Page three- the stepping stone to the heavens

And what else? I promised three crises; for the third, let us move still farther forward in time, to a point at which human civilization was in full career.

By the third millennium B.C. the first great civilization, that of the Sumerians in the downstream reaches of the Tigris-Euphrates Valley, was at its peak. In that dry climate the night sky was uniformly and brilliantly visible, and there was a priestly caste that had the leisure to study the heavens and the religious motivation to do so.

It was they, in all likelihood, who first noticed that although most of the stars maintained their configurations for night after night indefinitely, five of the brighter ones shifted position steadily, night after night, relative to the rest. This represented the discovery of the planets, which they distinguished by the names of gods, a habit we have kept to this day. They noted that the Sun and the Moon also shifted position steadily with references to the stars, so they were considered planets too.

The Sumerians were the first (possibly) to begin to follow the motions of all the planets rather than of the Moon only, and to attempt the far more complicated task of generalizing and systematizing planetary motion rather than lunar motion. This was continued by the later civilizations inheriting their traditions, until the Chaldeans, who ruled the Tigris-Euphrates Valley in the sixth century B.C., had a well developed system of planetary astronomy.

The Greeks borrowed astronomy from the Chaldeans and elaborated it further into a system, which Claudius Ptolemy put into its final form in the second century A.D.

This Ptolemaic system placed the Earth at the centre of the universe. Earth was supposed to be surrounded by a series of concentric spheres. The innermost held the Moon the next Mercury, then Venus, the Sun, Mars, Jupiter, and Saturn, in that order. The outermost held the fixed stars. Many subtle modifications were added to this primary scheme.

Now let's consider the objects in the heavens, one by one, and see how they would impress the early observer. Suppose first that only the stars existed in the sky.

In that case there would be no reason whatever for any astronomer, whether Sumerian or Greek, to assume that they were anything other than what they appeared to be: luminous dots of light against a black background. The fact that they never changed their position relative to one another, even after long periods of observation, would make it reasonable to suppose that the sky was a black solid sphere enclosing the Earth and that the stars were imbedded in that solid sky like tiny, luminous thumbtacks.

It would be further reasonable to suppose the sky and its embedded stars to be a mere covering, and that the Earth, and the Earth alone, made up the essential universe. It had to be the world, the only existent thing man could inhabit.

When Mercury, Venus, Mars, Jupiter, and Saturn were discovered and studied, they added nothing startlingly new to this picture. They moved independently, so they could not be affixed to the sky. Each had to be embedded in a separate sphere, one inside the other, and each of these spheres had to be transparent, since we could see the stars through them all.

These planets, however, were merely so many more stars to the primitive observer. They were brighter than the others and moved differently, but they had to be only additional luminous points. Their existence did not interfere with the view of the Earth as the only world. What about the Sun, though ?

That, it would have to be admitted, is unique in the heavens. It is not a dot of light, but a disc of light, many millions of times as bright as any star. When it was in the sky, it painted the sky blue and washed out any mere dot of light.

And yet, although the Sun was much more, it was not much different. All the stars and planets, and the Sun too, were composed of light, while the Earth was dark. The heavenly bodies were changeless, while all on earth corrupted, decayed, and changed. The heavenly bodies moved around and around, while objects on Earth either rose or fell. Heaven and Earth seemed fundamentally different.

About 340 B.C., Aristotle set the distinction in a fashion that held good for two thousand years. The Earth, he said, was made of four basic constituent elements: earth, water, air, and fire. The heavens, however, and everything in them, were made of a fifth element, peculiar to itself and completely different from the four of Earth. This fifth element is 'ether', from a Greek word meaning 'glowing'.

This glowingness, or luminosity, which seemed so fundamental to heavenly bodies as opposed to earthly ones, extended to temporary denizens of the heavens too. Meteors existed only momentarily, but they were flashes of light. Comets might come and go and have strange shapes, but those shapes were luminous.

Everything, it seemed, conspired to show the heavens to be separate and the Earth to be the only world.

. . . Except the Moon.

The Moon does not fit. Like the Sun, it is more than a mere dot of light. It can even be a full disc of light, though it is then hundreds of thousands of times less bright than the Sun. Unlike the Sun or anything else in the heavens, how- ever, the Moon changes its shape regularly.

Sooner or later the question must have arisen, Why does the Moon change its shape?

Undoubtedly, man's first thought would be that what seemed to happen did happen: that, every month, a new Moon was born from the fires of the Sun. Some unnamed Sumerian might have had his doubts, however. The complete and careful study of the Moon's position in the sky as compared to the Sun must have made it quite clear that the luminous portion of the Moon was always 1he portion that faced the Sun.

It would appear that as the Moon changed position relative to the Sun, progressively different portions were illuminated, and this progressive change resulted in changes of phase as seen from the Earth.

If the phases of the Moon were interpreted in this fashion, it appeared that the Moon was a sphere that shone only by light reflected from the Sun. Only half the sphere was illuminated by the Sun at any one time, and this illuminated hemisphere shifted position to produce the succession of phases. If any proof were needed to substantiate this, it could be found in the manner in which, at the time of the crescent Moon, the rest of the Moon's body could sometimes be made out in a dimly red luminosity. It was there but was simply not being illuminated by the Sun.

