The Sun Goes Dark, the Moon Becomes Blood

The ancients had another, far more dramatic, celestial irregularity to contend with. On rare occasions, the moon would gradually dim and turn a deep red for a time, only to reemerge in its full glory after a short time. On even rarer occasions, daylight would fade as a great shadowy disk stole across the sun

As we saw in the previous section, eclipses were events that could cause great fear, and governments and rulers put tremendous pressure on astronomers (or soothsayers or astrologers or whatever they called their official sky watchers at the time) to come up with dependable ways of predicting when eclipses would occur. Here was yet another set of celestial events that certainly weren’t random, yet, without a complete understanding of how the various parts of the solar system were put together, they were hard to predict accurately.

Time on Our Hands

Why did the ancients concern themselves about things moving in the sky when they were stuck here down on Earth? Chalk it up in part to human curiosity. But their interest also had even more basic motives.
You’re walking down the street, and a passerby asks you for the time. What do you do?
You look at your watch and tell him the time.
But what if you don’t have a watch?
If you still want to be helpful, you might estimate the time, and you might even do this by noting the position of the sun in the sky.

The ancients had no wrist watches, and, for them, time—a dimension so critical to human activity—was measured by the movement of objects in the sky, chiefly the sun and the moon. What, then, could be more important than observing and explaining the movement of these bodies?

Heavens on the move

The next time you’re outside doing yard work in the sun, put a stick in the ground, call it a gnomon, and watch the motion of its shadow. Believe it or not, you have made a simple sundial, which was one of the earliest ways that human beings kept track of time. In fact, keeping time was one of the two major reasons that early civilizations kept a close watch on the skies.

The other reason, of course, was to use the motions of the planets through the constellations to predict the future for the benefit of kings and queens and empires. Well, the first practice (keeping track of time) has continued to this day. The U.S. Naval Observatory is charged with being the timekeeper for the nation, using technology a bit more advanced than a stick in the ground. The second practice (predicting the future) is also alive and well, but astronomers have turned those duties over to The Psychic Network—at least for the time being.

In the days before movies, television, video games, and the Internet, the starry sky (untouched by city lights and automobile exhaust) was truly the greatest show on Earth. Generations of sky watchers looked and imagined and sought to explain. Common sense told many of these early watchers that they were on a kind of platform overarched by a rotating bowl or sphere that held the stars. We have seen that in various cultures, other explanations surfaced from time to time. It doesn’t matter right now whether these explanations were right or wrong (well, many of them were wrong). What matters is that the explanations were, to many astronomers, unsatisfying. None of the explanations could account for everything that happened in the sky.

For example, if the stars were all fixed in this overarching bowl, how did the planets break free to wander among the stars? And they didn’t wander randomly. The planets were only found in certain regions of the sky, close to the great circle on the sky called the ecliptic. Why was that? The sky is filled with thousands of bright points of light that move, and none of the ancient explanations adequately explained all of these movements.

Eratosthenes Sizes Up the Earth

Anaxagoras’s explanation of eclipses was a bold exercise in the use of science to understand a phenomenon well beyond everyday experience. One other such exercise came from Eratosthenes of Cyrene (ca. 276–ca. 194 B.C.E.). A careful observer, Eratosthenes noted that at the town of Syene (present-day Aswan, Egypt), southeast of Alexandria, the rays of the sun are precisely vertical at noon during the summer solstice. That is, a vertical stick in the ground would cast no shadow. He further noted that, at Alexandria, at exactly the same date and time, sunlight falls at an angle of 7.5 degrees from the vertical.

As we’ll see in the next chapter, small differences and apparently inconsequential discrepancies often have profound implications in astronomy. Eratosthenes instinctively understood the importance of details. Assuming—correctly—that the sun is very far from the earth, he reasoned that its rays are essentially parallel when they strike the earth. Eratosthenes believed (as did Aristotle, whom we’ll meet in the next chapter) that the earth was a sphere. He further reasoned that the angle of the shadow cast in Alexandria (7.5 degrees) was equal to the difference in latitude (see Chapter 1, “Naked Sky, Naked Eye: Finding Your Way in the Dark”) between the two cities. How did he figure that? Think of it this way: Imagine poking a stick vertically into the earth at the equator and at the North Pole, and imagine the sun is directly over the stick at the equator. The stick at the equator will have no shadow, and the stick at the North Pole will cast a shadow at an angle of 90 degrees from the stick.

