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Chapter 1:
Something in the Way She Moves
The striking gleam of Venus hanging low and bright in our morning and evening skies, outshining all but the Sun and Moon, has demanded the attention of ancient and modern skywatchers, inspiring religious and scientific wonder. As long as we have been human, Venus has led us to worship, fear, sing, speculate, invent, model and calculate. It is like no other celestial object in brilliance, and in the strangeness and complexity of its motions. It is not surprising that this brightly shining, strangely looping, disappearing and resurrecting object has been personified, deified, given personality and purpose. Watching the intense gaze of Venus as it moves slowly, intricately, through the seasons and years, you get the feeling that there is a mind at work, and certainly there is. It is your own. Venus first appears as a faint jewel in the evening, barely out-shining the glare of dusk, setting quickly on the heels of the Sun. At first, she steps gingerly into night, departing before darkness sweeps the sky. Then each evening she rises slightly further from the sun, glowing longer and brighter. After 30 weeks she reaches her "maximum elongation", the farthest she gets from the Sun. Now, no tentative intruder into the night, she beams into the blackness, lingering long after the sun has departed, reluctantly setting three hours later. At this phase Venus is so brilliant that if you get to a dark place on a clear evening you can see your surroundings dimly illuminated by Venus-light and even find your Venus-shadow. After seeming to pause here for a week or so, she reverses course, diving down for the next 10 weeks to catch the Sun, setting closer and closer to sunset until she is again consumed by the sun's glare, nine months after first appearing. Venus hides in the solar brilliance for a week and then re-emerges on the other side of night, appearing just before sunrise to dance the same routine through our morning skies, rising earlier each week until, three hours ahead of the sun, she heralds the coming day. Again she pauses before plunging back towards the sun, disappearing this time for about eight weeks, only to re-emerge in the west, peering again through the evening glare, beginning another cycle. Through countless millennia, for much longer than anyone has been watching, at least from around here, Venus has been jitterbugging with the sun, rhythmically swinging back and forth between the morning and evening skies of Earth, just as surely as Earth has been skipping, seen or un-seen, across the nighttime skies of Venus. Eventually, who knows where or when, we began to take notice. Venus in the sky, burning through the twilight, is a dazzling presence, brighter than all other planets combined. She commands our mornings and evenings, moving to her own unique rhythms, dancing with the moon as it passes. I have seen Venus hundreds of times in the evening and morning and even once at high noon during a total eclipse, yet every single time I am stunned by its radiance. Who would have believed that one day we would send a fleet of robots, driven by remote commands, flying across space at unimaginable speeds to plunge right into that brilliance and report home on what lies inside? This great brightness and complex, enigmatic behavior led many ancient cultures to a great interest in--sometimes an obsession with--Venus. The Maya of Central America, for instance, were adept at tracking and predicting the date and direction of Venus' first re-appearance in the morning or evening--an event of great religious significance. Today we tend to regard the planets as part of the "mechanical universe." We have learned that we live on a planet that is a large spinning ball following Newtonian law in an endless orbit around our home star, and we know that the other planets are other balls obeying the same laws on orbits around our shared sun. This picture provides a simple, elegant and accurate explanation for the strange cyclic appearances, loops and disappearances of Venus and the other planets in our night sky. But much more than that, it is a correct description of the physical layout and motions of our solar system. We know this, most viscerally, because we have "been there, done that." We have gone to the planets and found them exactly where we expected them to be. Viva the Copernican Revolution!1 Our physics works. And yet, with the pride we take in our modern understanding of the dimensions and motions of the larger universe in which we live, let's not forget that the knowledge and insights of earlier generations and civilizations of sky watchers were impressive in their own right, and even surpassed our own in some ways. Before we discuss the Venus-cosmology of some earlier societies, let's review our modern picture of the motions of the planets, the lights that wander.2 Have you ever been waiting in a train at a station and thought, for a moment, that the train next to you had begun moving backwards, only to realize that the forward motion of your own train was causing this sensation? Or been sitting in a car at a stop-light, and sensed the car next to you moving forward, but then realized that it was you rolling backward? These common experiences illustrate how observing from a moving platform can alter our perceptions of