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The Ancient Sky
The fortuitous alliance of two agents led to the birth of astronomy: curiosity and necessity. From savannas, mountaintops, and forest clearings, the first celestial observers looked up at the nighttime sky and beheld a vast, pitch-black bowl covered with sparkling pinpoints of light. While likely awed at first by this jewel-like canopy, imagining it as a vaulted roof through which the fires of the gods flickered, prehistoric peoples eventually learned there were practical benefits to studying the sky’s incessant motions and cycles.
Tracing out patterns of stars—constellations—became a useful procedure for establishing a coordinate system across the heavens, and the leisurely parade of these stellar figures over the seasons served as valuable markers for navigation, agriculture, and timekeeping. As the Greek poet Hesiod advised in the eighth century b.c., “When the Pleiades, daughters of Atlas, are rising, begin the harvest, the plowing when they set.” Here the farmer was instructed to reap winter wheat in the spring, when the Pleiades rise with the Sun, and to plant seeds in the fall, when the notable constellation sets in the west before sunrise. In ancient Egypt observers noticed that the brilliant star Sirius rose in the east right before dawn, at the very time that the Nile river experienced its annual flooding.
In the high northern latitudes it was the Sun’s recurrent passage that held particular significance. As winter approaches there, the Sun’s path moves steadily southward, just as the days and nights get colder. Primitive megaliths were built to mark the pivotal moment—winter solstice—when the Sun would (to much thanksgiving) turn back and once again rise higher in the sky.
Relics from the first days of civilization showcase the ancients’ intense intellectual curiosity about the nighttime sky. Inscriptions on Chinese oracle bones recorded the appearance of bright comets and “guest stars”; Mayan hieroglyphic books documented the movements of Venus with remarkable precision; clay tablets in Babylonia, dating back nearly four thousand years, chronicled the cyclic movements of the Moon and the “wanderers”—the planets—among the fixed stars. With Alexander the Great conquering Persia in 331 B.C., Babylonia’s tradition of keen skywatching merged with Greece’s focus on geometric models of the universe’s workings.
It was the ancient Greeks who were most influential in moving contemplation of the cosmos from pure mythology to a more reasoned cosmology. They began to wonder about the essential nature of heavenly bodies: how they moved, what they were made of. The first challenge was explaining why that small, elite group of wanderers—the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn—moved at differing speeds and in some cases even stopped and moved backwards in the sky. The Pythagoreans, so enamored of numbers and harmonic relationships, influenced Greek astronomers to solve this problem by thinking of the heavens as a geometric system. With beauty and harmony requiring uniform motion, imaginative models were devised to have the planets move via a set of nested spheres. It was the first attempt at a grand unified theory: explaining celestial motion with a single, all-encompassing mechanism. At the same time, these early astronomers came to understand the source of the Moon’s light, the cause of eclipses, and the true shape of the Earth. They also used their knowledge of geometry to tackle such other questions as the size of our planet and the distances to the Sun and Moon.
There were some prescient speculations on the nature of the solar system in this ancient era. In the fourth century b.c. Heraclides of Pontus suggested that night and day were due to the rotation of the Earth. Aristarchus of Samos later put the Sun at the center of his model of the universe. But these ideas never flourished, as they were overshadowed by the authoritative cosmology espoused by the noted philosopher Aristotle. At the center of his cosmos was the Earth, composed of one of the four basic elements. Surrounding this were water and air. The last element, fire, extended outward to the Moon. In this realm, life was mortal and imperfect. The heavenly bodies, on the other hand, inhabited a domain that was flawless and eternal—the celestial spheres in perpetual circular motion. This model held sway for nearly twenty centuries, and astronomy progressed only when observers such as Hipparchus and Ptolemy dared to tinker with its precepts. Hipparchus discovered the precession of the equinoxes, and Ptolemy cleverly amended Aristotle’s standard model to make it agree better with observation. From these early creative attempts to understand the star-studded sky, a science was born.