By Greek times, the fact that the Moon shone only by reflected light from the Sun was accepted without question.

This meant that the Moon was not an intrinsically luminous body, as all the other heavenly bodies seemed to be. It was a dark body, like Earth. It shone by reflected light, like Earth. (In fact, the dim, red glow of the dark Moon at the time of the crescent resulted from the bathing of that part of the Moon in earthlight.)

Then, too, the Moon's body, unlike that of the Sun, showed clear and permanent markings, dark splotches that marred its luminosity. This meant that, unlike the other heavenly bodies, the Moon was visibly imperfect, like the Earth.

It was possible to suppose, then, that the Moon, at least, was a world as the Earth was one; that the Moon, at least, might bear inhabitants as Earth did. Even in ancient times, then, the Moon (and the Moon alone) gave man the notion of a multiplicity of worlds. Without the Moon, the notion might never have arisen before the invention of the telescope.

Aristotle, to be sure, did not put the Moon in a class with the Earth, but considered it to be composed of ether. One might argue that the Moon was closer to Earth than any other heavenly body was, and therefore absorbed some of the imperfections of earthly elements, developing stains and losing the capacity for self-illumination.

But then Greek astronomy advanced further. About 250 B.C., Eratosthenes of Cyrene used trigonometric methods for calculating the size of the Earth. He came to the conclusion that the Earth had a circumference of twenty-five thousand miles and therefore a diameter of eight thousand miles. This was essentially correct.

In 150 B.C., Hipparchus of Nicaea used trigonometric methods to determine the distance to the Moon. He decided the distance of the Moon from the Earth was about thirty times the diameter of the earth. This, too, was essentially correct.

If the work of Hipparchus and Eratosthenes was combined, then the Moon was 240,000 miles from Earth, and to appear to be its apparent size, it had to be a little over two thousand miles wide. It was a world! Whatever Aristotle said, it was a world in size at least.

It is not surprising, then, that by the time Claudius Ptolemy was publishing his grand synthesis of Greek astronomy, Lucian of Samosata was writing a popular romance involving a trip to an inhabited Moon. Indeed, once the Moon was recognized as a world, it was an easy further step to assume that other heavenly bodies were worlds as well.

Yet it is only the Moon - only the Moon - that is close enough to Earth for its distance to be estimated by trigonometric methods based on unaided-eye observations. Without the Moon, it would have been impossible to gain any knowledge whatever of the distance and size of any heavenly body prior to the invention of the telescope. And without the nudge of knowing the Moon's distance and size, might there have been quite the urge to explore the heavens even after the telescope was invented and used for military purposes?

Then, in 1609, Galileo did press the telescope into astronomic service for the first time.

Galileo studied the heavens and found that, through his telescope, the planets, which seemed to be dots of light when viewed by the unaided eye, appeared to be distinctly formed spheres of light. What's more, Venus, at least, was so located with respect to the Earth as to show phases like those of the Moon; phases, moreover, plainly related to its position with respect to the Sun.

The conclusion seemed inevitable. All the starlike planets: Mercury, Venus, Mars, Jupiter, and Saturn; were worlds like the Moon. They appeared as mere dots of light because they were so much farther from us than the Moon was.

This in itself was not fatal to the Aristotelian view, for it could be argued that the planets (and the Moon), however large they were, and however non-luminous, were nevertheless composed of ether.

What really destroyed the ethereal concept once and for all however, was Galileo's observation of the Moon. (1ndeed, he looked at the Moon first of all.) On the Moon, Galileo saw mountains, and dark, smooth areas he interpreted as seas. The Moon was clearly, visibly, a world like Earth: imperfect, rough, mountainous.

It is no wonder, then, that with this second blow dealt by the Moon, the concept of the plurality of worlds took another giant step forward. The seventeenth century saw the beginning of a set of novels dealing with manned voyages to the Moon that grew steadily more sophisticated and have not ceased right down to the present day.

Of course you say that Galileo would have demonstrated the plurality of worlds, by telescope, even if the Moon had not existed, and that the resistance of the Aristotelians would have broken as telescopes improved and as other tools were invented.

Suppose that were the case: Science-fiction writers might then have dreamed of flights to Mars or Venus instead of to a non-existent Moon . . . But dreams are only dreams, after all. Would man have attempted to make space flight a reality if the Moon had not existed?

The Moon is less than a quarter of a million miles from us. Venus, on the other hand, is 25 million miles away even when it is at its closest (at intervals of a year and a half). It is then a hundred times as far from us as the Moon is. Mars at its times of closest approach is farther still. Every thirty years or so, when it is particularly close, it is 35 million miles away.

It takes three days to reach the Moon. It would take at least half a year to reach Venus or Mars. It has taken heroic measures for men to reach the Moon. Would it have been reasonable to expect them to have made the many-times-multiplied heroic measures necessary to reach Venus or Mars from scratch? No, it is the Moon - the Moon only - that made space flight possible. It did so first by letting us see that there are other worlds than our own and then by offering us an easy stepping-stone by means of which we can sharpen our techniques and from which, as a base, we can eventually make the much greater assault on the more distant worlds.