| Looking Up:
A Musical Interlude.
As you came into the lecture room today, I was playing for you a piece of music with an accompanying choral setting. As I explained earlier, I will be doing this from time to time, for a number of reasons. Partly, I must admit, it is self-indulgence: I like to listen to a great variety of music, and this gives me an opportunity to pass some of my enthusiasm on to you. But there is also a serious reason: some of the pieces or excerpts I play for you will have an astronomical connection - not always very direct! - which may help you remember some important facet of the subject.
I am always interested to see if there is in the class who can identify any music I may happen to use, and explain my choice and its relevance. Here are the words which were set to today's music:
"O vast Rondure, swimming in space
Covered all over with visible power and beauty,
Alternate light and day and the teeming spiritual darkness,
Unspeakable high processions of sun and moon and countless stars above,
Below, the manifold grass and waters,
With inscrutable purpose, some hidden prophetic intention,
Now first it seems my thought begins to span thee.
The poetry is by Walt Whitman, the American poet who provided the words I used in the first lecture; and the music is from the `Sea' Symphony, the first symphony by the great English composer Ralph Vaughan Williams. You can see why I find the words appropriate: we are indeed now at the point at which our `thoughts will begin to span' the mysteries of the cosmos. Let us begin!
How Does Information Reach Us from the Universe?In this and several subsequent lectures, we will be skipping back and forth between two tracks. On the one hand, I will need occasionally to tell you the right answers now, so that you have a correct understanding of the phenomena you are studying (why we see eclipses, for instance). But on the other hand, part of my motivation for this course is to give you some idea of how astronomical understanding developed through the ages, and how and why ancient ideas gave way to modern. I will ask you, from time to time, to put yourselves in the place of the ancients, a few thousand years ago, and to ask yourselves what you might hope to learn (how far away is the sun? does the Earth move through space? ) and then to think about how you might achieve those objectives. How do we know anything at all about the universe (by which I mean things outside the Earth, beyond the atmosphere)? Information comes to us in four ways: 1 in the form of ordinary matter, including meteorites, cosmic rays, lunar rocks brought back by the astronauts, and so on. 2 in the form of neutrinos, special sub-atomic particles which are the focus of much research interest here at Queen's (and about which we will learn more in Phys 016 when we consider the nuclear reactions which are occurring within stars). 3 as gravitational "waves," which were predicted by Einstein. These `ripples' in the very structure of space are understood to spread out at the speed of light whenever there are significant changes in the spatial distribution of big chunks of matter, as for instance when two massive stars in a binary pair are orbiting each other rapidly. Although such waves have not yet been directly detected, there is strong inferential evidence (from the behaviour of binary stars) that they must indeed exist. Again, we will look at this subject later, in the Phys 016 'Stars and Galaxies' companion course. 4 as electromagnetic radiation - that is, light, of a vast range of wavelengths. In other words, we merely look out into space -- but please note that we use not just our eyes and optical telescopes, but also radio telescopes, infrared detectors, X-ray telescopes, and so on. It is important that you understand that there is more to light than just what the eye is sensitive to ('visible light'). This is discussed on pages 158-159 of your text [pp 156-158 in the second edition]. Of course, the ancients had no knowledge of neutrinos or gravitational waves, and must have been mystified by the occasional arrival of large meteors (although, as we will see, there are indications that at least some people realised that meteors were coming in from spaces outside the usual earthly realm). But even though they were restricted to observations in visible light -- and moreover only those which could be carried out with the unaided eye, since there were no telescopes until the 1600's -- the intelligent thinkers of several millennia ago were able to make amazingly good progress in developing sensible notions of what must be going on. I hope that you will come to appreciate just how scientifically sophisticated these people were. Of course, there was often a strange mix of mysticism and irrationality in the thinking of ancient times, at least by today's standards, but it does our forebears a great disservice to represent them as hopelessly primitive. Let us begin, therefore, by putting ourselves in the place of an inquisitive person of several thousand years ago. What would we have seen? And what obvious and fundamental questions would have leapt to our minds as needing explanation?
