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Stars and Planets: April 2005

Posted on: Wednesday, 30 March 2005, 06:00 CST

IT'S THE clash of the titans this month - the two biggest planets in the solar system, Jupiter and Saturn, are both brilliantly on view in the night sky.

People often ask us if it's possible to see these planets, but in April there's no getting away from them. They're the brightest objects on view, and the key to recognising them is that they don't twinkle like stars.

Jupiter is particularly bright this month. On 3 April it's at "opposition" - meaning that it's opposite the Sun in the sky, and at its closest to the Earth. "Close" is a relative term, however - the planet is still more than 600 million kilometres (373 million miles) away. But Jupiter is so vast - at 143,000 kilometres (88,860 miles) in diameter, it could contain 1,300 Earths - and as it's made almost entirely of gas, it's very efficient at reflecting sunlight.

Although Jupiter is so huge, it spins faster than any other planet in the solar system. It rotates every nine hours 55 minutes, and as a result its equator bulges outwards - through a small telescope, it looks a bit like a tangerine crossed with an old- fashioned humbug. The humbug stripes are cloud belts of ammonia and methane stretched out by the planet's dizzy spin. You can actually watch the cloud formations changing minute by minute.

Space missions to Jupiter have revealed how active the planet is. It has a fearsome magnetic field that no astronaut would survive; huge eruptions of lightning; and it radiates more energy than it receives from the Sun. Its core is so squashed by Jupiter's mighty bulk that it simmers at a temperature of 20,000C.

In some ways, Jupiter has a lot in common with a star. It commands its own "mini-solar system" - a family of some 60 moons. The four biggest are visible with good binoculars, and even - to the really sharp-sighted - to the unaided eye. We remember a 78-year- old lady at an evening class asking "what are those little dots either side of Jupiter?".

These are worlds in their own right - Ganymede is even bigger than the planet Mercury. But the "star" has to be Io, with its incredible geysers, which erupt plumes of sulphur dioxide 300km (186 miles) into space.

Compared to Jupiter, Saturn - the next planet out - is a picture of calm.

With its serene rings, it looks quite unreal - like a model world, suspended in the blackness. Like Jupiter, Saturn is another gas-giant.

The second-biggest planet in the solar system, Saturn is so insubstantial that, were you to put it in a vast cosmic ocean, it would float. Its glorious rings are believed to be quite recent. They're made of billions of chunks of ice - the remains of a moon which came too close and paid the penalty by being torn apart.

Saturn, like its bigger neighbour, also has a large family of moons - more than 30 at the last count. At this distance from the Sun, the moons are largely made of ice. Currently, the hugely successful Cassini probe is in orbit around Saturn, making close passes by its moons. Its most famous encounter was with Titan, where it released the European Huygens probe - which sent back the astonishing images of the clouded moon's surface.

What's up?

Mighty Jupiter is the most brilliant object up in the night sky this month (after the Moon). It sits in the

Y-shaped star-pattern of Virgo (the virgin), with the constellation's brightest star, Spica, to Jupiter's lower left.

To the upper right of Jupiter you'll find Leo (the lion), shaped like a crouching feline, with Regulus marking its heart. To the right again - over in the west - Saturn shines alongside the twin stars of Gemini, Castor and Pollux.

Around 22 April, you may spot some shooting stars diverging outwards from the constellation Lyra (the lyre). The Lyrid meteors consist of debris from a comet called Thatcher. This year, bright moonlight will drown out all but the brightest meteors.

On the other side of the world, there's a eclipse of the Sun on 8- 9 April. It starts in New Zealand as an annular eclipse, with the Moon not quite covering the Sun, but leaving a bright ring of sunlight around the Moon's silhouette. Over most of the Pacific the eclipse is total. It becomes annular again at the end of the eclipse, visible from Panama and northern Colombia.

Source: Independent, The; London (UK)

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