Holy Hyakutake, that's close

A comet heading our way in March will miss our planet by a mere 10 million miles, write Nigel Henbest and Heather Couper

Keep a eye open in March to spot the most exciting sky sight for many a year. Britain will have a grandstand view of the newly discovered Comet Hyakutake, as it skims past the Earth at the end of the month.

This unexpected celestial visitor was found by Yuji Hyakutake, a Japanese amateur astronomer, on 30 January. He was scanning the skies with a huge pair of binoculars with lenses 6in across. He found a faint smudge of magnitude 11 to 12 - 100 times fainter than the naked eye can see - between the constellations of Hydra (the water snake) and Libra (the scales). It was a previously unknown comet, then beyond the orbit of Mars as it headed in towards the Sun.

Just five weeks earlier, Mr Hyakutake had found another comet in the same region. While the previous Comet Hyakutake was never destined for greatness, astronomers realised that this new comet was on track for the record books. Brian Marsden, of the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts, calculated that the second Comet Hyakutake was coming towards the Earth. It would miss our planet by less than 10 million miles.

The last time a comet came so close was in 1983, when Comet IRAS-Araki- Alcock buzzed the Earth. But it was a puny specimen. Comet Hyakutake is a much more impressive creature, and promises to shine as brightly as the most brilliant stars.

On the other hand, Comet Hyakutake will cover an area of sky bigger than the full Moon, so its light will be spread out to a large misty glow. We are lucky that there will be no bright moonlight to drown out the comet as it passes the Earth, but you will need to get well away from street lights to see this rare visitor at its best. Let your eyes get used to the dark for 10 minutes in order to see its true glory.

The comet will be at its best between 22 March and the end of the month, at the positions marked on the "Looking North" map. For a comet, it is moving at a rapid pace. (Contrary to popular belief, comets do not whip through the sky like a firework rocket.) And it passes near the familiar stars of Ursa Major - better known as the Plough - and Polaris (the Pole Star), which can be used as "signposts" to Hyakutake.

On 22 March identify the brilliant star Arcturus by following the "handle" of the Plough towards the horizon. The comet is to the lower left of Arcturus. Over the next two nights, it brightens considerably as it heads towards the Plough, from magnitude 2 (equal to the Pole Star) until it almost rivals Arcturus in brightness.

Comet Hyakutake passes closest to the Earth in the early morning of 25 March, as it moves through the constellation of Draco (the dragon) between the Plough and the Pole Star. It may be sporting a faint tail of glowing gas, though the bright dusty tail that makes a comet so spectacular does not develop until after it has passed the Sun, on 1 May.

Binoculars will show some details in the comet's large fuzzy "head". It will take a pretty large telescope to show much more - in particular, the tiny central nucleus where the action lies. This is a frozen snowball a few miles across. It began life in the chilly reaches of the outer solar system and, as it comes within the orbit of Mars, the Sun's heat is boiling away the ice to create the comet's gaseous head and tail.

Professional astronomers are quickly preparing plans to try to make out the shape and size of the nucleus. Some will use the Hubble Space Telescope to take the sharpest possible pictures. Others will bounce radio waves off the nucleus and try to work out its properties from the radar "echoes" they receive.

For astronomers, the observations planned for Comet Hyakutake will be an excellent dress rehearsal for an even more spectacular comet due to grace our skies next year: Comet Hale-Bopp. Watch this space.

What's up

Venus, usually only visible at dusk and dawn, makes a rare appearance on the star chart this month. By the end of March, it will not be setting until midnight (BST), and it is a stunning sight against a dark sky. A small telescope will show Venus about three-quarters lit up by the Sun at the beginning of March, although it will look more like a "half-moon" towards the end. As it and the Earth draw closer together, Venus grows even brighter. At the end of March, it is magnitude -4.2, and the most brilliant object in the sky apart from the Sun and Moon.

Otherwise, there is a paucity of planets on show. The only other planet visible is Jupiter, which rises about 3.30am mid-month. But Comet Hyakutake should more than make up for the lack of planets.

The stars on view are very definitely those of spring, with Leo the lion riding high in the south. Below Leo is Hydra, the largest constellation in the sky but by no means easy to see, because its stars are faint and the constellation is low in the sky as seen from the UK.

Diary (all times GMT):

4 March: Mars at conjunction.

5 March, 9.22am: full Moon.

12 March, 5.15pm: Moon at last quarter.

17 March: Saturn at conjunction.

19 March, 10.44am: new Moon.

25 March, 7am: Comet Hyakutake closest to Earth.

27 March, 1.30am: Moon at first quarter.

28 March: Mercury at superior conjunction.

31 March, 2am: British Summer Time begins.