Tuesday, February 26, 2008

Understanding Galileo's Work

While Kepler was theorizing from Copernicus’s data, an Italian astronomer, Galileo Galilei (1565–1642), directed his gaze skyward, amplifying his eyesight with the aid of a new Dutch invention (which we’ll meet in the next chapter), the telescope. Through this instrument, Galileo explored the imperfect surface of the moon, covered as it was with craters, “seas,” and features that looked very much like the surface of the earth. Galileo was even more surprised to find that the surface of the sun was blemished. These “sunspots”, he noted, changed position from day to day. From this fact, Galileo did not conclude that the spots changed, but that the sun was rotating, making a complete revolution about once each month.

His telescope also revealed for the first time that moons orbited Jupiter—another observation that strongly supported the notion that the earth was not the center of all things. He observed that the planet Venus cycled through phases, much like the moon, and that the size of the planet varied with its phase. From this, he concluded that Venus must orbit not the earth, but the sun.

Galileo published these many independent experimental proofs of a heliocentric solar system in 1610.Six years later, the Catholic Church judged the work heretical and banned them, as well as the work of Copernicus. Galileo defied the ban and, in 1632, published a comparison of the Ptolemaic and Copernican models written as a kind of three-way discussion. He was so bold as to write in Italian, instead of learned Latin, which meant that common folk (at least those who were literate) were being invited to read a theory that challenged the teaching of the Church.

Galileo was silenced by the Holy Inquisition in 1633, forced to recant his heresy under threat of death, and placed under house arrest for the rest of his life.

Three Kepler Laws

Kepler had predicted that with Tycho’s data, he would solve the problem of planetary motion in a matter of days. After almost eight years of study, trial, and error, Kepler had a stroke of genius. He concluded that the planets must orbit the sun not in perfect circles, but in elliptical orbits (an ellipse is a flattened circle). He wrote to a friend: “I have the answer … The orbit of the planet is a perfect ellipse. Kepler was able to state the fundamentals of planetary motion in three basic laws.

That planets move in elliptical orbits, with the sun at one focus of the ellipse, is known as Kepler’s First Law. It resolved the discrepancies in observed planetary motion that both Ptolemy and Copernicus had failed to explain adequately. Both of those great minds had been convinced that in a perfect universe, the orbits of planets had to be circular.

Another apparent attribute of elliptical orbits determined Kepler’s Second Law. It states that an imaginary line connecting the sun to any planet would sweep out equal areas of the ellipse in equal intervals of time. The Second Law explained the variation in speed with which planets travel. They will move faster when they are closer to the sun. Kepler did not say why this was so, just that it was apparently so. Later minds would confront that why. The first two of Kepler’s laws were published in 1609.

The third did not appear until ten years later and is slightly more complex. It states that the square of a planet’s orbital period (the time needed to complete one orbit around the sun) is proportional to the cube of its semi-major axis. Since the planets’ orbits, while elliptical, are very nearly circular, the semi-major axis can be considered to be a planet’s average distance from the sun.

Understanding Kepler's work

When King Frederick died in 1588, Tycho lost his most understanding and indulgent patron. Frederick’s son Christian IV was less interested in astronomy than his father had been, and, to Christian, Tycho was an unreasonably demanding protégé, who repeatedly sought more money. At last, the astronomer left Denmark and ultimately settled in Prague in 1599 as Imperial Mathematician of the Holy Roman Empire. In the Czech city, he was joined by a persistent younger German astronomer, Johannes Kepler (1571–1630), who, after writing several flattering letters to Tycho, became his student and disciple.

Tycho Brahe, a colorful character, who lost part of his nose in a duel (he replaced it with a golden prosthesis), died ingloriously in 1601, apparently from a burst bladder after drinking too much at a dinner party. After a bit of a struggle, Kepler got a hold of the mass of complex observational data Tycho had accumulated. While Tycho had been a brilliant observer, he was not a particularly good theoretician. Kepler, a sickly child, had grown into a frail adult with the mind of a brilliant theorist—though with very little aptitude for close observation, since he also suffered from poor eyesight. Indeed, Tycho and Kepler were the original odd couple, who argued incessantly; yet their skills were perfectly complementary.

