HOW WE SEE THE SKY

A Naked-Eye Tour of Day & NightBy THOMAS HOCKEY

The University of Chicago Press

Copyright © 2011 The University of Chicago
All right reserved.
ISBN: 978-0-226-34577-2

Contents

Preface…………………………………………………ixIntroduction…………………………………………….11 Bowl of Night………………………………………….52 This Big Ol’ Wheel Keeps Rolling…………………………213 A Globe of Stars……………………………………….414 Of Precession, Planispheres, and Patience…………………595 The King of Day………………………………………..776 Solstices, Equinoxes, and More…………………………..977 Around the World with the Sun……………………………1178 Many Moons…………………………………………….1339 Living Month to Month…………………………………..15110 Facing Up to the Moon (and the Sun, Too)…………………16711 Eclipses……………………………………………..18112 Placing Planets……………………………………….199Notes…………………………………………………..223Recommended Reading………………………………………227Index…………………………………………………..229

Chapter One

Bowl of Night

And the first movement in the morning was to open the window—again to examine the sky…. I discovered a star—a solitary star—twinkling dimly in the sky which had now changed its hue to a pale grayish twilight, while vivid touches of coloring were beginning to flush the eastern sky. There was absolutely no other object visible in the heavens—cloud there was none, not even the slightest vapor. That lonely star excited a vivid interest in my mind. I continued at the window gazing, and losing myself in a sort of day-dream. That star was a heavenly body, it was known to be a planet, and my mind was filling itself with images of planets and suns. My brain was confusing itself with vague ideas of magnitude and distance, and of the time required by light to pierce the apparent illimitable void that lay between us—of the beings who might inhabit an orb like that, with life, feeling, spirit, and aspirations like my own.

JAMES FENIMORE COOPER, “The Eclipse,” unpublished manuscript circa 1831

This is a book about astronomy without optical instruments, the way astronomy was practiced throughout most of human history. I will introduce some simple tools that people have used when studying the sky. However, an instrument, narrowly defined, is a device that extends human senses (a lens, a microphone, etc.). Here we will be aided by our eyes and minds alone.

CONFRONTING THE SKY

Before discussing what is in the sky, let us begin with the sky itself. We experience the sky as an imaginary inverted bowl, the center of which is directly over our heads. Its rim is defined by the horizon. When asked to look straight up, many of us do not bend our necks back a full ninety degrees. This and other psychological factors give us the impression that the sky is a somewhat out-of-hemispherical bowl, squashed at the top.

The sky-as-bowl (maybe a shallow bowl) is a practical way of thinking about the sky; after all, it really looks like that. And it is somehow comforting. Look up at the night sky, and try to force your eyes to see it for what it is: a black expanse as near to infinity as you ever will see, punctuated by stars at so vastly different distances from you that terrifically luminous stars appear dim and barely luminous stars appear bright. What did you feel in that instant before your brain again overtook your imagination? A bit of panic? Maybe a tinge of vertigo?

Thinking of the sky in “3-D” this way is difficult; it is contrary to any other human experience (where our environment is bounded). It can be exhilarating. It also can be scary. Our minds tell us that a bowl is a much more comfortable construct. There is nothing wrong with that. The idea of a black opaque bowl over our head is a useful fiction for describing things in the sky at any moment in time.

For instance, it aids communication. You can tell another person where to look at some shared sight in the sky by providing him or her with just two pieces of information. One is how high to look, the other is where (in which direction) to look.

By “height” and “where” I do not refer to physical distance. I just mean how far one must tip one’s head, and how to turn, to see the star in question. (I will quantify this at the beginning of the next chapter.) Without prior knowledge, it is impossible to know how far away a star—or anything, for that matter—is. After all, once an object is too high to reach (e.g., an apple in a tree), would you know how far away it is without first knowing how big an apple is? Likewise, how far are the stars if you do not know how big a star is? You have never picked up and examined a star. You have read in textbooks that a star is exceedingly large, but would you necessarily assume that without being told?

On the earth we may get lucky: There are clues to distance that are familiar to artists who deal in perspective. A landscape feature nearly lost in the haze is likely farther away than a similar one in clear view. But we have none of these clues in the sky.

The bowl of the sky has been an important philosophical concept in Western society. For instance, in the Bible, it is called the Heavenly Firmament. It served to separate the earth from the heavenly waters. It is not outrageous to think that there is water above us: What color is the sea? Blue. And, of course, water does sometimes fall to the earth—in the form of rain.

(In popular culture, the idea of the sky as a physical thing has always had great traction. “It’s like the sky is falling” remains a simile for calamity.)

