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Chapter 8 the Celestial Sphere

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Chapter 8 the Celestial Sphere Chapter 1 Naked eye observations 1.1 Introduction The etymology of the word ‘Astronomy’ implies that it was the discipline involved in ‘the arranging of the stars’. Today we might say that astronomy is our attempt to study and understand celestial phenomena, part of the never-ending urge to discover order in nature. We do not know who were the first astronomers—what we do know is that the science of astronomy was well advanced in parts of Europe by the middle of the third millennium BC and that the Chinese people had astronomical schools as early as 2000 BC. In all ages, from the burgeoning of man’s intelligence, there have been people fascinated by the heavens and their changing aspect and these people, as far as their cultural environment has allowed them, have tried to formulate cosmologies. We are no different today. Nowadays, the word ‘Astrophysics’ is also used to describe the study of the celestial bodies. In fact, many astronomers use both terms quite generally and it is not infrequent to find Departments of Astronomy and Astrophysics within educational establishments. The question may well be asked ‘What is the difference between Astronomy and Astrophysics?’ Very loosely, Astronomy might be defined as the subject of the ‘where and when’ related to the description of a celestial body with the ‘why and how’ being covered more by Astrophysics. Rather than trying to provide a hard and fast rule for the terminology, we will simply use Astronomy to cover all aspects of the description of the skies and the Universe. If our current theories of the Universe are nearer the truth, it is probably not that our intelligence has increased in the past six millennia. It is more likely that the main factor has been the discovery and development of the ‘scientific method’, which has led to our present civilization based on the flood of technological advantages provided by this method. This has enabled scientists in far greater numbers than ever before to devote their lives to the study of the heavens, assisted by telescopes, computers, space vehicles and a multitude of other equipment. Their attempts to interpret and understand the wealth of new information provided by these new instruments have been aided by allied sciences such as physics, chemistry, geology, mathematics and so on. We must remember, however, that for more than nine-tenths of the last five thousand years of our study of the heavens, we have had to rely on the unaided eye. The Mediterranean people who set the constellations in the sky, the Babylonians, Egyptians and Greeks, the Arabian astronomers who flourished during the Dark Ages of Post-Roman Europe, the Chinese, the Mayan and other early American astronomers, all built their theories of the Universe on naked eye observations. And so we begin by following in their footsteps and seeing what they saw as they observed over a few minutes (see section 1.2), over a few hours (see section 1.3), over a month (see section 1.4) or over at least a year (see section 1.5). In this way, we will find it easier to understand why their cosmological theories were formulated in their particular ways. 3 4 Naked eye observations 1.2 Instantaneous phenomena 1.2.1 Day During the day a variety of phenomena may be seen. In a particular direction lies the Sun, so bright it is impossible (and dangerous) to look directly at it. In general, the sky background is blue. The Moon may also be visible, having a distinct shape though certainly not circular. If the Sun has just set or if dawn is not far away, there is sufficient daylight to see clearly. We call this condition twilight. On the horizon opposite to the twilight glow, a dark purple band is sometimes seen. This area corresponds to a zone on the sky which is cut off from the direct sunlight by the Earth and is receiving very little light by scattering from the atoms and molecules in the atmosphere. It corresponds, in fact, to the shadow of the Earth in the sky. Its presence tells us of the extreme purity and low humidity of the local atmosphere. Needless to say, it is very rarely seen in Britain. To the ancients, clouds, wind, rain, hail and other atmospheric phenomena were inadequately distinguished from what we term celestial events. Our civilization includes them in meteorology,a science quite distinct from astronomy, so that we need not consider them further, except to remark that astronomers’ observations have, until recently, been dependent entirely upon good weather conditions being available. With the development of radio telescopes and the fact that other equipment can be placed in artificial satellites and operated above the Earth’s atmosphere, this dependence is no longer complete. 1.2.2 Night If seeing conditions are favourable, a view of the night sky provides a far wider variety of celestial phenomena. If the Moon is visible, its brightness will dominate that of all other objects. Its shape will be crescent or gibbous or even circular. At the last condition, its apparent diameter is very close to that of the Sun. To anyone with reasonable eyesight, its surface will not be evenly bright. Areas darker than their surroundings will be noticed, so that the fancy of primitive man could see a ‘Man in the Moon’, a ‘Beautiful Lady’ or a ‘Rabbit’, sketched out by these features. In addition to the Moon, some two to three thousand tiny, twinkling points of light—the stars—are seen, ranging in brightness from ones easily visible just after sunset to ones just visible when the Moon is below the horizon and the sky background is darkest. Careful comparison of one bright star with another shows that stars have different colours; for example, in the star pattern of Orion, one of the many constellations, Betelgeuse is a red star in contrast to the blue of Rigel. The apparent distribution of stars across the vault of heaven seems random. With the eyes becoming accustomed to the darkness, a faint band of light, the Milky Way, catches the observer’s attention. Modern astronomers, with the aid of telescopes, know that this luminous region stretching from horizon to horizon across the sky in a great circle is made up of a myriad of stars too faint to be resolved with the naked eye. To the ancient observer, its presence inspired all kinds of speculations, none of them verifiable. One or two of the tiny points of light may draw a closer scrutiny. They shine steadily, in contrast to the twinkling of the stars and they are among the brightest of the star-like objects. There must be some reason why they are different. If our observer is going to watch for a few hours, attention will be returned to these objects. 1.3 A few hours 1.3.1 Day The heavens are never static. The slowly-moving shadow cast by an upright rod or a boulder or tree reveals the Sun’s movement across the sky. If observation is kept up throughout the day, the Sun is A month 5 seen to rise above the eastern horizon, climb up the sky in a circle inclined at some angle to the plane defined by the horizon and culminate, i.e. reach a maximum altitude above the line joining the north to the south points, then descend in a mirror image of its forenoon path to set on the western horizon. If the Moon is seen during that day, it will appear to imitate the Sun’s behaviour in rising and setting. 1.3.2 Night As darkness falls, the first stars become visible above the eastern horizon. With the ending of twilight the fainter stars can be seen and, as the hours pass, the stellar groups rise from the eastern horizon, reach their maximum altitude like the Sun, then set or become dim and invisible as daylight returns. The impression of being on a flat plane surmounted by a dark revolving bowl to which the stars are attached is strong, especially when it is seen that there are many stars in a particular region of the sky that revolve, never rising, never setting, about a hub or pivot. These stars are said to be circumpolar. It is then clear that those other stars that rise and set do so simply because their circular paths about this pole are so big that they intersect the horizon. The Moon also revolves across this upturned bowl. Although the Moon appears to have an angular motion across the sky similar to that of the stars, careful observation over a few hours reveals that it moves slightly eastwards relative to the star background. Occasionally a bright object, called a meteor, shoots across the sky in a second, looking like a fast-moving or ‘falling star’. It may be too that faintly luminous sheets are seen, hanging down the bowl of the heavens like great curtains. These are the auroraeW1.1. If our observer is watching at any time after October 4, 1957, it is quite likely that one or more faint specks of light will be seen to cross the sky, taking a few minutes to do so, their presence giving reminder that man-made satellites are now in orbit about the Earth. Indeed, one of the latest satellites— the International Space StationW1.2—is exceedingly bright—as bright as the brightest planet Venus— and bears testament to the continual development of manned orbiting laboratories. 1.4 A month The month is the next period of any significance to our watcher.
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