Prominent Phenomena Making Regular Appearances.The ancients would certainly have been aware of the regular coming and going of certain very prominent and conspicuous astronomical phenomena, including: the sun, which is mostly unchanging in appearance, although not in position. It must be noted that prominent dark sunspots can sometimes be seen with the unaided eye. Ancient Chinese astronomical records describe such sunspots. (Please note, however, that you should never stare at the sun, except through carefully designed filters. Quick glances out of the corner of your eye can reveal the sunspots if they are big enough. They are more easily and safely seen when the sun is considerably dimmed, as in the late evening when you see the setting sun through thin cloud or haze.) the moon, which varies through its phases - sometimes full, sometimes crescent, etc. Figures on pages 41 and 42 of the text [page 52, seoond edition] make clear why these phases occur. Please note that it has nothing to do with the Earth's shadow falling on the moon's surface, which happens only rarely (and produces what is called a lunar eclipse). The Earth's shadow can fall on the moon only when the Sun, Earth, and Moon are perfectly lined up: that is, a lunar eclipse can only happen at the time of a full moon. (See page 45 [p55, second edition].) stars, which are not all of the same brightness. The fact that the stars are not all the same brightness has a couple of immediate implications: If the stars are all identical, they cannot all be at the same distance from us. (Those which are somewhat farther away would look somewhat fainter, of course.) Conversely, if the stars are all at the same distance, like points of light painted on the flat `plane' of the sky, they cannot all be identical: some must be intrinsically brighter that the others. Indeed, it is possible that both of these things are true: there may be stars of a variety of brightnesses, located at a variety of distances from us. The problem, of course, is to find a way to determine those distances. With a little imagination, one can see patterns: the constellations. The `pictures' they are seen to make are not the same in all cultures, and indeed most of them don't look much like their namesakes, to my eyes at least. (Scorpius and Orion are good examples of constellations which do, however.) In fact, the stars in a constellation are usually completely unrelated to each other (see Figure 2.2 on page 28 of the text [Fig 2.3, page 41 of the second edition]), and the different motions of the stars through space will eventually lead to new patterns being formed. In general, though, the stars seem not to change in position with respect to one another for many years - if, for instance, you live to be five hundred years old, you will not notice any perceptible change in the constellations. This stability is not to be confused with the fact that the whole pattern of stars moves across the sky during the night, for the same reason that the sun moves across it in the day: the Earth is spinning (rotating) on its axis. The unchanging nature of this pattern is why the ancients spoke of the `fixed stars,' although even the ancient Greeks were aware that at least some of the individual stars do slowly change in position. You can think of the stars as providing a fixed backdrop against which we can watch the changing positions of the sun, moon, and planets; but the backdrop itself wheels overhead every day. By the way, the constellations have no real significance to astronomers except insofar as they provide a reference frame for knowing where to find interesting objects and phenomena. In late 1995, for instance, we were advised to look `near the Big Dipper' to see Comet Hyakutake. But professional astronomers like myself don't need to know the star patterns in detail (and in fact I don't know them as well as I should) since our telescopes are computer-controlled and can be directed very precisely towards the targets we have chosen without any reference to the boundaries of constellations. Moreover, the targets of our research are usually far too faint to be seen with the unaided eye. By contrast, astrologers do assign a significance to the constellations - in particular, those in the Zodiac, the dozen or so constellations through which the sun appears to pass as we orbit it during the year. (See the figure on page 35 of the text [p 49, second edition].) If time permits in the course, I will discuss why it is that I believe astrology to be a worthless exercise, except perhaps if you appreciate it purely for its entertainment value. I find it disturbing, however, to discover that people will predicate important and sometimes life-defining personal decisions on the dubious advice provided by astrologers. `Wandering' points of light which are sometimes present and sometimes not to be seen. From the Greek word for wanderers, these are called planets. In total, the ancients would have known of exactly five