And when Tycho died, instead of using his instruments to make new observations, Kepler dived into Tycho’s data, seeking in its precise observations of planetary positions a unifying principle that would explain the motions of the planets without resorting to epicycles. It was clear, especially with Tycho’s data, that the Copernican system of planets moving around the sun in perfectly circular orbits was not going to be sufficient. Kepler sought the missing piece of information that would harmonize the heliostatic solar system of Copernicus with the mountain of data that was Tycho’s planetary observations.

Friday, February 22, 2008

The Man with the Golden Nose

The astronomer Tycho Brahe (1546–1601) was born in Denmark just three years after the publication of De Revolutionibus. As a youth, he studied law, but was so impressed by astronomers’ ability to predict a total solar eclipse on August 21, 1560, that he began to study astronomy on his own. In August 1563, he made his first recorded astronomical observation, a conjunction (a coming together in the sky) of Jupiter and Saturn, and discovered that the existing ephemerides were highly inaccurate. From this point on, Tycho decided to devote his life to careful astronomical observation.

On November 11, 1572, he recorded the appearance of a new star , brighter than Venus, in the constellation Cassiopeia. The publication of his observations (De Nova Stella, 1573) made him famous.

King Frederick II of Denmark gave him land and financed the construction of an observatory Tycho called Uraniborg (after Urania, the muse of astronomy). Here Tycho not only attracted scholars from all over the world, but designed innovative astronomical instruments and made meticulous astronomical observations—the most accurate possible before the invention of the telescope.

A Revolution of Revolutions

For centuries, astronomy had been a science in which errors of a few degrees in planetary position on the sky were acceptable. But to Copernicus, Kepler, and others who would follow, errors of that magnitude indicated that something was seriously wrong in our understanding of planetary motion. They were driven to discover their origin.

Now we turn to the details of Copernicus’s model. The first of the six sections of De Revolutionibus sets out some mathematical principles and rearranges the planets in order from the sun: Mercury is closest to the sun, followed by Venus, Earth (with the Moon orbiting it), Mars, Jupiter, and Saturn.

The second part applies the mathematical rules set out in the first to explain the apparent motions of the stars, planets, and sun. The third section describes Earth’s motions mathematically and includes a discussion of precession of the equinoxes, attributing it correctly to the slow gyration of the earth’s rotational axis. The last three parts of the book are devoted to the motions of the moon and the planets other than Earth.

While Copernicus’s purpose in reordering the planetary system may have been relatively modest, it soon became apparent that the new theory required the most profound revision of thought.

First: The universe had to be a much bigger place than previously imagined. The stars always appeared in the same positions with the same apparent brightness. But if the earth really were in orbit around the sun, the stars should display a small but noticeable periodic change in position and brightness. Why didn’t they? Copernicus said that the starry celestial sphere had to be so distant from Earth that changes simply could not be detected.

Building on this explanation, others theorized an infinite universe, in which the stars were not arranged on a celestial sphere, but were scattered throughout space.
The second required revision, while not as obvious, was even more basic.
Why do things fall? Aristotle explained that bodies fell toward their “natural place,” which, he said, was the center of the universe.

That explanation worked as long as the earth was considered to be the center of the universe. But now that it wasn’t the center, how could the behavior of falling bodies be explained? The answers would have to wait until the late seventeenth century, when Isaac Newton published his Principia, including a theory of universal gravitation.

Profound as were the astronomical and other scientific implications, the emotional shock of the Copernican universe was even greater. Suddenly, the earth, with humankind upon it, was no longer at the center of all creation, but was instead hurtling through space like a ball on a string. Why did we stay on the ball? What was the string? These were all unanswered questions that must have been very unsettling for those who thought about them.

