Collins Night Sky. Wil Tirion

people find them easier to understand than maps and charts, especially when they are first beginning to recognize the constellations.

      Nearly all cameras are digital nowadays, but there are advantages in using film, notably in the reproduction of the colours of stars and certain objects, such as the Orion Nebula, where the distinctive pink tint visible in the (film) photographs opposite and MONTHLY CHARTS is lost by most digital cameras. Film cameras with a ‘B’ setting also provide a basically unlimited length of exposure for recording star trails or very faint objects. Most digital cameras (even digital SLRs) often have a maximum duration of a few minutes – rarely more than 30 minutes – so long, unbroken star trails are impossible to achieve. Film also has the advantage of being less sensitive to light pollution during long exposures.

      Digital imaging has advantages in that it is possible to examine an image immediately after it has been obtained to check framing or whether the image shows the desired amount of detail; several images may be ‘stacked’ to reveal fainter objects and features; and computer-processing is able to remove background colour casts and reduce the effects of light pollution.

      Tunc Tezel

      A powerful telephoto lens or a telescope is required to obtain large images as in this photograph of the crescent Moon and Venus.

      In short, either film or digital imaging may be used. At the time of writing, Ektachrome, an excellent transparency film for constellation images and star trails, is still available, and is recommended. For either type of camera you will, of course, require a stable tripod, and a suitable mechanical or electrical/electronic remote shutter release, with a locking mechanism to allow long exposures.

      With 35-mm film cameras, a standard 50-mm lens covers a field of about 35° ∞ 47°, a useful area to include the whole of Orion and similar-sized constellations. The standard lenses on many digital cameras cover a similar field. Such a lens is often suitable for photographs of planetary conjunctions taken at twilight, when you will probably want to include some foreground objects to provide a more attractive picture. Remember, however, that images of the Moon and planets are extremely small with all ordinary lenses (with a 50-mm lens, the Moon occupies approximately one-hundredth of the width of a 35-mm frame). For large images, you need to use a telescope, driven to follow the object in the sky.

      Zoom lenses allow the area covered to be altered but always involve some loss of light when compared with fixed focal-length lenses. A wide aperture (f/2.8 or better) is desirable to gather plenty of light. Focussing may be a problem with digital cameras, because most rely upon automatic focus, so manual focussing should be chosen if available.

      If using film, it is important to specify it should be returned to you uncut, because automated machinery will sometimes cut through the centre of each frame. Standard ideas of film speeds do not fully apply to astronomical subjects, but films in the range ISO 100–400 will generally give good results, and similar ISO-equivalents should be used with digital cameras.

      With a fixed tripod and camera position, the Earth’s rotation causes everything to record as a trail. With short exposures the trailing may be too small to see. Experiment with exposures of 10, 15, 20 and 30 seconds. The apparent rotation of the sky is fastest near the equator, so longer exposures may be used near the poles. Long star trails look dramatic, but they make it difficult to recognize the constellations that have been photographed.

      Storm Dunlop

      A trailed photograph of Orion taken with a standard 50-mm lens, a fixed 35-mm camera, and an exposure of 5 minutes.

       THE CELESTIAL SPHERE

      The circumpolar stars are the key to starting to identify the constellations. Not only are they visible at any time of the year, but nearly everyone living in the northern hemisphere is familiar with the seven stars of the Plough – known as the Big Dipper in North America – an asterism that forms part of the large constellation of Ursa Major. This is where we start. There are five main constellations to identify.

      Because of the movement of the stars caused by the passage of the seasons, Ursa Major lies in different parts of the evening sky at different periods of the year. The diagram shows its position for the four main seasons to give you a guide as to where to look. The seven stars remain visible throughout the year anywhere north of latitude 40°N. At that latitude, however, they may be very low on the northern horizon, especially in autumn. Much of the remainder of the constellation is then hidden below the horizon for part of the night.

      Ursa Major is a large constellation, but initially few people are familiar with the extended groups of stars that form part of it and lie well to the south and east. In the centre of the curve of stars that form the ‘tail’ of the Bear (or the ‘handle’ of the Dipper) lies the small constellation of Canes Venatici, which consists of two moderately bright stars and a scattering of fainter ones.

      Ursa Major at 22:00 in winter (right), spring, summer and autumn.

      Constellations in the northern circumpolar region.

      POLARIS AND URSA MINOR

      Once you have identified the seven bright stars, locate the two stars, α and β UMa farthest away from the ‘tail’. These two, named Dubhe and Merak, respectively, are known as the ‘Pointers’. A line from Merak to Dubhe, extended to about five times their separation, leads to a fairly isolated bright star. This is the Pole Star, Polaris, or α Ursae Minoris. All the stars in the northern sky appear to rotate around it. In fact, it lies slightly less than one degree away from the true pole, and a trailed photograph of the northern sky shows that it traces a tiny circle (about 1.5 degrees across) around the pole.

      Apart from this small variation, the elevation of Polaris above the northern horizon is always equal to the observer’s latitude. This was an extremely useful property, used as a guide to navigation by generations of sailors from the time of the Greeks to comparatively modern times.

      Finding UMa and Polaris.

      In contrast to Ursa Major, Ursa Minor is a small constellation, consisting of little more than the relatively faint, stars that form the ‘Little Dipper’ (as it is known in North America). Strangely. the asterism has no common name in Britain, other than the Little Bear, despite being superficially similar to the seven stars of the Plough. Five stars – including a moderately close pair – form the body of the constellation, with another three forming the ‘tail’ at the tip of which lies Polaris – the end of the ‘handle’ of the Little Dipper. The two stars farthest from the pole, β and γ UMi (Kochab and Pherkad, respectively) are known in English-speaking countries as ‘The Guards’.

      CASSIOPEIA

      On the opposite of the North Pole from Ursa Major lies Cassiopeia. It has a highly distinctive shape, appearing as five stars that form a letter ‘W’ or ‘M’ depending on its orientation. Provided the sky is reasonably clear of clouds, you will nearly always be able to see either Ursa Major or Cassiopeia, and thus be able to orientate yourself on the sky.

      Finding Cassiopeia from the Plough and Polaris.

      To find Cassiopeia from Ursa Major, start with UMa (Alioth), the first star in the tail of the Bear. A line from this star extended through Polaris points directly towards γ Cas (Cih), the central star of the five. Cassiopeia