ASTRONOMY OF THE GREEKS

Anyone looking at the sky in a completely unsophisticated manner, without benefit of any astronomical training whatever, and willing to judge by appearances alone, is very likely to conclude that the Earth is covered by a smooth and flattened dome of some strong and solid material that is blue by day and black by night.

Under that solid dome is the air and the floating clouds. Above it, he may decide, is another world of gods and angels where the immortal souls of men will rise after the body dies and decays.

As a matter of fact, this is precisely the view of the early men of the Near East, for instance. On the second day of creation, says the Bible: "God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament" (Genesis 1: 6‑7).

The word "firmament" is from the Latin word flrmamentum, which means something solid and strong. This is a translation of the Greek word stereoma, which means something solid and strong, and that is a translation of the original Hebrew word raqia, which refers to a thin metallic bowl.

In the biblical view there was water below the firmament (obviously) and water above it, to account for the rain. That is why in the time of Noah's flood, it is recorded that ". . . the fountains of the great deep [were] broken up, and the windows of heaven were opened" (Genesis 7:11). The expression might be accepted as metaphor, of course, but I'm sure that the unsophisticated accepted it literally.

But there's no use laughing from the height of our own painfully gained hindsight. About 700 B.C., when the material of Genesis was first being collected, the thought that the sky was a solid vault with another world above it was a reasonable conclusion to come to from the evidence available.

What's more, it would seem reasonable about 700 B.C. to suppose that the firmament stretched over but a limited portion of a flat Earth. One could see it come down and join the Earth tightly at the horizon. Few people in ancient times ever traveled far from home and the world to them was but a few miles in every direction. Even soldiers, merchants, who tramped longer distances, might feel the Earth was larger than it looked but that the world to the enlarged horizon was still flat, and still enclosed on all sides by the junction of firmament and ground. (This was also very much the medieval view and probably that of many unsophisticated moderns.)

The Greek philosophers, however, had come to the conclusion, for a number of valid reasons, that the Earth was not a more or less flat object of rather limited size, but a spherical object of sufficient size to dwarf the known world to small dimensions.

The firmament, then, must stretch all around the globular Earth, and to do so symmetrically, it must be another, but much larger, sphere. The apparent flattening of the firmament overhead had to be an illusion (it is!) and the Greeks spoke of what we would call the "heavenly sphere" as opposed to the "terrestrial sphere."

None of this, however, altered the concept of the firmament (or heavenly sphere) as made up of something hard and firm. What, then, were the stars?

Naturally, the first thought was that the stars were exactly what they appeared to be: tiny, glowing disks embedded in the material of the firmament ("Look how the floor of heaven is thick inlaid with patens of bright gold").

The evidence in favor of this was that the stars did not fall down, as they would surely do if they were not firmly fixed to the heavenly sphere. Secondly, the stars moved about the Earth once every twenty‑four hours, with the North Star as one pivot (the other being invisible behind the southern horizon), and did so all in one piece without altering their relative positions from night to night and from year to year.

If the stars were suspended freely somewhere between the heavenly sphere and the Earth, and for some reason did not fall, surely they would either not move at all or, if they did, would move independently. No, it made much more sense to suppose them all fixed to the heavenly sphere, and to suppose that it was the heavenly sphere that turned, carrying all the stars with itself.

But alas, this interpretation of the heavens ‑ beautiful and austerely simple ‑ did not account for everything.
As it happened, the Moon was clearly not imbedded in the heavenly sphere, for it did not maintain a fixed position relative to the stars. It was at a particular distance from a particular star one night, farther east the next night, still farther east the one afterward. It moved steadily west to east in such a way as to make a complete circuit of the starry sky in a little over twenty‑seven days.

The Sun moved from west to east, too, relative to the stars, though much more slowly. Its motion couldn't be watched directly, of course, since no stars were visible in its neighborhood by which its position might be fixed. However, the nighttime configuration of stars shifted from night to night because, clearly, the Sun moved and blotted out slightly different portions of the sky from day to day. In that manner it could be determined that the Sun seemed to make a circuit of the sky in a little over 365 days.

If the Sun and the Moon were the only bodies to be exceptional, this might not be too bad. After all, they were very much different from the stars and could not be expected to follow the same rules.

Thus the Hebrews, in their creation myth, treated the Sun and Moon as special cases. On the fourth day of creation, "God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also" (Genesis 1: 16).

It seems amusing to us today to have the stars dismissed in so offhanded a fashion, but it makes perfect sense in the light of the Hebrew knowledge of the day. The stars were all imbedded in the firmament and they served only as a background against which the motions of the Sun and the Moon could be studied.

