Nakedeye astronomy

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To our ancestors, the sky was both a calendar and a clock. For better or worse, improvements in the calendar and the invention of clocks have rendered even the most rudimentary knowledge of the stars, or the movements of the Sun and the Moon, more or less superfluous. These days, there is no practical incentive for anyone, other than an astronomer, to take an active interest in what is going on in the night sky. Consequently, most of us are not in the habit of looking at it closely as a matter of course.

Another reason why we don't look at the sky is the belief that most celestial objects and events are so remote that they can be seen only with a telescope. It's true that every single advance in astronomy since the early years of the seventeenth century, when a telescope was turned skyward for the first time by Galileo, has relied one way or another on the telescope. Without telescopes, our view of the Universe would probably be as circumscribed as that of the ancient Greeks. We would still believe that the Solar System was at the centre of the Universe, that there was only one Moon, that there were only five planets, and only a few thousand stars. Although Copernicus worked out his ideas without the benefit of telescopic observations, without the precise measurements of planetary motion for which a telescope is essential, the Copernican hypothesis might have remained just that: an unprovable theory. Telescopes have allowed astronomers to discover things that no one could have guessed at, and without which astronomy would have not have advanced much beyond Copernicus.

Compared with a telescope, the naked eye seems a puny thing. And so it is. Yet there is no better way to become acquainted with the workings of the night sky, i.e. with the movement of stars, planets and the Moon. With the naked eye you can scan the entire sky in a matter of seconds, and take in with a single glance the spectacle of a starry sky, the majesty of the Milky Way, or the thrill of a meteor shower. These are not things you can do with a telescope. Unencumbered by a telescope, you are free to roam where you will and look at whatever takes your fancy - straining to make out the Moon's surface features at one moment, noticing a halo about the Moon at the next, while delighting in the metallic and deeply shadowed landscape illuminated by the bluish light of a full Moon.

Although most of us don't look at the night sky in the hope of making discoveries, for which a telescope is essential, everyone should look at the Moon and the planets through a telescope at least once. The sight of the Moon's cratered surface, or the mottled disc of a planet, are breathtaking sights. But to become acquainted with the sky as a whole it really is necessary to look at it without a telescope. There is more than enough on view to keep anyone occupied for a lifetime. The time to think about getting a telescope is when you've exhausted all there is to see without one, or if you become particularly interested in seeing things that can't be seen with the naked eye. Even so, a telescope is not for the casual observer. Quite apart from the expense, are you going to make good use of it? Will you use it a few times and then leave it to gather dust in a cupboard? My advice is to get a good pair of binoculars instead. These are cheaper, more portable, and can be used night and day.

The important thing is not to feel that you have to approach the night sky as a budding astronomer. Naked-eye astronomy is a personal odyssey for knowledge, for spectacle and, above all, for a closer engagement with nature, as in the moment when you notice the unblinking dot of light that is a planet, and realise that there in the sky is another world accompanying Earth in an never-ending race around the Sun, and not just some abstraction in a book. An unexpected glimpse of a planet can be as exciting as an hour spent looking at it through a telescope. It all depends on your frame of mind and on what you want from the sky. Just keep in mind that you are free to look at whatever interests you, and ignore everything else.

I think that the greatest challenge we face when we look at the night sky, and the most exciting, is to free ourselves from the illusion that the Earth sits immobile at the centre of the Universe. In this sense, our experience of the night sky is no different from that of our pre-Copernican ancestors. When we look up at the stars for any length of time we become aware that the entire sky appears to rotate very slowly from east to west. This gives us the impression that it is the Earth that is at rest, and that the heavens revolve around it. Yet, at the same time, we know that this experience is an illusion. We accept, at least intellectually, that the Sun is at the centre of the Solar System, and that the Earth and planets orbit around it. Nevertheless, the geocentricism implicit in the apparent motion of the heavens is a powerful illusion, and to free oneself from it requires a considerable mental effort.

If you are to succeed in this it is not enough merely to be acquainted with the heliocentric, or Sun-centred, theory of the Solar System. This theory, at least in its essentials, is extraordinarily simple, so simple that even a child can grasp it. Yet, when confronted with a starry sky, we have no sense of the Earth's motion. Instead, the Sun and Moon appear to chase one another endlessly around the sky, and planets to weave their way among the fixed stars from one day to the next, all apparently circling the Earth. The challenge is to make the link between your first hand, Earth-centred experience, and the heliocentric reality; and this requires considerable imaginative effort. What follows is intended, in part, to help you make the necessary connections between what you see, and the reality behind it.

It has to be said that if you want to become familiar with the night sky you must be prepared to be a constant, if informal, skywatcher. Through sustained and thoughtful naked-eye observation - yes, you must think about what you see - you will gradually become familiar with the motions of celestial bodies, so that sooner or later you come to understand them for yourself, without the need to have them explained by someone else, or to refer to sky maps or data concerning planetary positions. Of course, this demands a degree of commitment. And while you don't have to go outside on every clear night, you should get into the habit of looking at the sky from time to time to keep track of what's going on 'up there'.

Every time you look at the night sky keep in mind that one of the things you are trying to understand is how the apparent Earth-centred motions of celestial bodies are linked to their real Sun-centred motions. The test of whether you have succeeded in this is whether, from the comfort of an armchair, you can picture in your mind's eye, at any time of the year, the approximate relative positions of the planets, including the Earth, and the direction in which they are moving around the Sun.

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