Prepare yourself for a once-in-a-lifetime spectacle

April will see a total lunar eclipse and the passing of a new comet, report Heather Couper and Nigel Henbest
There's an action-packed drama in the heavens this month. Even as you read this, Comet Hyakutake is skimming past the Earth, far closer than any other heavenly body except the Moon, in a celestial show that will continue through to the end of April. It's joined by a dazzling appearance from Venus and - a one-night special - a total eclipse of the Moon.

In the last week of March, Comet Hyakutake heads almost directly over the Earth's north pole on its way towards the Sun. It's conveniently close to the Plough (Ursa Major) and to the Pole Star, Polaris. To find the Pole Star, follow the last two stars of the Plough downwards.

Comets do not zoom through the sky like shooting stars. They move gradually. The chart shows just where to find Hyakutake at 10pm each evening.

This comet could the brightest for 20 years. Astronomers can predict exactly "where" comets will be, but it's rash to predict how bright they will be. There have been many splendid comet "flops", the most famous being Kohoutek in 1973. Billed as "comet of the century", it was scarcely visible to the naked eye.

Hyakutake should do better. The International Astronomical Union reckons it will reach magnitude 1, matching the first magnitude stars shown as star-symbols on the chart. The British Astronomical Association thinks it could be several times brighter, more brilliant than the brightest star, Sirius.

But don't expect too much from Hyakutake. These days, we are treated to firework displays and laser shows that have given us a heightened expectation of what a celestial display should look like. There may not be much of a tail this week, either, so Hyakutake will look more like a fuzzy ball than most people's idea of a comet. And it is passing so close that its light is spread out into a patch several degrees across. The slightest illumination from streetlamps will drown Hyakutake. So make every effort to get to a dark site. As we move into April, the Moon comes round to full: you'll need to wait until the Moon has set to see the comet at its best.

If you have a clear northwestern horizon, you can witness be a once-in- a-lifetime event on the night of 3-4 April. The full Moon will drown out the comet, but as the Moon enters eclipse (see below), the sky will darken and the comet will seem to appear out of nowhere.

Comet fever will abate for a few days, as Hyakutake moves away from the Earth. As it swings towards the Sun, however, the increasing heat on the comet will boil away more of its frozen ices into shining vapour. Moving steadily down towards the north-western skyline, the comet will brighten from Easter onwards. It will grow a narrow tail of shining gases, and probably an even brighter fan-shaped tail of dust particles.

The British Astronomical Association calculates the comet will surpass Venus in brightness around 23 April. The brilliant planet and glorious comet will put on a display the like of which has not been seen since Halley's Comet performed a double act with Venus back in 1910.

Eclipse: Taking top billing on3-4 April is the Moon. We are due for the first total eclipse of the Moon to be seen from this country since November 1993, so it'll be worth staying up till half past midnight to watch the full Moon fade from sight as it moves into the Earth's shadow. The eclipse starts at 11.21pm on 3 April, and the Moon is fully eclipsed by 00.26am (4 April). The Moon starts to reappear at 1.53am, and the eclipse is over by 2.59am.

Astronomers can predict the instant the eclipse will occur, but not how it will look. During some eclipses, the Moon disappears completely. Sometimes, it glows a dull copper even in mid-eclipse, lit by sunlight bent round in the Earth's atmosphere. The amount of illumination depends on the state of the Earth's atmosphere: it must be clear of clouds and dust if sunlight is to reach the Moon.

The last two total lunar eclipses in 1992 and 1993 were unusually dark (the Moon seemed to disappear) because the Earth's atmosphere was polluted with ash from the eruption of Mount Pinatubo in 1991. This dust has settled, so we may see a reddish ghost of the Moon throughout the eclipse.

Without comet and eclipse excitement, Venus would be the star of the month. It is the dazzling object in the west after sunset, some 15 times brighter than the most brilliant star. Low down in the evening twilight, later in April, you may catch a glimpse of the other "evening star", the tiny planet Mercury.

Jupiter is rising in the southeast around 2am, while Saturn and Mars are too close to the Sun to be seen this month. Around 21 April we'll be treated to shooting stars radiating outwards from the constellation Lyra. It won't be a meteor storm, though, more a light shower.

Leo dominates the southern sky with Virgo to the lower left. Its brightest star, Spica, lies near to the Moon during the lunar eclipse on 3 April. The bright star above is Arcturus in the constellation Bootes (the herdsman).

Diary (all times BST)

1 April: Venus at greatest eastern elongation

3-4 April, 11.21pm-1.53am: total eclipse of the Moon 4 1.07am: full Moon

11 April, 0.36am: Moon at last quarter

17 April, 11.48pm: new Moon

21 April: maximum of Lyrid meteors

23 April: Mercury at greatest eastern elongation

25 April, 9.40pm: Moon at first quarter

Hyakutake on the Internet, Section Two, page 12