Science: Close call with a passing giant: Heather Couper and Nigel Henbest suggest unusual features to look for in the king of the planets

LOOK up on any clear night in April and you cannot fail to spot the star of the show - the biggest planet, Jupiter. This month it is the most brilliant object in the sky after the sun and moon and is visible all night.

The reason for its prominence is that Jupiter moves into 'opposition' on 30 March, lying directly opposite the Earth in its orbit, and therefore it is at its closest to our planet. At 680 million kilometres (420 million miles), it is hardly a stone's throw away, but its combination of high reflectivity and enormous size makes Jupiter a striking sight - the brightest planet after our neighbour-world Venus.

Unlike the other planets, Jupiter is well worth looking at through a reasonable pair of binoculars. Support them steadily - by resting your elbows on a fence, for instance - and you will see two unusual things.

First, it isn't round. Like a tangerine, it conspicuously bulges around its middle. Despite its enormous size (1,300 Earths could fit inside), it is the fastest-spinning of all the planets. Its 'day' is less than 10 hours long and the breakneck rate of spin forces the equatorial regions outwards.

The second thing you will notice are tiny specks of light - up to four - lined up either side of the equatorial bulge. These are the largest of Jupiter's enormous family of 16 moons.

The first person to see Jupiter's biggest moons, through the newly invented telescope, was the German astronomer Simon Marius in 1609. He also gave them their names - Io, Europa, Ganymede and Callisto, after four of Jupiter's lovers - but his name is no longer associated with them. The far more outspoken Galileo observed the moons a few days after Marius and used his discovery that they actually circle Jupiter to support his assertion that the sun - and not the Earth - lay at the centre of the solar system.

Here was proof that some bodies in space were not in orbit about the Earth, as the orthodox teachings had maintained, and Galileo held the moons up as a cornerstone of his theory. As a result, the four are known as the Galilean satellites.

Galileo would be amazed to discover what a fascinating bunch his moons have turned out to be. All of them are larger than Pluto, and Ganymede is even bigger than Mercury. In 1979 the two Voyager space probes swept past them and returned detailed images of their surfaces.

As well as looking at Jupiter's moons, the Voyagers scrutinised the planet itself in unprecedented detail. They discovered a world made almost entirely of gas - mainly hydrogen and helium - whose cloud tops are twisted into fantastic loops and whorls by violent storms. The planet has a powerful magnetic field thousands of times stronger than the Earth's, with radiation belts that are a hazard to interplanetary craft.

Because Jupiter is so huge, its central regions are highly compressed and, like the compressed gas in a bicycle pump, very hot. The temperature at Jupiter's core is 35,000C. Had Jupiter been 50 times heavier, its core would have become hot enough to fuse hydrogen, and the planet would have turned into a star.

For the past three and a half years the Voyagers' successor - named after Galileo himself - has been en route to Jupiter. Due to arrive in December 1995, the Galileo probe will go into orbit about the giant planet, becoming - appropriately - Jupiter's first artificial moon.

While the first generation of planetary craft made only a nodding acquaintance with their targets, the new probes are designed to monitor and survey the worlds of our solar system over much longer periods. Galileo is scheduled to spend two years 'touring' the Galilean moons, following up the Voyager discoveries. As well as the main probe, the spacecraft carries a bullet-shaped 'atmosphere probe' whose fate is to be fired into the storm-tossed clouds of the giant planet.

Galileo is not without its problems. One is its lengthy, zigzag trajectory through the solar system, which means it will take six years to get to Jupiter compared with two years for the Voyagers. This is because the launching device on the space shuttle was replaced by a safer, less powerful model after the Challenger disaster.

More seriously, its main antenna - furled like an umbrella during storage and launch - has never properly unfurled, despite repeated attempts. A recent try, following Galileo's swing-past of the Earth last December, was unsuccessful.

The project's scientists have all but conceded defeat - but all is not lost. There is a much smaller antenna on board that can relay signals back to Earth quite efficiently, but at a much slower rate. This means that, although scientists will eventually be able to get their hands on almost all the data they are anticipating, there will be far fewer spectacular pictures.

The planets

AT BEST, Mercury rises just half an hour before the sun this month and it will not be visible in the dawn twilight. Venus, however, is another matter. Having dominated our winter skies at dusk, this month it swings round in front of the sun to take up residence in the skies of dawn. By the end of April it will be rising an hour before the sun and, being close to the Earth, will appear near its brightest at magnitude minus 4.4

In contrast, Mars is fading fast as it and the Earth draw apart. The relative motion between the two planets is causing Mars to drift into line with the twin stars Castor and Pollux - the position in which it started off last autumn. Mars is visible as soon as the sun goes below the horizon, and does not itself set until 3am BST.

Jupiter is on view nearly all night and is the brightest object in the sky after the sun and moon. On 6 April the full moon will lie just a few moonwidths below the giant planet in the constellation of Virgo.

Ringworld Saturn moves at its crawler pace from Capricornus into Aquarius this month. Dedicated Saturn-spotters will see it as a brightish 'star' rising at about 3.30am BST by the end of the month.

The stars

WITH Leo riding high, Arcturus at the end of the Great Bear's tail and Virgo climbing in the south-east - while Orion flees westwards - it is safe to say that spring is really here.

Virgo, the second largest constellation in the sky, plays host to a motley crew of celestial bodies this month. First there is the full moon on 6 April, 384,000km away. Then there is Jupiter - considerably more distant at 680 million kilometres, but well within our solar system.

Spica, the brightest star in Virgo, is much more of a giant leap - 260 light years, or about 2,500 million million kilometres, away. Scan the 'bowl' of Virgo's Y-shape with binoculars and you should spot some fuzzy blobs - just a handful of the thousands of galaxies (star-cities) that go to make up the giant Virgo cluster of galaxies.

The nearest giant cluster of galaxies to our Milky Way, the Virgo Cluster is 50 million light years, or about 500 million million million kilometres, away. The 'depth' you can see in this small patch of sky covers a ratio of 1,000 million million to one.

Put in more Earthly terms, if you represent the distance to the Virgo Cluster by the distance between London and Carlisle, the Earth and moon would be separated by a space no larger than the diameter of an atom.

Diary (all times BST)

1 Venus at inferior conjunction

6 7.44pm Full moon

13 8.39pm Moon at last quarter

22 12.49am New moon

29 1.41pm Moon at first quarter.