A stormy giant with pretty ringlets: Look out for Saturn and its many moons as Heather Couper and Nigel Henbest chart the September skies

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The Independent Online
Look among the dim stars of Aquarius and Capricorn, and you'll spot a brightish, untwinkling 'star' low in the south. It is the planet Saturn, at its closest to the Earth this month and visible all night - but at a distance of 1,300 million kilometres, it is hardly just around the corner.

Saturn is quite unremarkable to the unaided eye. But through even the smallest telescope, it looks like a tiny model of a planet: a creamy-yellow globe circled by wide, bright rings. All the outer gas giants in our Solar System have rings, but Saturn's are the widest, brightest and most beautiful. They span a distance almost equal to the separation of the Earth from the Moon - and yet they are only tens of metres thick. Beautiful images from the two Voyager probes, which visited Saturn in 1980 and 1981, revealed that the rings are divided into thousands of separate ringlets, made up of millions of chunks of ice ranging in size from an ice-cube to a car.

Inside its dazzling rings, Saturn itself is unexciting. A smaller version of the gas giant Jupiter, it has occasional storms (the last in 1990), and very high winds (1,800 kph at the equator). But it does have the biggest family of moons of any planet in the Solar System. There are 18 'definites' and six more suspected - briefly glimpsed on Voyager images. With one exception, they are all made of ice.

These crystal worlds come in a bewildering range of shapes and sizes. Hyperion, for instance, is hamburger-shaped, and tumbles over and over as it orbits Saturn. The outermost moon, Phoebe, is as black as coal, like a giant version of the solid core of Halley's Comet. Iapetus looks like a twin-scoop ice-cream - black on one side, dazzling white on the other.

Many of the moons are heavily cratered, with the exception of Enceladus, whose super-smooth surface may be the result of constant outpourings from ice volcanoes. Two of the cratered moons, Mimas and Tethys, bear scars fully one-third their diameters. The impacts that blasted out these craters must have almost broken these moons apart. In fact, two jagged, tiny moons closer to Saturn - Janus and Epimetheus - are almost certainly halves of a world that was smashed in two by a giant meteorite.

Four of Saturn's moons circle the planet inside its rings. Their presence explains why some of the rings are so narrow: the ring particles are herded into line like sheep by the gravity of these tiny 'shepherd' moons, each measuring less than 100km across.

Saturn's largest moon, Titan, with a diameter of 5,150km, is bigger than the planets Mercury and Pluto, and second only in size to Jupiter's moon Ganymede. Unlike its fellow ice-worlds, Titan is made of rock - and, unique among moons, it is swathed in an atmosphere 50 per cent thicker than that of the Earth.

Titan is the only world to have an atmosphere largely composed of nitrogen - the gas that makes up most of our air. Some scientists have suggested that Titan may resemble the Earth in the far-off days when life was forming. But any prospects of making life on Titan now are on hold; the world is in deep-freeze.

Titan's clouds are so thick that Voyager 1 could not see its surface. But we may glimpse its landscapes within the next 10 years, when the US spacecraft Cassini flies to Saturn. It will carry the European probe Huyghens which will plunge into Titan's atmosphere. We should then be able to see what scientists suspect: that Titan's surface is punctuated with seas of liquid methane, into which falls a gentle rain of orange, organic 'gunge'.

The planets

Towards the end of the month, Mercury is an evening star - but it sets less than half an hour after the Sun so it will be hard to spot. Venus is ending its domination of our evening skies, although it achieves its greatest brilliancy (magnitude minus 4.6) on 28 September. Mars is now rising just after midnight BST, and early in the month it makes up a line with the two bright stars in Gemini, Castor and Pollux.

Jupiter is also on the way out. By the end of September it sets at 8pm BST. On the 9th, look out for the Moon below Jupiter, and only three moonwidths away. Saturn is at opposition (at its closest point to the Earth in its orbit) on 1 September, and is visible all night. A small telescope will reveal the rings, and its largest moon Titan.

The stars

The tiny constellation of Delphinus reaches its highest elevation in our skies this month. The constellation originated in Greek times, when, according to myth, dolphins were messengers of the sea god Poseidon. Poseidon's son, Arion, was once attacked on a ship - but a school of dolphins saved him. In gratitude, Poseidon placed a dolphin in the sky.

The two brightest stars in Delphinus bear the strange names Sualocin and Rotanev. Written backwards, they spell Nicolaus Venator, the latinised version of Niccolo Cacciatore. Thus a very junior assistant astronomer at the Palermo Observatory in the early 19th century found a way to immortality.

Diary (all times BST)

September 1 Saturn at opposition

5 7.33pm New Moon

12 12.34pm Moon at First Quarter

19 9.01pm Full Moon

23 7.19am Autumn Equinox

28 1.24am Moon at Last Quarter

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