Imagine yourself as one of your ancestors, say ten thousand years ago. Your reality consists of a few tools, household utensils, perhaps buildings (the city-states were beginning to appear along the Tigris) and, of course, all that nature has to offer: trees, hills, plants, rivers, streams—and the sky. The sky is the biggest, greatest, most spectacular object you know. During the day, the sky is crossed by a brightly glowing disk from which all light and warmth emanate. Announced in the predawn hours by a pink glow on the eastern horizon, the great disk rises, then arcs across the sky, deepening toward twilight into a ruddy hue before slipping below the horizon to the west. Without electric power, your working hours are dictated by the presence of the sun’s light.

Flat Earth, Big Bowl

As the sun’s glow fades and your eyes become accustomed to the night, the sky gradually fills with stars. Thousands of them shimmer blue, silvery white, some gold, some reddish, seemingly set into a great dark bowl, the celestial sphere, overarching the flat earth on which you stand.

Thousands of stars in the night sky?

Maybe that number has brought you back through a starlit ten thousand years and into the incandescent lamp light of your living room or kitchen or bedroom or wherever you are reading this: “I’ve never seen thousands of stars!” you protest.

We said earlier that, from many locations, our sky is spoiled. The sad fact is that, these days, fewer and fewer of us can see anything like the three thousand or so stars that should be visible to the naked eye on a clear evening. Ten thousand years ago, the night sky was not lit up with the light pollution of so many sources of artificial illumination. Unless you sail far out to sea or travel to the high, dry desert of the Southwest, you might go through your entire life without really seeing the night sky, at least not the way our ancestors saw it.

Man in the Moon

Even in our smog- and light-polluted skies, however, the Moon shines bright and clear. Unlike the Sun, which appears uniform, the surface of the Moon has details we can see, even without a telescope. Even now, some three decades after human beings walked, skipped, and jumped on the Moon and even hit a golf ball across the lunar surface, the Moon holds wonder. Bathed in its silver glow, we may feel a connection with our ancestors of 10 millennia ago. Like them, we see in the lunar blotches the face of the “Man in the Moon.”

If the face of the Moon presented a puzzle to our ancestors, they were also fascinated by the way the Moon apparently changed shape. One night, the Moon might be invisible (a new moon); then, night by night, it would appear to grow (wax), becoming a crescent; and, by one week later, be a quarter moon (which is a half moon in shape).

Through the following week, the Moon would continue to wax, entering its gibbous phase, in which more than half of the lunar disk was seen. Finally, two weeks after the new moon, all of the lunar disk would be visible: The full moon would rise majestically at sunset. Then, through the next two weeks, the Moon would appear to shrink (wane) night after night, passing back through the gibbous, quarter, and crescent phases, until it became again the all-butinvisible new moon.

The cycle takes a little more than 29 days, a month, give or take, and it should be no surprise that the word “month” derived from the word “moon.” In fact, just as our ancestors learned to tell the time of day from the position of the Sun, so they measured what we call weeks and months by the lunar phases. The lunar calendar is of particular importance in many world religions, including Judaism and Islam. For those who came before us, the sky was more than something to marvel at. It could also be used to guide and coordinate human activity.

Lights and Wanderers Ten thousand years ago, family time at night was not occupied with primetime sitcoms followed by the news and David Letterman. Our ancestors were not glued to television screens, but presumably to the free show above, the celestial sphere. Early cultures noticed that the bowl above them rotated from east to west.

They concluded that what they were seeing was the celestial sphere—which contained the stars—rotating, and not the individual stars. All the stars, they noticed, moved together, their positions relative to one another remaining unchanged. (That the stars “move” because of Earth’s rotation was a concept that lay far in the future.)

The coordinated movement of the stars was in dramatic contrast to something else the ancient sky watchers noticed. While the vast majority of stars were clearly fixed in the rotating celestial sphere, a few—the ancients counted five—seemed to meander independently, yet regularly, across the celestial sphere. The Greeks called these five objects planetes, “wanderers,” and, like nonconformists in an otherwise orderly society, the wanderers would eventually cause trouble. Their existence would bring the entire heavenly status quo into question and, ultimately, the whole celestial sphere would come crashing down.