motion. Similarly, our views of celestial motions are dominated by the motions of our spinning and orbiting platform, the Earth. The path of Venus is easy to understand with our modern sun-centered model of the Solar System. However, we must first "cleanse the doors of perception," understanding and thereby circumventing a few persistent optical illusions that make this motion seem more complex and mysterious than it is. After all, our senses and brains have adapted well for finding food, shelter and companionship here on Earth, not for the (until recently) useless task of fathoming the heavens. So, let's help ourselves out with a little visualization exercise to shed some "common sense" perceptions, that can only get in the way. It's always best to start at the beginning. First we must step off the Earth and watch it spin. Everything in the sky--sun, moon, stars, and planets--appears to "rise" and "set," because we are on the surface of a round world that is spinning in space. Of course we all know this, have been weaned on it, but to really understand the picture I am about to draw it helps to be able to feel it, to know it on a deeper, non-analytical, gut level. So, next time you watch a sunset, try this. As the reddening sun "goes down" towards the horizon, use your mental powers to stop it in its tracks, and instead force the horizon to rise up and slowly obscure the sun. With a little effort, using the sun's position as a steady reference point, you can, like the fool on the hill, see and feel the world spin.3 Now, after the sun is blocked by your Western horizon and the light fades, observe the night sky slipping by as you spin beneath it, and feel yourself still turning away from the Sun. At midnight, sense the sun far below your feet, obscured only by the turning Earth on which you stand. Several hours later, as the sky in the East begins to brighten and you anticipate, and then observe, the sun "rising," picture the horizon falling away, revealing the sun in exactly the same place where you left it the previous evening. This is the hard part. Here the altered-state of sleep-deprivation may come in handy as you try to loosen up the synapses and integrate old knowledge into new perspective. It's easy enough to say that the Earth has spun around overnight, but harder to really believe, on the level of a gut feeling, that West has become East. (We are used to thinking that "never the twain shall meet.") Nevertheless, as you turn your head around 180 degrees to look back at the rising sun, you are looking in the exact same direction as you were at the moment when the Western horizon rose up and swallowed it the night before.4 If you can feel this, you have successfully internalized the Copernican Revolution. So much for the Earth's spin on its axis. Now, lets think about orbits. The Earth is, of course, on a nearly circular orbit around the Sun with a period of one year (by definition). Our distance from the sun is 93 million miles, 150 million kilometers, 8 light minutes, or 1 Astronomical Unit (by definition), whichever is easiest for you to picture. (Good luck!) Viewed from this position in the solar system, the motions of all other planets fall into two different classes. Those we call "superior" planets, meaning that they are farther from the sun than us, travel slowly across the whole night sky, moving eastward against the far more distant background stars from night to night. Mars, Jupiter, and Saturn are the ones bright enough to see with your naked eyes, and thus were known before telescopes were invented. Each of these worlds pauses near the anti-sunward position, where it is high overhead at midnight, and reverses direction for a few months. This "retrograde loop" occurs when we, in our orbit, pass one of these slower-moving, more distant worlds, just as a car in a slower lane appears to move backwards against the trees on the horizon as you pass it. Each world executes its loop, bowing to the Earth as we pass, and then resumes the slow trip from eastern to western horizon. Eventually each disappears at sunset as it travels behind the sun, only to reappear a few weeks later in the morning, resuming a slow circle dance around the night sky. Our view is quite different for the two "inferior" planets, Mercury and Venus which, since they orbit closer to the Sun than us, are always near it in our skies. These planets appear only as evening or morning stars. They are never seen in the middle of the night, when your side of Earth is turned away from the Sun. They travel across our daytime sky, unseen because of the intense scattered light of the nearby Sun.5 We can usually see them only near the horizon, right after sunset if they are east of the Sun, or else just before sunrise. The closer a planet is to the Sun, the faster it orbits. This is not just because they are traveling on smaller, shorter loops. Closer planets, caught more deeply in the sun's mighty gravitational spell, actually move faster in their orbits.6 So, just as the Earth keeps lapping the outer planets, Venus constantly passes Earth like a runner on an inside lane. [drawing showing Venus and Earth orbits, inferior conjunction, etc.] Now there is another persistent illusion, interfering with our intuitive comprehension of sky motions, that we should name, and thereby help to transcend: the loss of the third dimension. The positional