Collins Night Sky. Wil Tirion

THE NIGHT SKY

WHAT TO USE

      If you have only just become interested in astronomy, resist the temptation to rush out and buy a telescope. The majority of cheap telescopes offered in many shops and magazines are generally not worth buying. You do not need any equipment at all to learn to find your way around the sky, identify the brightest objects, and begin observing.

      Once you can find your way around, you can consider using some form of optical equipment. Old-fashioned opera glasses are actually quite useful, because they have a low magnification – so you do not become lost – but are not particularly common nowadays. It is probably best to invest in a good pair of binoculars. These are always designated by their magnification and aperture, such as ‘8 × 42’ or ‘7 × 50’, where the first figure indicates the magnification (8 and 7 times, respectively) and the second the aperture – the diameter of the main lens (the objective or object glass) through which light enters the instrument. As a general rule in astronomy, the larger the aperture the better, because more light is being gathered.

      Binoculars have one disadvantage: if too large or heavy they are difficult to keep steady. You can buy or make suitable mountings for binoculars, but these tend to be cumbersome and make life difficult for beginners. Avoid very high magnifications, because these restrict the field of view and amplify every minor movement. (The modern image-stabilized binoculars are marvellous to use, but extremely expensive.) Generally, magnifications of 7×, 8×, and 10× are best, while one of 12× becomes difficult to hold steady. Some simple tricks that help with the problem of steadiness are sitting in a garden chair that has arms to support the elbows; resting the binoculars on top of a broom, a wall, or something similar; and tying a length of string or light chain to the binoculars, and standing on one end while gently pushing upwards to put the string or chain in tension. When trying to observe objects right overhead, it is best to lie flat on the ground on a suitable mattress.

      You must also avoid too low a magnification. In the dark, the pupil of the eye expands to a maximum diameter of about 7 mm – slightly less for older people. The light leaving any optical instrument does so as a circular beam, the diameter of which equals the aperture of the instrument divided by the magnification. Put in simple terms, the diameter of the exit pupil should not be larger than the maximum diameter of the pupil of the observer’s eye. With 7 × 50 binoculars, the exit pupil is about 7 mm, which is about the maximum for useful work. (Technically this is simplification and other factors affect an instrument’s performance, but it is a useful rule-of-thumb.)

      Monoculars, many of which essentially consist of half of a pair of binoculars, are also useful. The types used by birdwatchers often come with a tripod, but observation of objects high in the sky may be difficult. The magnification on many monoculars, however, is often too high for most astronomical purposes.

      If you become sufficiently interested in astronomy to want to observe fainter objects, there are many different types of telescope that you can buy (or even build). Details about choosing between the different types are given in some of the publications mentioned under ‘Further Information’. Although some amateurs have highly sophisticated equipment, including computer-controlled telescopes and electronic (CCD) cameras, it is worth remembering that serious scientific work is carried out by many observers with nothing more than a pair of 7 × 50 binoculars.

      Measure the circular exit pupil by holding the binoculars in front of an evenly illuminated surface (such as the blue sky). If the exit pupil has flattened sides, the prisms inside the binoculars are cutting off some of the light.

      View a horizontal line on a distant object, and move the binoculars away from your eyes until you see twin images. They should be perfectly aligned (TOP). If one is slightly higher than the other (CENTRE), your eyes may be able to correct the difference, but if they are tilted (BOTTOM), eyestrain will result.

STARTING OUT

      There are a few points to remember when you start observing. You need to find somewhere fairly close by where it is dark and you have a clear view of the sky. (When you become more experienced, you may be happy to travel considerable distances to suitable observing sites, but initially you need somewhere that is convenient.) Because of the problems of light pollution it may be difficult to avoid a general glow of light in the sky, but try to find somewhere that is at least out of any direct lighting. You may need to use more than one position to see the whole sky. Ideally, find somewhere that is also sheltered from the wind.

      Choose the darkest position possible, because this will help your eyes to become dark adapted. Unlike the expansion in size of your pupils which happens almost instantaneously when you move into a dark area, dark adaptation depends upon the concentration of a particular pigment in the retinas of your eyes. This accumulates after a period in the dark and may take 15–20 minutes (or even longer if you are tired), and enables you to see faint objects. Although it varies between individuals, most people are actually able to see quite well – enough to move about without falling over things – by starlight alone. It help greatly if garden paths, patios, or similar areas are light in colour, especially because dark-adapted vision is colourless, so everything just appears as a shade of grey.

      Stephen Pitt

      The Moon, moving from west to east (right to left) is just leaving total eclipse.

      Unfortunately bright light destroys dark adaption, so astronomers use dim red lights to illuminate their charts, books, and notebooks, because this colour has the smallest effect. If you do not have a dim red light, cover a torch (flashlight) with red plastic or cellophane. For some types of observation it is not essential to be able to see what one is writing – if it consists of figures, letters, words or times, perhaps, rather than drawings – so some astronomers develop the habit of writing in the dark, using a dark pencil and drawing a thick line under each observation. They then transcribe their notes later – exactly, with no changes whatsoever, but more neatly – into their observing notebooks.

      Stephen Pitt

      Two Leonid meteors frame Orion. Jupiter is the bright, over-exposed object in Gemini, and the tiny red dot in the centre is the Rosette Nebula in Monoceros.

      It is also important to keep warm and dry. Depending on where you are, it may become cold at night even in summer, so warm clothing is always important. Roughly a quarter of body heat is lost through the head, so wear a proper hat. Make sure you have adequate footwear and, if it is likely to be very cold, wear two pairs of socks and have proper gloves or mitts. Try to avoid standing on wet grass and, if you do have to lie down to make observations high overhead, use a groundsheet and a thick piece of foam. An adjustable garden chair with arms is extremely helpful, but even here try to avoid sitting over wet grass. In some climates, it may be warmer at night, but you may then have to think about wearing clothing that protects you against insect bites, or using insect repellent.

      Finally, it is useful to keep a note of your observations. Use a notebook with fixed leaves, rather than a loose-leaf type. Keeping records need not be a chore. At first you may have little to record, but even a simple note of your observing sessions can prove very useful. Make a habit of recording the date and time of any observation. This is particularly important when you see anything unusual – such as an aurora or extremely bright meteor – and we will discuss dates and times shortly.

EXPLORING THE SKY

      It is not difficult to start finding your way around the sky, but