Science: When a halo appears in the heavens: Heather Couper and Nigel Henbest take a shine to the Moon's glowing circle and other celestial light shows

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The Independent Online
On a good dark night it's not just the stars, planets and Moon that are on view. Let your eyes become accustomed to the dark, and take a look for some of the natural 'glows'.

The commonest of these is a lunar halo, a complete circle of light surrounding the Moon. Usually a halo is at a radius of 22 degrees from the Moon (roughly a spread-out hand at arm's length), but occasionally it is twice as large. The Moon's light, passing through Earth's atmosphere, encounters high altitude icy clouds - cirrus to the meteorologist. The ice crystals refract the Moon's rays to produce the halo.

According to old weather traditions, a lunar halo portends bad weather. And cirrus clouds usually build up ahead of a warm front, so the appearance of a halo round the Moon does indicate rain on the way.

On a wet but moonlit night, look out for a ghostly ring on the other side of the sky to the Moon. The 'moonbow' looks like a rainbow, but pale and washed of all colour. The moonbow is actually formed in exacty the same way as the rainbow, as the light of the Moon is bent through drops of rain and reflected back.

Occasionally you can spot a far more brilliant display in the northern part of the sky. The Northern Lights - or aurorae - take several forms. Sometimes you see just a bright arch on the northern horizon. Or there may be brilliant rippling curtains and flashing rays of light, in red and green. If you are right underneath - most likely in Scotland - the aurorae can stream out in all directions, as a crown of light directly overhead.

Some sky-glows come from well beyond our own planet. At this time of year the Milky Way, the collected light of the billions of distant stars that make up our galaxy, forms an arch in the western sky.

April is an ideal time for spotting a much rarer glow. Choose an evening when the Moon is out of sight, as it is for the first couple of weeks. Wait till the twilight glow has faded and look to the west, where you may notice a narrow cone of light reaching upwards from the horizon. This is the Zodiacal Light, the line in the sky that marks the solar system seen edge-on in our skies. The evening glow this month lies in the constellations of Aries and Taurus.

The Zodiacal Light is proof that the space between the planets is not a complete vacuum. This faint band consists of billions of microscopic dust particles reflecting the Sun's light.

At this time of year it appears slightly brighter in the constellation Virgo, at a point exactly opposite in our skies to the direction of the Sun. This is the gegenschein, which occurs because the interplanetary dust is good at reflecting light exactly back along its path, so the regions of dust opposite the Sun appear brighter than the other regions of interplanetary dust. Some of these particles are specks of dust shed by old comets. Others are chips of rock broken off when asteroids collided in the asteroid belt between Mars and Jupiter.

The planets

Venus and Jupiter are the 'stars' of the month. Look for Venus in the western sky at twilight - by the end of the month it sets at nearly 11pm BST. It is getting brighter as it draws closer to the Earth, and at magnitude -3.8, it outshines everything in the sky except the Sun and Moon. The Moon itself will be close to Venus on 12 April.

At the end of the month, Jupiter is in 'opposition' - opposite the Sun in its orbit as seen from the Earth, and at its closest - and it dominates the sky all night. Although it is still more than 400 million miles away, its flattened disc is obvious in binoculars; as are its four brightest moons.

The other naked-eye planets - Mercury, Mars and Saturn - are all features of the morning sky, rising less than an hour before the Sun.

Shooting stars

Shooting stars from the Lyrids meteor shower enter Earth's atmosphere between 19 and 25 April, peaking in numbers on 22 April. However, because of bright moonlight, this year's display is not favourable.

The stars

Whatever the weather may be doing, two sure signs of spring are on view high in the sky. One is the constellation of Leo the lion; the other is Bootes, the herdsman. These are two star patterns the Earth turns to face in the April-May sector of its orbits.

You can find Bootes by extending the line of the 'tail' of the Great Bear. Most of the herdsman's stars are relatively faint, but several are double or triple when viewed through a small telescope. The exception is orange Arcturus - the 'bear driver' in Greek. Arcturus is the fourth brightest star in the whole sky, partly because it is only 36 light years away. It is a red giant nearly 30 times wider than the Sun. Its colour is a result of it having expanded and cooled as it slowly approaches the end of its life.

Leo needs no signposts: it is the obviously leonine pattern of stars riding high in the south. Its brightest star, Regulus, lies 85 light years away. Just above it, at the 'nape' of the lion's neck, is the yellow star Algieba ('lion's mane'). Seen though a small telescope, it is an attractive double star, with a companion only slightly fainter than the main star.

Diary (all times BST)

3 April 3.55am Moon at last quarter

11 1.11am New Moon

19 3.34am Moon at first quarter

22 10am maximum of Lyrids meteor shower

25 8.45pm full Moon

30 Jupiter at opposition; Mercury at superior conjunction

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