relationships and motions we observe in the sky certainly are three dimensional. The most distant thing we can see with the unaided eye, the Andromeda Galaxy, is 5 x 1013, or 50,000,000,000,000, times as far away as the Moon, which is the closest sky object (barring Sputniks, spooks and space junk). But your stereo vision senses depth from the slight difference in direction between your left and right eyes, and everything in the sky is too far away for this to work. So we are stuck with a very powerful illusion that everything up there is all the same distance away. We see the sky in only two dimensions. Thus the "dome of night," or what astronomers call the celestial sphere. What we actually observe are the changing angles between objects on this imaginary sphere, and we see the planets seemingly wandering among the stars, which we have learned are actually millions of times farther away. We know now, thanks to the powerful insights provided by modern astronomy, that there is an incredibly large third dimension to these motions. But we simply do not see it under ordinary circumstances. This is one of the reasons why a total solar eclipse is such a moving experience, provoking religious awe and fear through the ages. If you are fortunate enough to be in the right place and time for one of these spectacular events, you will see the moon pass in front of the sun and block its light, transforming your surroundings into sudden eerie darkness. At this moment, you do get a brief glimpse of the 3D structure of the sky usually hidden from us by the limitations of our senses. When we look at the motions of Venus relative to the Sun's position, what we see is the flat projection of Venus' 3D movements on this illusory celestial sphere. The constant running back and forth between evening and morning skies with disappearances in between, is simply the nearly edge-on view of an inside circular orbit. Running faster than us on her inside track, Venus is constantly overtaking and passing us. When we see her coming up in the evening, we are looking back over our left shoulder as she chases after us in our orbit, to overtake us at "inferior conjunction" when she disappears from our view, passing between us and the sun. A week later when she re-emerges into morning she is already off and running, leaving us in her cosmic dust, receding towards the other side of the sun. [1.3, part I, 1.3, part II] This cycle is easier to comprehend if you mentally stop the daily rotation of the Earth, and just watch Venus for a few years. If, as the Earth spins on its axis, you fix your gaze on the sun like a pirouetting dancer trying to maintain equilibrium, you can watch Venus swing from one side of the sun to the other, bouncing back and forth between morning and evening. This may be easier said than done, but with my students in the planetarium I can dim the sun and fix it steady in the sky, turn the time knob "up to eleven" and watch Venus yo-yo back and forth across the Sun as together they navigate yearly around the zodiac. Here the yo-yo string is the Sun's gravitational pull, which holds Venus in an orbit about 3/4 of the distance to the Earth. In fact, if you do not have access to a planetarium, you can simulate this effect by getting a friend to swing a glow-in-the-dark yo-yo (or any luminous object on the end of a string) in a circle over her head, in the dark across the room from you. The yo-yo will appear to move back and forth in a line. So Venus seems to move as the Sun pulls it across the darkness of night.7 Footnotes: 1 The acceptance of the Earth as not the center of the universe, but an astronomical body among others, spinning and traveling through space, is often referred to as the Copernican revolution, after Nicolas Copernicus, the visionary Polish astronomer who advocated the heliocentric (sun centered) universe in the 1500's. Of course, we now know that the Universe is no more sun-centered than it is Earth-centered. The sun is in orbit around the Milky Way Galaxy, which is itself in motion. The profound philosophical implications of this re-assessment of the Earth's importance have not yet completely sunk in among the Earth's inhabitants 400 years later. The revolution continues. 2 The word "planet" comes from a greek word meaning "wanderer". 3 This activity is done at your own risk. The author and publishers of this book take no responsibility for injuries resulting from loss of equilibrium due to a sudden internalization of celestial dynamics. 4 Actually, its not quite exactly the same direction. Since the sun itself must travel around our entire sky as we orbit in 365 days, in 12 hours it has moved half of 1/365 of a circle, or about half a degree, against the background of distant stars. Half a degree is roughly the sun's angular diameter in our sky. This angle corresponds to roughly two minutes of the Earth's spin. 5 Venus is so bright that you can sometimes see it in the daytime if you know exactly where to look. 6 The exact relationship is that the square of the orbital period is proportional to the cube of the average distance from the sun. This is one of Kepler's laws of planetary motion. Kepler was an early follower of Copernicus, before it was popular or safe. 7 Seen from Mars or Jupiter, Earth is an inferior planet. Someday people living on Mars may see Earth swing between morning and evening star. |