But then it turned out that certain of the brighter stars were also anomalous in their motions and shifted positions against the background of the other stars. In fact, their motion was even stranger than that of the Moon and the Sun, for, though they moved west to east most of relative to the stars, as the Moon and the Sun occasionally would turn about and move east to west. puzzling!

The Greeks called these stars planetes, meaning "wanderers," as compared with the "fixed stars." The Greek word has become "planet" to us and seven of them were recognized. These included the five bright stars which we now call Mercury, Venus, Mars, Jupiter, and Saturn, and, of course, the Sun and the Moon.

What to do with them? Well, like the stars, the planets did not fall and like the stars they moved about the Earth. Therefore, like the stars, they had to be embedded in a sphere. Since each of the seven planets moved at a different speed and in a different fashion, each had to have a separate sphere, one nested inside the other, and all nested inside the sphere of the stars.
Thus there arose the notion not of the heavenly sphere, but of the heavenly spheres, plural.

But there was only one heavenly sphere that could be seen‑the blue sphere of the firmament. The fact that the other spheres were invisible was no argument, however, for their non‑existence, merely for their transparency. They were sometimes called "the crystalline spheres," where the word "crystalline" was used in its older meaning as "transparent."

The Greeks then set about trying to calculate where the different spheres were pivoted and how they must turn in order to cause each planet to move in the precise fashion in which it was observed to move. Endless complications had to be added in order to match theory with observation, but for two thousand years the complicated theory of the crystalline spheres held good, not because men of thought were perversely stupid, but because nothing else so well fit the appearances. 

Even when Copernicus suggested that the Sun, not the Earth, was the center of the universe, he didn't abolish the spheres. He merely had them surrounding the Sun, with the Earth itself embedded in one of them. It was only with Johannes Kepler --

But never mind that. The details of the motions of the crystalline spheres don't concern us in this article. Let us instead consider an apparently simpler question: In what order are the spheres nested? If we were to travel outward from Earth, which sphere would we come to first, which next, and so on. 
The Greeks made the logical deduction that the closest sphere would be smallest and would therefore make a complete turn in the briefest time. Since the Moon made a complete circle against the stars in about four weeks (a far shorter time than any other planet managed to run the course), its sphere must be closest.

Arguing in this manner, the Greeks decided the next closest sphere was that of Mercury; then, in order, Venus, the Sun, Mars, Jupiter, and Saturn. And finally, of course, there was the sphere of the stars.
And how far apart were the spheres and what were their actual distances from the Earth?

That, unfortunately, was beyond the Greeks. To be sure, the Greek astronomer Hipparchus, about 150 B.C., used a perfectly valid method (after the still earlier astronomer Aristarchus) for determining the distance of the Moon, and had placed it at a distance of thirty times the Earth's diameter, which is correct, but the distance of no other heavenly body was determined with reasonable accuracy until the seventeenth century.

Now the scene switches. About 520 B.C., the Greek philosopher Pythagoras was plucking strings, and found that he could evoke notes that harmonized well together if he used strings whose lengths were simply related. One string might be twice the length of another; or three strings might have lengths that were in the ratio of 3:4:5.

The details are irrelevant, but to Pythagoras it seemed highly significant that there should be a connection between pleasing sounds and small whole numbers. It fit in with his rather mystical notion that everything in the universe was related to simple ratios and numbers.

Those who followed in his footsteps after his death accentuated the mysticism and it seemed to the Pythagoreans that they now had a way of deciding not only the how of planets, but the why as well. Since numbers governed the universe, one ought to be able to deduce the way in which the universe ought to be constructed.

For instance, 10 was a particularly impressive number. (Why? Well, for one thing, I + 2 + 3 + 4 = 10, and this seems to have some mystical value.) In order, then, for the universe to function well, it had to be composed of ten spheres.

Of course, there were only eight spheres, one for the stars and one for each of the seven planets, but that didn't stop the Pythagoreans. They decided that the Earth moved around some central fire of which the Sun was only a reflection, and worked up a reason for explaining why the central fire was invisible. That added a ninth sphere for the Earth. In addition, they imagined another planet on the opposite side of the central fire, a "counter‑Earth". The counter‑Earth kept pace with the Earth and stayed always beyond the central fire and was thus never seen. Its sphere was the tenth.

In addition, the Pythagoreans thought that the spheres were nested inside each other in such a way that their distances of separation bore simple ratios to one another and produced harmonious notes in their motion as a result (like the plucking of strings of simply related lengths). Originally, I imagine, the Pythagoreans may have advanced this notion of harmonious notes only as a metaphor to represent the simply related distances, but later ' mystics accepted the notes as literally existent. They became "the music of the spheres."