“More Pleasing to the Mind”

This uncertainty in the calendar bothered Copernicus deeply. Many astronomers thought that any errors in the Ptolemaic system might be due to the many small “typos” that had crept into the manuscript with centuries of copying by scribes. However, connections between east and west at this time meant that Copernicus was able to have a nearly pristine copy of Ptolemy’s work Almagest. Any errors had to be errors in the model itself.

Ptolemy’s complex geocentric system of epicycles Copernicus questioned the Ptolemaic system on the very basis that a modern scientist might. The model had become too complicated, and scientists tend to seek simplicity (where possible) in their models of the universe. The printing presses that were firing up across Europe at the time made it possible for many more scholars to read good copies of ancient works.

As Copernicus started reading Greek manuscripts that had been long neglected, he rediscovered Aristarchus’s old idea of a heliocentric (sun-centered) universe. He concluded that putting the sun at (or near) the center of a solar system with planets in orbit around it created a model that was “more pleasing to the mind” than what Ptolemy had proposed and medieval Europe accepted for so many centuries.

But he did not rush to publish, only after much hesitation privately circulating a
brief manuscript, De Hypothesibus Motuum Coelestium a Se Constitutis Commentariolus
(A Commentary on the Theories of the Motions of Heavenly Objects from Their Arrangements)

in 1514. He argued that all of the motion we see in the heavens is the result of the earth’s daily rotation on its axis and yearly revolution around the sun, which is motionless at the center of the planetary system.
Sound familiar?

Aristarchus had suggested it almost 2,000 years earlier, but no one had listened. The earth, Copernicus explained, was central only to the orbit of the moon. For almost two more decades he refined his thought before consenting, in 1536, to publish the full theory. But largely because of opposition from Martin Luther and other German religious reformers, De Revolutionibus Orbium Caelestium (On the Revolutions of the Celestial Spheres) wasn’t actually printed until 1543.

As close as Copernicus’s model came to representing the motion of planets in the solar system, it insisted on the perfection of circular orbits, so that it actually had no better predictive ability for planetary motions than the Ptolemaic model it replaced. For all its creakiness, the Ptolemaic model still predicted, for example, where Mercury would be on a particular night about as well as Copernicus’s model did.

Sunday, February 17, 2008

The ‘Heretical’ Polish Priest

In Europe, astronomy—as a truly observational science—did not revive until the Middle Ages had given way to the Renaissance. The German mathematician and astronomer Johann Müller (1436–1476) called himself Regiomontanus, after the Latinized form of Königsberg (King’s Mountain), his birthplace. Enrolled at the University of Leipzig by the time he was 11, Regiomontanus assisted the Austrian mathematician Georg von Peuerbach in composing a work on Ptolemaic astronomy.

Regiomontanus took his job seriously and, in 1461, journeyed to Rome to learn Greek and collect Greek manuscripts from refugee astronomers fleeing the Turks, so that he was able to read the most important texts, including the Greek translation of Almagest. In the meantime, his mentor Peuerbach had died and Regiomontanus completed the master’s work in 1463. Three years later, he moved to Nürnberg, where a wealthy patron built him an observatory and gave him a printing press. Beginning in 1474, he used the press to publish ephemerides, celestial almanacs giving the daily positions of the heavenly bodies for periods of several years.

The publications of Regiomontanus, which were issued until his death in 1476, did much to reintroduce to European astronomy the practice of scientific observation.
He was so highly respected that Pope Sixtus IV summoned him to Rome to oversee revision of the notably inaccurate Julian calendar then in use.

Regiomontanus began this work on the calendar, but then died mysteriously—possibly from plague or from poison, perhaps administered by enemies resentful of his probing the cosmos too insistently. At that time, it could be dangerous to question accepted ideas, especially where the heavens were concerned. Nikolaus Krebs (1401–1464), known as Nicholas of Cusa, wrote a book called De Docta Ignorantia suggesting that the earth might not be the center of the universe.

Fortunately for Nicholas (who was a cardinal of the Catholic church), few paid attention to the idea. Another Nicholas (actually spelled Nicolaus)—Copernicus—was born in eastern Poland in 1473, almost a decade after Krebs’s death. A brilliant youth, he studied at the universities of Kraków, Bologna, Padua, and Ferrara, learning just about everything that was then known in the fields of mathematics, astronomy, medicine, and theology.