Now, move the stick from the North Pole to a latitude of 45 degrees. The shadow will now fall at an angle of 45 degrees from the stick. As the stick that was at the North Pole gets closer and closer to the equator (where the other stick is), the angle of its shadow will get smaller and smaller until it is beside the other stick at the equator, casting no shadow. Noting that a complete circle has 360 degrees, and 7.5 degrees is approximately 1/50 of 360 degrees, Eratosthenes figured that the two cities were separated by 1/50 of the earth’s circumference, and that the circumference of the earth must simply be fifty times the distance between Alexandria and Syene.

The distance from Syene to Alexandria, as measured in Eratosthenes’s time, was 5,000 stadia. He apparently paid someone (perhaps a hungry grad student) to pace out the distance between the two cities. So he calculated that the circumference of the earth was 250,000 stadia. Assuming that the stadion is equivalent to 521.4 feet, Eratosthenes calculation of the earth’s circumference comes out to about 23,990 miles and the diameter to about 7,580 miles. These figures are within 4 percent of what we know today as the earth’s circumference—24,887.64 miles—and its diameter, 7,926 miles. And he figured that out with only a few sticks—and one long hike. Eratosthenes made other important contributions to early astronomy. He accurately measured the tilt of the earth’s axis with respect to the plane of the solar system, and compiled an accurate and impressive star catalog and a calendar that included leap years.

We may consider Eratosthenes the first astronomer in the modern sense of the word. He used careful observations and mathematics to venture beyond a simple interpretation of what his senses told him. This combination of observation and interpretation is the essence of what astronomers (and all scientists) do. It is a cruel irony that Eratosthenes lost his eyesight in old age. Deprived of his ability to observe, he committed suicide by starvation.

Anaxagoras Explains Eclipses

Anaxagoras (ca. 500–ca. 428 B.C.E.) believed that the earth was flat, but speculated that the sun was a large, red-hot body and that the moon was much like the earth, complete with mountains and ravines. Most important, Anaxagoras theorized that solar eclipses were caused by the passage of the moon between the sun and the earth. His was the first explanation of an eclipse that didn’t involve the supernatural and certainly didn’t summon up any dragons.

Aristarchus Sets the Sun in the Middle and Us in Motion

Living in a technology-driven society, we’ve become accustomed to thinking of linear progress in science, a movement from point A, which takes us to point B, then to point C, and so on. We don’t think that steps backward can ever occur. If this were the way knowledge actually was built, the model of a geocentric (or earth-centered) universe would have died during the second century B.C.E.

Far in advance of his peers, Aristarchus of Samos (ca. 310–230 B.C.E.) proposed that the earth is not at the center of the universe or the solar system, but that it orbits the sun while also rotating.

This theory sounds completely reasonable to our modern ears, but it did not sit well with the Greek philosophical establishment, nor with common sense. Why, one might ask, if the earth is orbiting the sun, and spinning on its axis do we not all go flying into space as we would if the earth were a large merry-go-round? Without a theory of gravity (which keeps everything stuck to the surface of the earth as it spins), there was no good answer to this valid question.

One philosopher, Cleanthes the Stoic, went so far as to declare that Aristarchus should be punished for impiety. Maybe if he had been punished, becoming a martyr to his idea, the heliocentric (sun-centered) solar system would have caught on much sooner than it did. But it didn’t. The geocentric model of Aristotle and others held sway for millennia.

Pythagoras Calls Earth a Globe

Anaximander and Anaximenes may no longer be household names, but a lot of us remember Pythagoras (ca. 580–ca. 500 B.C.E.) from high school geometry. We all heard about the man who is credited with the Pythagorean theorem (“The sum of the squares of the sides of a right triangle is equal to the square of the hypotenuse,” or A2 + B2 = C2). He also taught that the earth was a globe—not a cylinder and certainly not flat—and that it was fixed within a sphere that held the stars. The planets and the sun moved against this starry background.