1 / Mayan Venus Tables
Some 3,500 years ago the Maya came to occupy a large territory in Central America that now covers southern Mexico, Guatemala, and northern Belize. By A.D. 200 (or even earlier) these native Mesoamericans had advanced from a simple Stone Age existence cultivating maize and squash to a sophisticated civilization whose cities contained impressive stone temples, palaces, and pyramids.
Along with hieroglyphic writing, the Maya developed refined astronomical methods that were representative of astronomical techniques carried out by early societies in other parts of the world—for example, in ancient Egypt and Babylonia. Like the observations made by those other ancient cultures, Mayan stargazing focused on cycles. They viewed the cosmos as a repetitive machine whose operation could offer their society advance knowledge of its fate if the celestial movements could be accurately tracked. Their meticulous observations of the nighttime sky were closely linked with their ritualistic needs.
Of particular importance to the Maya was the planet Venus, whose appearance in the sky follows a distinct pattern. When Venus passes between the Earth and the Sun (a configuration known as inferior conjunction), it cannot be seen for eight days. Eventually Venus is spotted in the morning sky, after it proceeds in its orbit and rises just before the Sun. For 263 days on average it remains visible in the morning, until it passes behind the Sun (superior conjunction) and again disappears. Fifty days later it comes back into view but this time as the evening star, remaining in the night sky for another 263 days until it reaches inferior conjunction once again. The period from one inferior conjunction to the next totals 584 days.
The Maya followed this cycle and recorded their knowledge of its predictability in the Dresden Codex, one of three surviving Mayan hieroglyphic books transported to Europe as spoils of the Spanish conquest. In each book, intricate glyphs are displayed on a single sheet of paper, pounded from the inner bark of a wild ficus tree and folded into separate leaves like a screen. The Dresden Codex, nearly four yards long, has thirty-nine leaves (painted on both sides) and takes its name from the German city in which it now resides. It’s essentially a series of almanacs that chronicle upcoming astronomical events, including lunar and solar eclipses. The glyphs depict a number of gods—some benevolent, others auguring bad tidings. They include the rain gods, the god of maize, a merchant god, a sun god, and a moon goddess, as well as several deities associated with death. Astronomy in this case was being used for divine forecasting, to help farmers predict times of drought, fearsome storms, or an abundant crop.
The Venus tables are found on six pages of the Dresden Codex and tell the reader when Venus will appear and disappear in the morning and evening sky over time. One of the Maya’s greatest achievements in their tracking of Venus was recognizing that the planet’s cycle was not a full 584 days but slightly less (583.92 days). They adjusted their calendar for this difference with astounding accuracy. Concern for such precision is essentially what transformed an astrological endeavor into a science.
The Maya had names for units of time comparable to days, months, decades, and centuries, although on a far different counting system. The uinal (or winal), for example, consisted of 20 days, a sort of month. At times 5 extra days were added. A tun, close to a year, was 360 days. Twenty tuns made up a katun, while 20 katuns was a baktun. A listing of the number of these “centuries,” “decades,” “years,” “months,” and days since some day zero was one way that the Maya generated a calendar. The Mayan Venus tables, though, use another system, where each day is represented by a set of numbers (a dot is one; a bar is five) and names. These dates are listed on the upper left of a page. Notice in Figure 1.1 that every line in this section has four symbol groups. Each specifies an important date in one complete cycle of the Venus period: first the day when Venus will disappear at superior conjunction; next when it reappears as the evening star; then when it disappears at inferior conjunction; and finally when it becomes visible once again as the morning star. Continuing along a selected line across five of the tables (see Figures 1.2 and 1.3) covers a unique period of 2,920 days, over which five Venus cycles equal eight Earth years. At the end, the user of the table moves on to the next line of the five-table chart, where the cycle begins again.
2 / Proof That the Earth Is a Sphere
In 342 B.C. King Philip II of Macedonia brought the learned philosopher Aristotle to his court to tutor his son, who as a man would become Alexander the Great. Soon after Alexander assumed the throne, Aristotle established a school in Athens where he continued his wide-ranging studies in philosophy, logic, politics, physics, and biology.
