Beginner's guide

Naked-eye astronomy

A great deal of good astronomy can be carried out with the naked eye alone. However, there is one problem which has become more and more obvious in recent years: light pollution. People living in cities find that far from being black, the night sky has an unhealthy orange glow which hides all but the brightest stars. Efforts are being made by local councils to install properly screened lights, but meanwhile I am afraid there is nothing that can be done except drive to an area away from the glare.

I have already discussed the best way to start finding your way around the sky. Once you have learned the main constellations, look at some individual stars and note their colours; for example, Betelgeux is red, Arcturus orange and Vega blue. There are star clusters such as the Pleiades in Taurus (how many stars can you see in the cluster without optical aid?) and there are nebulae, such as the Sword of Orion, which are stellar nurseries where new stars are being formed out of dust and gas. Just occasionally a formerly faint star will flare up to naked eye visibility; this is termed a nova. Amateurs have a fine record of nova discovery because they watch the sky much better than the average professional, who relies solely upon instruments.

Identifying the planets is easy enough. Venus and Jupiter are always far more brilliant than any star. Mars is very red, and can be brilliant, though at other times can sink to second magnitude. Saturn does, admittedly, look like just a bright star (magnitude 0-1) but it is a slow mover, and remains in the same constellation for many weeks at a time. Moreover the planets keep to a definite band in the sky known as the Zodiac.

Of the other planets, Mercury always keeps close to the sun, and you will not see it unless you are deliberately searching for it; Uranus and Neptune are below naked eye visibility.

Occasionally comets can be spectacular - as with Hale-Bopp of 1997, though most are telescopic. A comet is millions of miles away (if you see something moving perceptibly across the sky, it cannot be a comet) but it leaves a 'dusty trail' behind it. if the Earth moves through a tail of cometery debris some of the particles will burn up in the upper atmosphere, creating a spectacular meteor shower. There are many showers every year, each named after the constellation from which the meteors appear to emanate. Thus, the August Perseids appear to come from Perseus while the December Ursids come from Ursa Major, the Great Bear. There are also sporadic meteors which may appear anywhere and at any time.

Aurorae, or polar lights, are due to electrified particles emitted by the Sun, which strike the upper atmosphere and make it glow rather in the manner of a spark coil. Because the particles tend to go to the magnetic poles, aurorae are not very common in England, but more so in Scotland, while a night in Iceland or Norway would be drab without them. From more southerly counties, with dark clear skies, look out for the Zodiacal Light soon after sunset; this is a crown-shaped glow rising from the horizon from the point where the Sun has set. I have to admit I have never seen it from my Sussex home, but other observers may well do better.

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