planets (not recognizing the Earth as one): Mercury, Venus, Mars, Jupiter, and Saturn. By the way, Uranus can sometimes just be seen with the naked eye in ideal conditions, but it was apparently not known to the ancients. Neptune and Pluto are too faint ever to be seen without a telescope. When I wrote the first version of these notes in September 2000, the most conspicuous planet in the night sky was Jupiter, an object which was brightly visible in the southern sky a few hours before sunrise. But if you see Jupiter in the middle of some familiar constellation now, you will discover it to be well outside the confines of that constellation a month or two later. Like all the planets, it is slowly on the move. There are also times in which we cannot see Jupiter at all. This is because it is on the far side of the sun, from our Earthly vantage point, and invisible in the brightly-lit daytime sky. As we will see, the planets do not follow simple paths over the backdrop provided by the 'fixed' stars. The complexities in the `wanderings' of the planets result from the fact that the Earth and the other planets orbit the sun in different paths and at different speeds, with confusing results. The ancient Greeks fell into the fairly obvious trap of believing the Earth to be at rest, an assumption which made the overall behaviour almost uninterpretable. It is important, by the way, to remember that the planets are so far away that the unaided eye can discern no details. Inevitably, they look like points of light, just as the stars do, and are distinguished from them only by their varying positions and brightnesses. ( You cannot, for instance, see the rings of Saturn without a telescope.) To even imagine that these points of light are bodies like the Earth would have taken a remarkable imagination! (Likewise, we all know now that the sun is the nearest star. But would it have been obvious to the ancients that the sun is a star at all? Would they necessarily have made the connection? Some imaginative ancients certainly speculated that this was so.)
Vestiges From the Past.It is interesting to note that various aspects of the earliest observations and speculations have been handed down through the succeeding generations in ways which are not merely scientific. For instance, certain everyday words and experiences come down to us from the astronomy of the ancient days we have been describing: The religious and superstitious importance of the number 7 may have its origin in the fact that there were seven wandering objects among the fixed stars: the Sun, the Moon, and five naked-eye planets. The names of the days of the week - again, seven of them! - in many languages have clear astronomical roots. Consider, for instance, mercredi, vendredi, mardi in French; lunes, martes, miercoles in Spanish; Montag, Sonntag in German; Sunday, Monday, Saturday in English. Words like disaster (bad star), arctic (from the Greek word for a northern constellation), influenza (if you have it, you are literally under the influence of a bad star), and many others have astronomical origins. Astrology (the casting of horoscopes). Please note that I am not saying that astrology works, but rather that the very existence of the subject is a descendent of long-ago times in which it was believed to be part of what determined how your life would unfold.
More Regularities.Let us finish our list of the regular, repeatable phenomena which the ancients would have seen in the night skies. They would have seen the Milky Way, a band of faint diffuse light stretching across the sky. We now recognize the Milky Way as a galaxy, and it is interesting to note that the word galaxy itself comes from the Greek for `milky way' - perhaps you recognize the stem of the word `lactic' (as in lactic acid or lactose intolerant), for instance. They would have recognized the existence of the ecliptic, which is a broad zone across the sky outside of which we never see the sun, moon, or planets. (The constellations along the ecliptic, a dozen in number, make up the Zodiac.) "There is a profound piece of astrophysics which flows from this apparently trivial observation, so let us pause for a moment to think about what the existence of the ecliptic means. The first thing to recognize is that, from our point of view, the moving objects in the Solar System are restricted as to what part of the sky they may visit! For instance, a person living on or near the equator will sometimes see the sun overhead, and also the various planets and the moon; but a person at the North Pole never sees that happen. Why not? In fact, there is nothing mysterious about the phenomenon, as an everyday analogy will show you. Imagine walking around town always in a vertical posture (you never lie down flat on a park bench, for instance). As you walk about, you will see cars to the left and right of you, as well as in front of or behind you, but never above your head or below your feet because they must drive on the solid