Strange moons and raging storms in clouds of blue: The twin planets Neptune and Uranus make a close appearance this month. Heather Couper and Nigel Henbest explain their characteristics

IF YOU are a dedicated planet-spotter, you may want to try searching this month for two worlds close to the very edge of our solar system - Uranus and Neptune. With binoculars or a small telescope, you can spot them in the constellation of Sagittarius. They appear a couple of moon's widths apart - this is one of their closest approaches in centuries - as they drift slowly westwards together during the course of the month. Both planets are newcomers to the solar system - at least as far as our knowledge about them is concerned. Like Earth, they have been around for nearly 5 billion years, but because they are so remote they came to light only relatively recently.

Uranus was discovered accidentally in 1781 by an amateur astronomer called William Herschel. King George III was so impressed that he instantly made Herschel his personal astronomer. Neptune came along in 1846, and owes its discovery to the power of mathematics. Astronomers realised that a mystery object was pulling Uranus off course in its orbit, and calculated where the culprit should lie. Neptune turned up bang on target, leading to an argument as to over who had actually had discovered it.

Until recently, very little was known about these remote worlds. Both turned out to be smaller versions of the gas giant Jupiter (but four times bigger than Earth, none the less), with small families of moons - Uranus with five, Neptune with two. They were often regarded as 'twin planets'. The only unusual feature of these worlds was Uranus's tilt - it circles the Sun on its side, tipped at 98 degrees from the vertical.

The two planets might have languished at the edge of the solar system for years had not the controllers of Nasa's Voyager 2 space probe pulled off a brilliant bit of 'interplanetary snooker'. By using the gravity of Jupiter and Saturn to catapult the probe towards a target, Voyager 2 was able to fly past Uranus in 1986, and Neptune in 1989.

Uranus was boring. It is a blue-green gas giant so featureless it took considerable computer-power to reveal one insignificant white cloud. However, Voyager 2 was able to confirm the existence of thin rings around the planet and imaged a handful more. But it was Uranus's moon system, now known to number 15, that stole the show. In particular, Miranda, at 500km (300 miles) across the smallest of the previously known moons, has a geology that defies description. It has chasms 10 times deeper than the Grand Canyon, and features that look like cosmic racetracks. Some scientists think Miranda was blasted apart by a huge impact, after which it reassembled itself in space.

As Voyager 2 drew closer to Neptune in the summer of 1989, it quickly became obvious that Neptune was far from being Uranus's twin. True, the planets are the same size; but Neptune turned out to be anything but boring. It is a beautiful turquoise world, its cloud-tops flecked with fleecy white 'cirrus' and midnight-blue spots. The biggest spot of all - the Great Dark Spot - is a raging storm the size of the Earth. Around it blow winds with speeds of up to 2,000kph (1,250mph), the fastest winds in the solar system.

The planet is surrounded by four rings, and circled by eight moons. One, Triton, is the coldest place in the solar system. Scientists were astonished to discover that it has volcanoes that spout nitrogen and black dust high into its thin atmosphere.

Why are Uranus and Neptune so different? It may be due to internal activity: Neptune actually radiates more heat than it receives from the Sun. But it may also be that Uranus was hiding its light under a bushel when Voyager 2 paid its visit.

Three months ago, the American astronomer Walter Wild used a technique that cancels the blurring effect of Earth's atmosphere to make images of Uranus with a multiple-mirror telescope in Arizona. He found a dark spot on Uranus at almost exactly the same latitude as the Great Dark Spot on Neptune, along with a bright region and a faint band near the pole. 'It probably just had a few bad weeks when Voyager passed by,' Mr Wild says.

THE PLANETS

It is not the best month for planets. Those with an extremely good western horizon might spot Jupiter and Mars (just two moon's widths apart 4-6 September) setting about half an hour after the Sun.

But the planet that holds centre stage at night is Saturn, shining brightly in the rather barren constellation of Capricornus. The angle the rings present to us is closing up, so take a look now (with a small telescope) because things won't get much better until 1998.

Early-risers will get gorgeous views of Venus, which rises two to three hours before the Sun. If you can catch it on 2 September, look at it through binoculars - it makes a very pretty sight.

If you think the chart looks strange, with two full moons - there isn't a mistake. Every two years or so we get two full moons in the same calendar month.

THE STARS

Lying just below the 'Summer Triangle' of Deneb, Vega and Altair is one of the smallest and most charming constellations in the sky - Delphinus, which looks like a tiny dolphin.

There is nothing terribly special about the stars of Delphinus. There is, however, a fascinating tale associated with its two brightest stars. Generally, the brighter stars have Arabic names, but these two hail by the decidedly un-Arabic Sualocin and Rotanev. If you read the names backwards, all becomes clear. They spell Nicolaus Venator - the Latinised form of the name Niccolo Cacciatore, who was assistant to the 18th- century Italian astronomer Giuseppe Piazzi.

SEPTEMBER DIARY

(all times BST)

1 3.33am Full moon

9 7.27am Moon at last quarter

16 4.10am New moon

22 8.32pm Moon at first quarter

23 1.22pm Autumn Equinox

30 7.54pm Full moon

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