Already visible by day, the mighty Hale-Bopp is putting on its best tails to trip the light fantastic this weekend. By Heather Couper and Nigel Henbest
Believe it or not, the best is still to come from Comet Hale-Bopp. Though it has now rivalled the brightest stars for a couple of weeks, and has just passed the Earth, over the Easter weekend it will put on a celestial show such as very few people alive have witnessed.

The last time Earth was treated to a comet of this magnitude was early in 1910, with the Great January Comet, which was easily seen in broad daylight - and, incidentally, upstaged Halley's Comet, which came along a couple of months later.

From a really clear site, Hale-Bopp is already visible in daylight. Astronomers at an observatory atop the Pic du Midi in the Pyrenees have photographed it from 3pm through to the early hours. Compiled into a movie, the 1,100 pictures reveal jets of steam within the comet's gassy head, which sweeps round like a giant Catherine wheel. The jets are bursting through cracks in the "dirty snowball" nucleus that lies at the comet's heart. The Pic du Midi movie indicates that the nucleus spins around once every 11.5 hours.

You may need time-lapse photography to see the comet spin, but binoculars - or a small telescope - will show you the curved jets in the comet's head. And even with the naked eye you can easily make out the comet's tails - yes, "tails", plural.

One tail, composed of gas atoms ripped apart by the Sun's radiation, is narrow and straight, swept out by the "solar wind" of hot plasma that continuously blows out from the Sun's surface. The second tail is broad and curved, made of tiny grains of dust - chips of rock and fragments of soot - pushed along by the pressure of the Sun's radiation.

With most comets, you need long exposure photographs to distinguish the two tails. Find a really dark site over the Easter weekend, and with the naked eye you'll be able to see both of Hale-Bopp's tails clearly - stretching over perhaps half the sky - and even distinguish their slightly different colours: the gas tail slightly blue, the curved dust tail yellow.

But even the slightest trace of light in the sky will drown the faint outer tendrils of the comet's tail, and rob the gassy head of its true splendour. Moonlight is as much a culprit as artificial illumination. If you've been following Hale-Bopp over the past couple of weeks, you'll have noticed it seems to be fading and shrinking. That's purely an illusion, as the waxing Moon has lit up the sky more and more.

Around Easter time, the Moon is out of the way and the comet is higher in the sky, above any low-lying mist that might obscure it and reflect light pollution. Finally, although the comet is gradually drawing away from the Earth, its magnificence will increase as it moves towards the Sun's heat, with closest encounter scheduled for 1 April.

We suspect that Hale-Bopp, officially named after its two American discoverers, will go down in folklore as the Great Easter Comet. And even that won't be the end of the spectacle. Hale-Bopp is well on show throughout April (see chart), though the Moon will dim its splendour for the first couple of weeks of the month.

Astronomers will be keeping a vigil on this unique cosmic visitor for months to come, not only with telescopes but also with instruments carried on rockets, satellites and even a Space Shuttle mission in July. They will be checking just what gases the Sun's heat is boiling out from deep in the icy nucleus. For this dirty snowball, some 20 miles across - four times bigger than the heart of Halley's Comet - is a deep-frozen, scarcely altered sample of the material that helped to form the planets of the solar system.

What else is up

While you're Hale-Bopp-watching in the early evening, keep an eye out for a bright "star" to its lower left, well down in the twilight glow. This is the planet Mercury, putting on its best evening show of the year.

The only other planet worth mentioning this month is Mars. At its brightest last week, it has really been upstaged by the comet on the opposite side of the sky. Now we are drawing away from Mars, and it is fading rapidly week by week.

Of the stars, the bright "winter constellations" of Orion and his entourage are now riding off into the sunset. Some large dull constellations are rising to greet the spring, graced by a few first magnitude stars such as Spica, in Virgo (the virgin) and Arcturus, in Bootes (the herdsman).

Diary (all times BST and 24-hour)

6 Mercury at greatest eastern elongation

7 1202 new moon

14 1800 moon at first quarter

22 2134 full moon

30 0337 moon at last quarter

Images and animations of Comet Hale-Bopp taken by the Pic du Midi observatory can be found on the Internet at: .