surface of the Earth and are restricted to the level ground around you. (Of course, if you walk into a parking tower you can get yourself into a situation where there may be cars overhead, on the floor above. Let us ignore that exceptional sort of circumstance, however.) The sun, moon, and planets obey a similar set of 'traffic rules', with the following consequences immediately implied: 1 The solar system is flattened, with all the planets and the moon moving around the sun in nearly the same plane (see the figure on page 199 of your text). This is the counterpart of saying that the cars must stay on the ground: that is, there is a restricted plane within which they must move. 2 Moreover, the Earth orbits the sun while spinning on an axis which points in a constant direction in space, almost perpendicular to this plane. (This is the analogue of you walking about in a vertical posture.) The Earth never `lies down.' [Digression: You already know, from earlier discussions, that the Earth's spin axis points towards a particular star, Polaris, which is also called the North Star. (This is why a person standing at the North Pole always sees Polaris nearly exactly overhead.) As we will learn later, this is merely a coincidence and will not be true forever: the Earth's spin axis very slowly changes its orientation in space, over many centuries, in a phenomenon known as precession.] Our conclusion that the solar system is flat may strike you as uninteresting or inconsequential, but in fact the implications are quite profound. As we will learn, this consideration tells us a great deal about the way the solar system came into existence. The second conclusion, the fact that the Earth's axis maintains a stable orientation in space, is also a fascinating piece of physics which I will explore a bit later on. The ancients would even have known about the regularity of eclipses, as rare as these are. Careful record-keeping over the centuries can show quite clearly that there is a certain periodicity in the eclipse record, and indeed in centuries BC people were able to correctly predict the coming of eclipses. There were, of course, societies in which such analyses had not been made, and to such people eclipses must have been unexpected and possibly terrifying. But many cultures had the power to predict and understand the reason for eclipses. Rare or Transient Phenomena The ancients would also have seen phenomena which come and go rarely and last only briefly. Remember, of course, that they would not have been able to make a clear distinction between astronomical phenomena (like comets) and atmospheric phenomena (such as rainbows, or an odd colour to the moon caused by lots of volcanic dust thrown up into the Earth's stratosphere). Until the middle ages, for instance, it was thought that comets were located high in the Earth's atmosphere. Meteors and the Northern Lights (Aurora Borealis) are, in a sense, a bit of both. The light is produced in the atmosphere in both cases, but the fundamental cause is astronomical - the falling in of stones (meteorites) or charged particles from the sun, respectively. The ancients would have seen: meteors ("shooting stars" or "falling stars") comet sstars which vary in brightness . One example of this is Algol, the name of which comes from the Arabic and means something like `the ghoul,' or `the mysterious one.' Every few days, this star dims perceptibly, something which is visible to the naked eye if you know where to look. The name may reflect the ancient puzzlement over its strange behaviour. stars which appear suddenly where no stars were to be seen before. Typically they get very bright, then fade away completely. These are novae and supernovae, in modern terminology; they are stars which are literally exploding. and sunspots.
What Conclusions Might The Ancients Have Drawn?I would suggest that one of the most important aspects of such astronomical work would be the realization that there is great order and reliability in the cosmos. This can be seen in a limited, almost technical sense: the sky provides a clock of great precision, and indeed astronomy still provides the basic underpinning of modern time-keeping. But it is just as true in a more profound sense: the Sun seems unchanging and eternal in its life-giving properties, the monthly fading and renewal of the moon can be counted upon, and so forth. This reliability must have been quite a contrast to the arbitrary nature of life in those days, with unpredictable illnesses, plagues, droughts, wars, etc. Even the most deserving and upright of citizens might easily be struck down by (say) some then-incurable disease, and life must have seemed very capricious and unfair in many respects. This profound reliability, coupled with the mysterious and grand spectacle of the skies and the dominant importance of the sun in particular (and importance of the moon for harvesting, hunting, etc) would naturally have led to a deep religious interpretation.