Copernicus earned great renown as an astronomer and in 1514 was asked by the church for his opinion on the vexing question of calendar reform. The great Copernicus declared that he could not give an opinion, because the positions of the sun and moon were not understood with sufficient accuracy.

Arabian Astronomers

From our perspective just beyond the cusp of the millennium, it is easy to disparage Ptolemy for insisting that the earth stood at the center of the solar system. But we often forget that we live in a unique age, when images of the earth and other planets are routinely beamed from space. These stunning pictures of our cosmic neighborhood have become so familiar to us that commercial TV networks wouldn’t think of elbowing aside this or that sitcom to show the images to the viewing public.

Informed as we are with “the truth” about how the solar system works, we wonder how Ptolemy’s complicated explanation could have been accepted for so long. There is no doubt that his model of the solar system was wrong, but, wrong as he was, his book contained the heart and soul of classical astronomy and survived into an age that had turned its back on classical learning. During the early Middle Ages, Ptolemy’s work remained unread in Europe, but his principal book found its way into the Arab world, and in 820 it was translated into Arabic as Almagest (roughly translatable as The Greatest Book). The circulation of Ptolemy’s work renewed interest in astronomy throughout Arabia, with centers of learning being established in both Damascus and Baghdad.

Abu ‘Abd Allah Muhammad Ibn Jabir Ibn Sinan Al-battani Al-harrani As-sabi’, more conveniently known as al-Battani (ca. 858–929), became the most celebrated of the Arab astronomers, although it took many years before his major work, On Stellar Motion, was brought to Europe in Latin (about 1116) and in Spanish translation (in the thirteenth century). Al-Battani made important refinements to calculations of the length of the year and the seasons, as well as the annual precession of the equinoxes and the angle of the ecliptic. Moreover, he demonstrated that the Sun’s apogee (its farthest point from Earth) is variable, and he refined Ptolemy’s astronomical calculations by replacing geometry with sleek trigonometry.

Another Arab, Al-Sûfi (903–986), wrote a book translated as Uranographia (in essence, Writings of the Celestial Muse), in which he discussed the comparative brilliance of the stars. Like the scale of the Greek astronomer Hipparchos, the system of Al-Sûfi rated star brightness in orders of magnitude. Relative star brightness is still rated in terms of magnitude.

Arab astronomers like Al-Sûfi also contributed star maps and catalogues, which were so influential that many of the star names in use today are of Arab origin (such as Betelgeuse, Aldebaran, and Algol), as are such basic astronomical terms as azimuth and zenith.

The prelude for Modern Astronomy

The “Dark Ages” weren’t dark everywhere, and astronomy didn’t exactly wither and die after Ptolemy. Outside of Europe, there were some exciting discoveries being made. The Indian astronomer Aryabh¯atta (born ca. 476) held that the earth was a rotating globe, and he correctly explained the causes of eclipses.

The Maya in particular created a complex calendar, prepared planetary tables, and closely studied Venus, basing much of their system of timekeeping on its movements. In Europe, however, astronomy was no longer so much studied as it was taught. Much Medieval learning discouraged direct observation in favor of poring over texts of recorded and accepted wisdom. Despite the work of Bishop Isidorus of Seville (570), who drew a sharp distinction between astronomy and astrology, medieval astronomy was mired in the superstition, which has its apologists and practitioners to this very day. But this chapter says little about astronomy’s “Dark Ages” and turns, instead, to its rebirth in the Renaissance.

When Night Falls

Even if today’s astronomers knew nothing of the solar system, they would likely reject the Ptolemaic model on the grounds of its unnecessary complexity. And for all of its intricacy, the Ptolemaic model was not a particularly accurate predictor of astronomical phenomenon. Indeed, the errors in the model became more glaring when better data on planetary positions (from Tycho Brahe and others) became available.

