April 30 1965 - Astronomy without a Telescope

Many viewers of The Sky at Night had written in to complain that most of the objects discussed were beyond the range of all but the telescope- owner. This, of course, is a perfectly valid criticism; the astronomer would be sadly handicapped without his telescopes. On the other hand, there is a great deal to be seen with the naked eye - when one knows where to look: and, with Henry Brinton, I did my best to point some of these out.

Subsequently, I invited viewers to make drawings of the Full Moon as seen with the naked eye, and send them in. In the next programme we showed several and I was surprised how good they were. Some of them came from schoolboys, who clearly had no access to published lunar map:, and on the whole I am surprised that pre-telescopic era charts of the Moon were not better than they actually were.

An astronomer is always pictured as a man with a large telescope. This is perfectly logical; without telescopes, our knowledge of the universe would be comparatively slight, and modern research has to be undertaken with the aid of very powerful instruments, j Yet there is a great deal of interesting observation open to the enthusiast who has no optical equipment at all, and naked-eye studies are certainly not to be despised.

The first step to be taken by the beginner is to learn the various constellation patterns, which is not nearly as difficult as might be thought. One good scheme is to select a few easily-found group: and then use these as direction-finders to more obscure constella­tions as well as bright stars. The two best 'sky marks' are Orion: and the Great Bear, both of which are prominent in the evening sky during winter, though by spring Orion has more or less disappeared in the evening twilight.

Ursa Major, the Great Bear or Plough, is circumpolar in Britain; that is to say it never sets, and is always to be seen when-1 ever the sky is sufficiently dark and clear. During May evening it is fairly high up, and makes a splendid guide. For instance, the tail of the Bear (or, alternatively, the handle of the Plough) shows the way first to the brilliant orange star Arcturus, in Bootes or the Herdsman, and then to Spica in Virgo, which is less striking "than Arcturus but which is nevertheless very prominent in the south-east. Also to be found from the Bear is Polaris, the Pole Star, which lies within a degree of the north pole of the sky, and which seems to remain almost stationary with the other celestial bodies moving round it.

Polaris is not placed exactly at the pole, as a simple experiment will show. If you take an ordinary camera, point it towards the celestial pole and then give a time exposure, the result will be a series of star-trails. The bright, short trail near the centre is that of Polaris, and the position of the true pole is easy to estimate. Pictures of this sort may be taken with no trouble at all; the only point to remember in aiming the camera is that the altitude of Polaris above the horizon is approximately equal to the observer's latitude on the Earth.

The two 'Pointers' in the Great Bear are named Dubhe and Merak; Dubhe is slightly brighter than the Pole Star, Merak a little fainter. If the line from them is extended in the opposite direction, away from Polaris, it will come to the constellation of Leo (the Lion) marked by a curved line of stars shaped rather like a question-mark twisted the wrong way round. Regulus is the brightest of these stars, which make up the so-called 'Sickle'.

It is often said that a star twinkles, but a planet does not. This is not strictly true, since a planet low on the horizon may twinkle violently, but it is correct to say that a planet twinkles less than a star, because it appears as a tiny disk instead of a mere point of light. Twinkling is due entirely to the effects of the Earth's atmosphere, so that it is most marked with objects low down in the sky.

Superficially one star looks very much like another, but the careful observer will soon notice that the colours are not all the same. Of the three brightest stars in the northern hemisphere of the sky, Arcturus is orange, Capella yellow, and Vega bluish. All three are visible during spring evenings (though Vega is ad­mittedly rather near the horizon), and it is instructive to compare them. Binoculars, of course, will bring out the different colours very well, and will also show various hues among the less brilliant stars; thus Dubhe, the senior of the two Pointers, is decidedly yellower than its companion Merak.

There are also some stars which are double, since they are made up of two components very close together. The best example of a naked-eye double is Mizar, the second star in the tail of the Great Bear, which has a much fainter star, Alcor, close beside it. There is a minor mystery about Alcor, since the old Arab astro­nomers of a thousand years ago stated that it could be seen only under good conditions and by keen-sighted observers. Nowadays this is not the case, and anyone with normal vision can see it whenever the sky is reasonably dark and there is no mist or cloud about. Telescopically Mizar itself is found to consist of two com­ponents, so close together that to the naked eye they appear as one, while one of these components is itself known to be double.

Other naked-eye pairs may be found, notably Nu Draconis in the Dragon's head and Epsilon Lyrae in the small but interesting constellation of Lyra, the Harp. Epsilon Lyra lies close to Vega, and is of special interest because it is a multiple object; each component is again double, so that we have a fine example of a quadruple star. With the naked eye, the appearance is that of two rather faint stars, very close together. Another easy pair L: Theta Tauri, in the Hyades cluster close to the brilliant reddish Aldebaran.

