to cover the seven principal stars that delin- eate the figure of Orion.
If we wish to be a little bolder we can relate the span of our outstretched hand to actual angular measure. It so happens that the span of our hand corresponds to just over 20 degrees (the symbol for degree is°), while the length of our thumb is about 7°. This latter figure conveniently matches the field of view of a low power pair of binoculars of the 7x50 or 8x30 variety (incidentally, all binoculars are labelled XX x YY, where XX is the magnifica- tion and YY is the diameter of the from lenses in millimetres).
The length of a thumb at arms length – 7°– it's not a great deal, is it? If you have a pair of binoculars then perhaps now you can appre- ciate just how small an area of sky is revealed to you at any one time, and why it can be a problem finding objects. Contrast this with the field of view of a typical astronomical tele- scope which is in the region of half a degree, or the apparent width of the Full Moon. It may surprise many of you to discover that the angular size of the Moon is so small (it can be covered by the equivalent of your little finger nail at arms length) – the proverbial man in the street would probably say that the Moon is as big as tennis ball or a small plate. If you don't believe me, try it!
Back to the constellations and naked eye viewing. Once we are confident that we have located, say, Orion (hint: the three
A further hand span to the upper left brings you to the celestial twins of Gemini, Castor and Pollux.
Another very familiar pattern visible through- out the year is the constellation of Ursa Major, or the Great Bear. Also known as the 'Plough' or the 'Big Dipper', the seven stars that com- prise this easily identified pattern lie low to the northern horizon in winter months. Like Orion, the main part of this constellation sub- tends about the width of an outstretched hand at arms length. In this fashion you can use the prominent seasonal star groups as a kind of framework, then you can fill in the fainter con- stellations as your knowledge grows – and rest assured it will.
Broadening your horizons: Assuming that you feel confident in identifying the stars visible in any particular season, preferably with the help of members of your local astronomical society and the star charts of a magazine, then you may have already been introduced to the pleasures of binocular or telescopic astronomy. Having 'served your apprentiship', so to speak, you will be in a better position to appreciate the true potential of any given optical instrument. As was mentioned earlier, the art of observing is a skill to be acquired with con- stant practice, and in some individuals this faculty has been developed to an extraordi- nary degree such that they can perceive a celestial interloper such as a nova or comet at a glance.
Streetlights and light pollution: If you are fortunate to live in a rural area that is not plagued by streetlighting, then your view of the heavens will not be impaired. However, for the observer living in city or suburban areas the subtle phenomena of the skies are largely lost – this is why the clear, dark skies of coun- try areas are such a revelation to many town dwellers: there is so much to see when the veil of light pollution has been lifted! So how can we alleviate the problem? Other than lob- bying your Local Authority or private compa- nies responsible, it is possible to invest in
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