 5260 km in diameter it is the largest moon in the Solar System, larger even than the planet Mercury. Ganymede and the fourth of the Galilean satellites, Callisto, 4800 km in diameter, are both balls of rock and ice, rather like giant muddy snowballs. Callisto is saturated with impact craters, the largest 300 km in diameter and called Valhalla; it is similar to the large basins on the Moon and Mercury, surrounded by wave-like ridges. Ganymede is also cratered by impacts, but less heavily than Callisto, and exhibits a strange grooving on much of its surface, apparently formed by faulting and stressing of the moon's icy surface layers. Bright spots on Ganymede and Callisto mark places where recent impacts have exposed fresh ice. The rest of Jupiter's 17 known moons are small and insignificant. Several of these - particularly the outer four, which move in a highly elliptical, retrograde orbits - were probably once passing bodies that were captured by Jupiter's gravity.