The naked-eye observer can also interest himself in variable stars, which brighten and fade over relatively short periods of a few days or a few months. Betelgeux in Orion is one such case sometimes it is little brighter than Aldebaran, though it has ah: been known to rival the brilliant Rigel. Other variables art Scheat in the Square of Pegasus, Mira in Cetus (the Whale), an: two of the stars in the famous W of Cassiopeia. With all these stars the variations are intrinsic, but things are different with Algol, the 'Demon Star' in Perseus, which seems to shine steadily for two and a half days at a time and then exhibits a long, slow 'wink' lasting for several hours before regaining its normal brilliancy. Strictly speaking, Algol is not variable, but is made up of two stars, one much more luminous than the other, moving round their common centre of gravity; when the fainter star passes in front of the brighter, the total light shows a decrease. With Algol, the whole cycle of changes is easy to follow with the naked eye.

Of the star-clusters, much the most celebrated is that of the Pleiades or Seven Sisters, which is prominent throughout the winter but which sets soon after the Sun by spring.

Normal-sighted persons can see seven separate stars in the group, so that the familiar nick-name is very appropriate. On the other hand, Eduard Heis, a last-century German astronomer, is reputed to have been able to see nineteen Pleiades without optical aid!

The Hyades, round Aldebaran, are brighter than the Pleiades, but are more scattered, so that the effect is not so spectacular. (Incidentally, Aldebaran itself is not a true member of the cluster; it simply happens to lie in much the same line of sight, and is only half as far away from us.) Another naked-eye cluster is Praesepe in Cancer; moonlight will drown it, but on a dark night it is by no means hard to see. It is a fine sight in a telescope, and has earned its unofficial name of’ the Beehive'.

Occasionally a bright nova, or temporary star, will appear in the sky. To be precise, a nova is not a new star; what happens is that a formerly very faint star suffers an outburst which makes it flare into short-lived prominence, though after a brief period of glory it fades back to its former obscurity. The outburst affects only the star's outer layers, so that no lasting damage is done, whereas in the much rarer supernovse the star's material is blown away into space, leaving nothing more than an expanding cloud of gas. Four supernova;, the stars of 1006, 1054, 1572, and 1604, have appeared in our Galaxy since records began; the first of these has left its debris in the form of the Crab Nebula, which is of the greatest interest to both optical and radio astronomers, but which is below naked-eye visibility. On the other hand, bright novae have been seen often enough, and some of them have been discovered by amateurs. There is always a chance that the naked-eye enthusiast will be fortunate enough to detect a nova, though it must be admitted that the chances are heavily against it.

Only three of the outer galaxies are visible without optical aid and two of these, the Nubecula; or Clouds of Magellan, are too far south to be seen in Europe. The third is the Great Spiral in Andromeda, which may be glimpsed as a dim misty speck.

So far as the Solar System is concerned, the naked-eye observer has considerable scope. He can see some of the waterless 'seas' on the face of the Moon, he can make useful observations of meteors and auroras, and he can study the artificial satellites such as the American Echo balloons.

Before the war, our knowledge of meteor paths depended almost entirely upon observations made without a telescope. The procedure was to plot the apparent track of the meteor against the stars, and estimate its magnitude and duration; if the same meteor were seen by another observer some way off the true height and path could be worked out. This sort of work is still useful, though it is only fair to add that radar methods have now been brought into play and yield more accurate results. Sporadic meteors may appear from any direction at any moment, but most of the bright shooting-stars belong to definite showers, so that some periods of the year are more favourable than others. The most consistent shower, that of the Perseids, is visible in early August; anyone who stares up at a clear, dark sky for a few minutes between, say, 28 July and 15 August will be unlucky not to see at least one meteor.

Auroras, on the other hand, are phenomena of the high atmos­phere, though their cause lies in the Sun. During 1963 and 1964 there were few bright displays, because the Sun was going through the quietest phase of its eleven-year cycle, but activity has now begun to increase again, and there should be more frequent aurora: during the next few years. Since the wonderful lights are due to particles sent out from the Sun, and these particles are magnetic, displays of aurora are best seen in high latitudes; a winter night in, say, Iceland or north Norway would be dull without them. Observers in the northern and central parts of Scotland are often able to see brilliant aurorae, though in England the opportunities are much fewer.

By now there are a great many artificial satellites circling the Earth, and a few of them are really bright, appearing as slowly- moving stars creeping across the sky. Most prominent of all are Echo I and Echo II, both American-launched and both of the balloon type; since they are larger than most of the man-made moons, and were deliberately coated with reflective material, they can hardly be over-looked, and predictions for them are given in various periodicals and daily newspapers. Amateurs have done excellent work in checking on the positions of bright artificial satellites. The procedure is to time the moment when the satellite passes between two known stars or else comes to a position which may be plotted easily on the star-chart. (Gases in which a satellite passes right in front of a star are convenient, but surprisingly rare.) The only essential equipment for this sort of work is a reliable stop-watch, together with a really good knowledge of the constellations.

Though the professional astronomer would be more or less helpless without his telescope, any casual watcher can learn much even if he has no optical aid at all. There is endless variety in the night sky, and there are many fascinating objects to be seen by anyone who knows where and when to look for them. One does not need a powerful telescope in order to take a real interest in astronomy.