Aristotle’s writings on astronomy were compiled in a four-volume text entitled De caelo, “On the Heavens.” The cosmology that he established within this work wielded a powerful influence on astronomers for nearly twenty centuries. Aristotle reasoned that the Earth was an arena of change and imperfection. Its basic elements, earth and water, moved downward, because they sought their natural place. The other essential elements, air and fire, moved upward. To Aristotle, though, the region inhabited by the planets and stars was far different. That was because celestial bodies did not move up or down but rather traveled in circles, an eternal path of perfection and uniformity. Given that difference, he concluded that the heavens had to be composed of another substance altogether, the aether.
There were irregularities in the heavenly movements that required clarification. To explain retrograde, the appearance of a planet’s traveling backward on the nighttime sky, Aristotle adopted a model of planetary motion devised by his contemporary Eudoxus of Cnidus.* Eudoxus, a geometer, introduced the idea of the planets and stars being moved by heavenly spheres rotating about the Earth. Each planet was attached to several spheres. The orderly motion of these spheres, once combined, produced a planet’s deceivingly irregular movement. Aristotle modified this system and advocated it so commandingly that it was difficult for celestial observers even to consider models that didn’t incorporate his vision of circular perfection. During the rise of Christianity, it was transformed into God’s chosen design. Modern astronomy would not emerge until astronomers were willing to break away from such Aristotelian notions and consider other possibilities to explain their observations (see Part II, “Revolutions”).
De caelo is more philosophy than true astronomy, but there is one section in which Aristotle does rally decent evidence in behalf of his conclusion: the sphericity of the Earth. That the Earth is round was likely recognized by Greek thinkers, such as the Pythagoreans, two centuries beforehand. For that matter, ancient seafarers saw far-off landmarks or ships dipping below sea level and probably realized that the Earth was curved. The earliest surviving proof, however, can be traced to Aristotle. The major part of his argument comes from his physics: a spherical shape, he theorizes, is naturally generated as the terrestrial elements fall downward to seek the center. But he doesn’t ignore observational data (including, as we shall see, the range of elephants). Absorbing the wisdom of others before him, he notes that the Earth’s shadow, as it passes over the Moon during an eclipse, is always circular. He adds to this by noting that travelers will see different stars come into view as they travel north and south, a change that would not occur if the Earth were flat. Such reasoning was a tremendous advance over earlier guesses on the Earth’s shape, such as Anaximander’s suggestion in the sixth century b.c. that it was a cylinder freely suspended in space. As described by Hippolytus in his Refutation of All Heresies, “Its form is rounded, circular, like a stone pillar; of its plane surfaces one is that on which we stand, the other is opposite.”
Aristotle’s astronomical commentary also includes the earliest recorded mention of the Earth’s circumference, 400,000 stades, although he doesn’t provide his source or the method of the calculation. Originally a stade was the length of a traditional Greek racetrack, but eventually different types of stades came into use. Values can vary from roughly 8 to 10 stades per mile. So Aristotle’s declared circumference is between 40,000 to 50,000 miles, not outrageously larger than the true measurement of nearly 25,000.
From De caelo
Translated by J. L. Stocks
The shape of the heaven is of necessity spherical; for that is the shape most appropriate to its substance and also by nature primary.
First, let us consider generally which shape is primary among planes and solids alike. Every plane figure must be either rectilinear or curvilinear. Now the rectilinear is bounded by more than one line, the curvilinear by one only. But since in any kind the one is naturally prior to the many and the simple to the complex, the circle will be the first of plane figures. Again, if by complete, as previously defined, we mean a thing outside which no part of itself can be found, and if addition is always possible to the straight line but never to the circular, clearly the line which embraces the circle is complete. If then the complete is prior to the incomplete, it follows on this ground also that the circle is primary among figures. And the sphere holds the same position among solids. For it alone is embraced by a single surface, while rectilinear solids have several. The sphere is among solids what the circle is among plane figures. Further, those who divide bodies into planes and generate them out of planes seem to bear witness to the truth of this. Alone among solids they leave the sphere undivided, as not possessing more than one surface: for the division into surfaces is not just dividing a whole by cutting it into its parts, but division of another fashion into parts different in form. It is clear, then, that the sphere is first of solid figures. . . .
From the Hardcover edition.