Unfortunately, the view that the heavens are separate and different from the earth obscured the fact that we have a great deal in common with the rest of the universe. Indeed, we are intimately related to it. The suggestion that the “rules” are different “up there” and “down here” retarded Western science for millennia.

Even in more modern times, since the idea of a physical star bowl was discarded, a sophisticated argument reinforces the sky-as-bowl metaphor. A bowl has a finite thickness.

Consider that most of the night sky is black. There is no star in most random directions. These two statements may seem trivial, but they are far from it.

Why are there not stars everywhere in the sky? A literal bowl of stars would seem to save us from this question. Its thickness is presumed to be small compared to its radius. The number of stars and the volume of the bowl have a fixed ratio that allows for many directions in which we look and encounter no star at all.

Yet as soon as you allow stars to be at greatly different distances away, and no longer require them to be embedded in a physical matrix, there is the possibility that the space of stars might extend forever. If so, every line of sight eventually must run into a star. Starlight dims but does not disappear with distance. An infinite space logically implies a sky completely ablaze with stars. This is not the case. So some sort of limit to the distances of the stars is sensible: Our metaphorical bowl of sky has a certain thickness. Notice that this argument works regardless if you believe that the universe is centered upon us, or whether you have made the Copernican leap in which the earth is not necessarily the center of everything.

After wrestling for hundreds of years with the paradox of a sky not infinitely populated with stars, astronomers finally did away with the requirement of celestial boundaries. We now know that 1) starlight is not instantaneous—light travels at a constant velocity, and 2) that we inhabit a universe of finite age. There is a distance so far away from us that we cannot see starlight originating at that distance. The travel time for this light would exceed the lifetime of the universe. Far from an elementary matter, a star-speckled (but not blanketed) sky sends a profound message to us about the nature of our universe.

THE ARCHITECTURE-SKY CONNECTION

When nearly everything else a society produces has disappeared, its buildings usually are what remain. This is why I will write about a lot of buildings and other monuments in my attempt to convince you that people have interacted with the sky for a long time. The study of prehistoric peoples’ interaction with the sky of sun, moon, planets, stars, and so on, is called archaeoastronomy.

I will use two words in this book that at first sound the same; however, my meaning for each will differ. One is “orientation.” An orientation occurs when two or more objects (or a plane) point in a particular direction. Orientation does not require intent. On my campus, two Spanish cannons are oriented with (aimed at!) the Methodist student center across the street. Nobody seriously thinks that this was intentional. It just happened. I will use the more subtle word “alignment” for an intended orientation. Warning: Some scholars juxtapose the definitions of these two words. However, to my ear, this usage sounds best.

An alignment or orientation requires both a backsight and a foresight. A sphere is worthless insofar as determining a preferred direction. We need an asymmetrical object with at least two discernable points (ends, edges, openings). The one closest to us is called the backsight, the one closest to the point to which we are to look is called the foresight. The notches on a rifle are effective back-and foresights.

The earliest purported use of astronomical foresights and backsights of which I know is at the Nabta Playa in Egypt. Here, sandstone blocks were moved into apparently intentional celestial alignments, by pastoral peoples as early as the fifth millennium BCE!

It used to be that to measure the orientation of architecture, one had to become familiar with the ways and means of the surveyor. Both direction and inclination must be taken into account. (A hillside has a different natural horizon than does a flat plain.) Today, though, the marvelous invention of GPS technology may greatly simplify the task.

Another example of archaeoastronomy can be found in the middle of America. About the year 1200, the Mississippian civilization built a city called Cahokia across the river from what would become modern-day Saint Louis. You can still visit it: It is on the United Nations list of World Heritage Sites. Evidence of the Cahokian people’s existence lies underground in excavated artifacts, and above ground in the world’s largest platform mounds made of earth.

West of the major Cahokian mound, one on which the supreme ruler of Cahokia is hypothesized to have lived, there was once erected a seventy-meter-diameter circle of tree trunks—forty-eight in all. In the middle was a lone post. Standing at the interior post (backsight), it is possible to see the point on the horizon where certain objects rise and set in the sky at particular times of year, behind three of the outer poles (foresights). I will discuss the solstices and equinoxes later. Right now, it is important to realize that what I have described so far is an orientation, but not an alignment.

With so many posts in the Cahokia sun circle, it is possible that these events occur by chance. We need further evidence to suggest an alignment. This evidence comes in many forms—from archaeology, other sciences, or somewhere else—and is somewhat subjective.

However, it is necessary to distinguish human accomplishments that are alignments from all the orientations that must (statistically) occur.