What Would You Like To Know?So far, I have described some of the regular, repeatable phenomena which ancient people would have seen in the sky. Now imagine yourself again one of those people, imbued with intelligence and curiosity. What would you next have wanted to know in trying to make sense of the universe? What are the questions which you might logically have asked at this stage? I believe that the obvious questions would be: What is the shape of the Earth? Is it flat, or spherical, or ...? What moves? Are we spinning, or does the whole universe rotate around us? Likewise, are we moving through space, or fixed in location? How big are things? How big is the Earth? (That might be moderately easy to measure.) How big is the sun? How far away are things? Is the moon farther or nearer than the sun? What can we find out about the planets? How far away are they - perhaps not in miles, but relative to one another and to the moon and sun? What are the stars like? Are they just very remote suns, or something quite different? How does it all function? (Think of trying to understand the positions and orbits of all the bits and pieces as you would try to work out the functioning of a mechanical clock.) In other words, we want to develop a cosmology, or cosmological model - a description of the universe (which, to the ancients, meant just the Solar System: the stars were just the backdrop against which everything else happened). There are two especially important things to note: 1 The question of the sizes of things is especially important if if we are to do any meaningful (astro)physics. If the sun is merely 100 metres over your head, it is little more than a bright bonfire. If, instead, it is 150 million kilometers away, it is a whole new kind of object, much more imposing than the Earth itself. (In a modern context, the sun would then also be recognized as so large that it is subject to a whole lot of different physics. Remember our earlier discussion of scaling things up to big sizes). 2 The determination of distances in astronomy is not easy! What we see in the sky is dots of light on a dark background, but it is very difficult to determine which dots are closer and which farther away, or to determine the true distances with any precision at all. You can determine the distance to a remote town by pacing it out, if necessary; but if you lived 4000 years ago, how would you ever have estimated the distance of the moon or sun? Here is a quote from Lucretius (writing in about the third century B.C.). He says "``...as to the size of the sun's blazing disc: it cannot in fact be either much larger or much smaller than it appears to our senses.''" How big did Lucretius think the sun was in real linear dimensions, like kilometers? He would probably have been astounded to learn that the Sun is actually one hundred times the diameter of the Earth, and has one million times the total volume. It is worth remembering that, unless you know something about the distance, how big across an object looks is utterly devoid of meaning. (This is a topic we explore in the first lab.) The interrelatedness of estimates of distance and size is often overlooked. Consider, for example, that UFO spotters are occasionally reported to say that they saw something ``about fifty feet long'' passing overhead. But unless you know how high up the object is -- a thousand feet? one hundred miles? -- the estimate is pure guesswork, and perhaps preconditioned by our expectation that a `flying saucer' would be about the size of a large aircraft. Previous chapter:Next chapter
0: Physics 015: The Course Notes, Fall 2004 1: Opening Remarks: Setting the Scene. 2: The Science of Astronomy: 3: The Importance of Scale: A First Conservation Law. 4: The Dominance of Gravity. 5: Looking Up: 6: The Seasons: 7: The Spin of the Earth: Another Conservation Law. 8: The Earth: Shape, Size, and State of Rotation. 9: The Moon: Shape, Size, Nature. 10: The Relative Distances and Sizes of the Sun and Moon: 11: Further Considerations: Planets and Stars. 12: The Moving Earth: 13: Stellar Parallax: The Astronomical Chicken 14: Greek Cosmology: 15: Stonehenge: 16: The Pyramids: 17: Copernicus Suggests a Heliocentric Cosmology: 18: Tycho Brahe, the Master Observer: 19: Kepler the Mystic. 20: Galileo Provides the Proof: 21: Light: Introductory Remarks. 22: Light as a Wave: 23: Light as Particles. 24: Full Spectrum of Light: 25: Interpreting the Emitted Light: 26: Kirchhoff's Laws and Stellar Spectra. 27: Understanding Kirchhoff's Laws. 28: The Doppler Effect: 29: Astronomical Telescopes: 30: The Great Observatories: 31: Making the Most of Optical Astronomy: 32: Adaptive Optics: Beating the Sky. 33: Radio Astronomy: 34: Observing at Other Wavelengths: 35: Isaac Newton's Physics: 36: Newtonian Gravity Explains It All: 37: Weight: 38: The Success of Newtonian Gravity: 39: The Ultimate Failure of Newtonian Gravity: 40: Tsunamis and Tides: 41: The Organization of the Solar System: 42: Solar System Formation: 43: The Age of the Solar System: 44: Planetary Structure: The Earth. 45: Solar System Leftovers: 46: The Vulnerability of the Earth: 47: Venus: 48: Mars: 49: The Search for Martian Life: 50: Physics 015 - Parallel Readings.
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(Wednesday, 20 January, 2021.)