IF YOU'VE never seen the innermost planet, Mercury, now's your chance. It's never far from the Sun in the sky, and moves pretty rapidly - but in the second and third weeks of March, it will put on a creditable display after sunset.

It looks like a bright star, setting up to one and a half hours after the Sun. If you have binoculars, try to find the planet on the evening of 10 March, when it will be a couple of moonwidths north of the planet Mars. The Red Planet is currently only one-tenth as bright as Mercury and rapidly disappearing into the glare of the Sun. (But DON'T look at the Sun with binoculars.)

After Pluto, Mercury is the tiniest planet in the Solar System - only 40 per cent larger than our Moon, and smaller than Jupiter's moon Ganymede. Orbiting just 58 million km (36 million miles) from the Sun, Mercury feels its gravity very strongly. Like a conker on a very short string, it has to travel very quickly - at 200,000 kph, a hundred times faster than Concorde - to avoid being pulled in.

Only one space probe, Nasa's Mariner 10, has ever flown past Mercury. The images sent back show a heavily-cratered world. Like the Moon, Mercury received a thorough battering from debris left over from the building of the Solar System. One severe impact occurred 3,850 million years ago, when an asteroid about 100km across struck the planet. The blast resulted in a huge crater, the Caloris Basin, and sent ripples through the solid rock to create a "bullseye" shape of concentric mountain ranges spanning 3,700 km. The impact was so severe that it raised mountains 2km high on the opposite side of the planet.

But Mercury has some features the Moon doesn't. Among these are low ridges that run for hundreds of kilometres across the planet's surface, utterly disregarding the underlying terrain. These "wrinkle ridges" are a sign that since it formed, Mercury has been shrinking - due to cooling - so its "skin" has become as creased and furrowed as an old, dried-up apple.

It may have decreased in size even more substantially. Mariner 10 revealed that Mercury has a disproportionately large iron core, making up nearly half of the planet. This is a strong indication that Mercury itself was once very much bigger - perhaps as much as 30 per cent. What made it grow smaller? Possibly the hot gases streaming off the Sun simply boiled away Mercury's outer rocky layers. Or perhaps (more sensationally) in the turbulent early years of the Solar System, Mercury was struck by a planet almost half its size. The resulting explosion blew off most of its surface layers, and destroyed the other planet altogether - except for its iron core, which merged with Mercury's to form a "supercore".

A few years ago, astronomers bouncing radar waves off the planet discovered that Mercury's poles contain highly reflective patches. This indicates something very smooth - more than any rock could be. Scientists concluded that they are observing deposits of ice, buried deep in dark craters that never get to see the rays of the Sun. On a world whose daytime temperature reaches 350C, it is an astonishing find.

What's up

At the beginning of March, the planet Venus rises a couple of hours before the Sun, although this is reduced to an hour by the end of the month. It reaches its greatest brightness this month (magnitude -4.5), and if you have a telescope, you'll notice it shrinks from a large crescent to a smaller "half moon" phase as the days progress.

Jupiter is concealed in the Sun's glare. Saturn is still visible in the western sky for a couple of hours after sunset - but it closes rapidly in on the Sun as the month ends.

Diary (24-hour, GMT)

5 March 0841: Moon at first quarter

13 March 0435: Full Moon

20 March 1955: Vernal Equinox (First day of spring)

21March 0738: Moon at last quarter

28March 0314: New Moon

29 March 0200: British Summer Time begins (clocks go forward)

- Heather Couper and

Nigel Henbest