A love of elegant simplicity also characterized the classical world, but Ptolemy’s era was already falling away from classical elegance and toward the cobwebbed mysteries that so appealed to people in the Dark Ages, when complexity and obscurity, not simplicity and clarity, were taken as the hallmarks of truth. Besides, Ptolemy’s model, while highly imperfect, agreed pretty well with actual observation; it kept Aristotle safely on his pedestal, and it let humankind stay right where the Church said that God had intended: at the center of everything.

There were others who came after Aristotle and before Ptolemy, most notably Aristarchus of Samos (310–230 B.C.E.), who actually proposed that the earth and the planets orbit the sun. But Aristotle, Ptolemy, and common sense drowned out such voices that, for some thirteen centuries, few wanted to hear. For the light of classical learning had been dimmed, and the spirit of scientific inquiry muffled (at least in the West) in a long age of orthodoxy and obedience.

Wednesday, February 13, 2008

Ptolemy’s Picture

Why didn’t the world’s astronomers just toss out Aristotle’s geocentric model of the solar system?
There are at least three reasons.
  • First, and most important, the geocentric picture appeals to common sense. On a given day, as we watch the sun and the stars rise and set, we do not feel like we are in motion. There are, for example, no great winds whipping at us as one might expect if the earth were tearing through space.
  • Second, human beings are egocentric creatures. We tend to see ourselves as being at the center of things. Extend this egocentric tendency into the heavens, and you have an explanation for our species’ geocentric tendency. If, individually, we feel ourselves at the center of things, we also feel this collectively, as a planet.
  • The final reason? Because Aristotle was Aristotle. His opinions were regarded with awe for centuries. Few dared—few even thought—to question his teachings. Instead, generations of European astronomers wrestled with Aristotle’s model in an effort to show how it was actually correct.
The most impressive of these wrestling matches was fought by Claudius Ptolemaeus, a Greek astronomer better known as Ptolemy, active in Alexandria during C.E. 127–145. Drawing on the work of the Greek astronomer Hipparchus (who died after 127 B.C.E.), Ptolemy developed the geocentric model into an impressively complex system of “deferents,” large circular orbits centered on the earth, and “epicycles,” small circles whose centers traveled around the circumferences of the deferents.

Some 80 deferents and epicycles came into play, along with several other highly complex geometric arrangements, which allowed Ptolemy to account for many of the perplexing planetary motions actually observed.

Aristotle Lays Down the Law

Fortunately for the hard-pressed astronomers of yore, total eclipses of the sun are relatively rare events. More immediately, they had nightly occurrences they couldn’t explain.

They could watch the sky all night and see the stars glide predictably across the sky, as if affixed to a moving celestial sphere. Likewise, the moon made its traversal with perfect regularity. But there were (so far as the ancients could see) five heavenly bodies that didn’t behave with this regularity. Mercury, Venus, Mars, Jupiter, and Saturn looked pretty much like stars, but, in contrast to stars, they wandered across the sky and so were called by the Greeks planetes, or “wanderers.” While the planets always remain near the ecliptic, they have strange motions. Relative to the stars, planets seem to slow down and speed up, moving (from an earthbound observer’s perspective) generally eastward (prograde motion), but sometimes westward (retrograde motion) with respect to the background stars.

How did the ancients explain this strange observation? The great Greek philosopher Aristotle (384–322 B.C.E.), tutor of Alexander the Great, formulated a picture of the solar system that put the earth at its center with all the other heavenly bodies orbiting around it. The orbits were described as perfect circles. Now, Aristotle was a smart man. He provided observational evidence that the earth was spherical rather than flat. Moreover, his descriptions of the orbits of the moon and the sun accorded very well with what people actually observed. As far as the stars went, the celestial sphere idea explained them well enough. The problem was the planets. Aristotle’s simple geocentric (earth-centered) model did not explain why the planets varied in brightness, why they moved at varying speeds, and why their motion was sometimes prograde and sometimes retrograde.

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.

Saturday, February 9, 2008

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.

Sunday, February 3, 2008

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.

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.

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.