It is easy to skip this warning about mistaking orientations for alignments because our minds are designed to 1) pick out patterns and 2) pick up on exceptions to normal life: We notice when we see “12:12” light up on the digital alarm clock, forgetting all the times that we have glanced at the same clock seeing no such interesting pattern. We remark on the coincidence, even though something like “12:12” must show up during one minute of every hour of every day.

At Cahokia, we suspect a real alignment because the observed orientations do not work if the middle pole is at the geometrical center of the circle—as it likely would be if somebody was merely building a “pretty” wheel of wood poles. Here we use the scientific method: We hypothesize that the sun circle is meant to mark natural phenomena; therefore, we figure out where one must stand for the alignment to work. This turns out to be a couple of meters east of the geometrical center. We then test our hypothesis by digging for the remains of an ancient post at this spot. It is there. (Notice that we did not dig a hole everywhere we could within the sun circle, hoping to find something; we dug in one—and only one—predicted location.) It looks like the builders of the Cahokia sun circle ignored the strong human urge for symmetry (placing the middle pole equally distant from the perimeter poles) and put function over form: The Cahokia sun circle may have looked a bit off, but it worked.

Is this enough evidence? Now the archaeologists weigh in. There seems to be a connection between one of the outer poles and the sun. They dig there. The excavators find what they know (because of their study of Cahokian culture through a myriad of other artifacts) to be a small ceramic vessel, the kind used to hold religious offerings, with a symbol representing the sun adorning it.

Do we have enough evidence now? We can never prove as a mathematical certainty that the Cahokian sun circle was aligned based on the astronomical knowledge and interest of its people. Questions remain: What were the other poles for, then? Still, there is enough evidence to make it unlikely that it was not aligned.

STARS AND CONSTELLATIONS

It is time to populate our sky. When we look up at the night sky, the moon may or may not be present; the same is true for the planets. We might see a meteor, comet, or some other rare astronomical phenomenon. However, we are guaranteed to see the stars on a clear night. Under optimal conditions, the naked eye may perceive thousands of stars above the horizon at one time. English amateur astronomer Thomas Backhouse (1842– 1920) published a book with the title Catalog of 9,842 Stars Visible to the Naked Eye (1911).

Sky watchers group the stars together into patterns called constellations. This is an old human habit. Some say that certain constellation patterns are tens of thousands of years old and survive until today largely by word of mouth, passed from generation to generation.

Constellations are patterns only, kind of a celestial connect-the-dots game. They have no physical significance. Some stars in a given constellation may be vastly farther away than others in the pattern. Thus, viewed from anywhere other than our solar system, the pattern would change. (Have you ever seen a rock or mountain with a profile that reminded you of a familiar shape, perhaps a human face? The particular pattern disappears when you step away from your viewing spot.) Still, constellations are handy landmarks for someone standing on the earth.

At one time constellations were named after creatures, heroes, and stories from popular mythology. Only the brighter stars were used. Some of the figures are obvious even today, though most take a great deal of imagination. Yet this was not a game such as that played by a child lying on her back and identifying elephants and mountains among the clouds. Remember that the creatures, heroes, and stories existed first. People assigned star patterns to honor these elements in their culture, not necessarily because the pattern looked that much like that for which it was named. (Someone once said that the constellations need look like their name sakes as much as the state of Washington resembles the United States’ first president.)

The constellations as established were more-or-less “stick figures,” with no agreed-upon boundaries. Moreover and somewhat obviously, different civilizations recognized different sets (and names) of constellations.

Modern astronomers have formalized the constellations, fixed their number (eighty-eight), and assigned them definite boundaries. The result is that every star is now a member of some particular constellation. You often can recognize the new constellations: For example, if you come across Antila (the Air Pump), you rightly will suspect that it is not of ancient origin. Smaller constellations may be so because they had to be wedged in between older, larger constellations. My least favorite constellation is Triangulum. Inasmuch as any three stars in the sky will form a triangle, I find this name singularly unimaginative.

A map of the constellations today looks a bit like that of the western United States: lots of east-west and north-south running borders. Now constellations can be used to give a general description of a location in the sky without resorting to a numerical coordinate system. Often this is all that is needed. If you are driving to a new city, you may not require its exact latitude and longitude on the globe. Knowing what county it is in might be all that you need to navigate your way there. If the city is big enough, you will see it once you get close.

Knowing what constellation an astronomical object occupies may be all that you need to find it, if the object is bright. The only requirement is a chart of the constellations.

Of course, you also need to know which constellations will be visible in the night sky at specific times, in order to know when and where to look for the object. Soon I will introduce this time element into our discussion.

(Continues…)

Excerpted from HOW WE SEE THE SKYby THOMAS HOCKEY Copyright © 2011 by The University of Chicago. Excerpted by permission of The University of Chicago Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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