Geocentric and Heliocentric Models of the Universe

Anna Marie Eleanor Roos. Scientific Thought: In Context. Editor: K Lee Lerner & Brenda Wilmoth Lerner. Volume 1. Detroit: Gale, 2009.


The development of geocentric (Earth-centered) to heliocentric (sun-centered) models of the universe spans time from the ancient Babylonians (4000 BC) to Nicolas Copernicus’ (AD 1473-1543) publication of his heliocentric system in 1543. There were advantages and disadvantages of each model to early astronomers until a sun-centered model was finally adopted by seventeenth-century scientists.

Historical Background and Scientific Foundations

The Babylonians

Ancient models of the universe dating from the first civilizations in Babylonia and Egypt were fundamentally geocentric. Presupposing that Earth was at the center of the universe, unmovable and steady, was a reflection of the human senses and experience. Babylonian clay tablets dating to 3800 BC evidence an established astronomical tradition; observations of the movements of the sun and moon around Earth provided calendars for agricultural purposes. The stars also were observed to revolve around Earth once a day, never changing their places in the constellation patterns. By contrast, the wandering planets with their own unique motions were thought to be gods that orbited Earth and influenced human activities. Tracking these god-planets via the Babylonian practice of astrology and astronomy was a means to determine their intents towards their human subjects.

The Beginnings of Greek Astronomy

The Babylonian’s theological practice towards astronomy was not shared by the ancient Greeks. Greek philosophers from the island of Ionia in the sixth century BC attempted to create models of the universe not by using gods as an explanation but by using natural laws. Thales of Miletos (c.624-546 BC), postulated that Earth was at the center of the universe, and that Earth was a flat disc floating on water. He also argued that all the material in the universe came from water, as water could be in several different phase states: ice (solid), water (liquid), and steam (gas). His colleague, Anaximenes (c.585-525 BC) had another idea. He proposed that the stars were attached to a transparent crystal sphere that rotated around Earth, an idea that ultimately impacted the cosmological models of Aristotle.

With the philosophy of Pythagoras (c.530-440 BC), the assumed shape of Earth changed from disc to sphere. The Greeks reasoned the moon was a sphere, and, by analogy, concluded Earth must be a sphere also. The Pythagoreans asserted that the planets, the sun, and the moon revolved around Earth in concentric circles, each fastened to a crystalline wheel. The rest of the universe—including the stars—also rotated around Earth. The daily rising and setting of the stars suggested that motions in the heavens were uniform, circular, and eternal. By the third century BC, the idea that Earth was a sphere was largely accepted in the Greek world. An accurate measurement of Earth’s circumference was made in the second century BC by Greek mathematician Eratosthenes of Cyrene (276-194 BC).

The Start of Heliocentric Models

Though the geocentric model was the predominant cosmological scheme in the Greek world, other systems also appeared. One of Pythagoras’ pupils, Philolaus (c. fifth century BC), postulated that at the center of the universe was a central fire around which all the other planets (including the sun) revolved. A counter-Earth (anticthon), revolving around the sun between Earth and the sun, protected Earth from being scorched by the central fire. Later astronomers such as Copernicus thought that the central fire was equivalent to the sun and that the Pythagoreans proposed the first heliocentric model.

According to the ancient commentator Sosigenes (fl. second century AD), Heraclides Ponticus (c.387-c.310 BC) was actually the first to propose a modified heliocentric system. Heraclides maintained that the heavens did not revolve daily around Earth, but that a daily rotation of Earth on its own axis was responsible for the phenomenon. Heraclides also noted that Venus (also known as the morning star) and Mercury tended to appear around the sun and that the two planets varied in brilliance. This variation in brightness can be explained only by assuming that the planets’ distance from Earth varies, and that neither Venus’ nor Mercury’s orbit centers upon Earth. Heraclides concluded that both planets revolved around the sun instead, catching different amounts of light during their different phases in rotation. Mars also varied in brilliance, so its orbit also could not be centered on Earth. So, Heraclides proposed a “halfway-house” system in which the planets revolved around the sun, and the sun with all its planets revolved around Earth.

Another puzzling celestial phenomenon concerned the orbital motion of the planets themselves. Most of the stars in the constellations retained their relative position night after night, but there were a few bright dots that moved against the stars’ fixed backgrounds. As mentioned earlier, the Babylonians thought these strangely moving dots were gods, but the Greeks sought a scientific explanation as to why these celestial bodies demonstrated their backwards or retrograde motion. The ancient Greeks dubbed them wanderers for their behavior: it is from this term that their modern name of planets is derived.

As an example, Mars usually moves from the West to the East compared to the “fixed stars,” but occasionally changes direction, moving from East to West for a few weeks before returning to its more normal motion. Mars appears to loop, moving forward then backwards across the sky under successive observations. Babylonian and Greek astronomers observed that all of the planets known at the time (Mercury, Venus, Mars, Jupiter, and Saturn) undertake similar retrograde motions. However, in a geocentric system, if the planets moved in perfect circular orbits around Earth, their passage across the sky should have been regular and uniform, not looping and retrograde.

However, Heraclides’ “halfway-house” system, in which all planets except Earth revolved around the sun, provided an explanation for retrograde motion. Planets farther from the sun are moving more slowly in their orbits than those closer to the sun. When a faster planet overtakes Earth in its rotations, its motion against the stars, as seen from Earth, reverses. Then, as Earth swings past the planet in its orbit, the observed planet appears to resume its normal motion west to east. The looping motion was thus explained if planets orbited around the sun rather than around Earth.

Around the same year Heraclides died, the astronomer Aristarchus of Samos (c.310-c.230 BC) was born.

He not only improved the estimates of the length of the solar year, but he was the first to propose a totally heliocentric universe. The advantages of the heliocentric theory for Aristarchus were:

  1. It explained the variations in brilliance of Venus and Mars.
  2. It removed the question of whether Venus and Mercury were above or below the sun.
  3. Instead of conceiving of the entire heavens as rotating around Earth daily, the same appearance could be achieved via Earth’s rotation.
  4. Aristarchus figured out that the sun’s diameter was 6fl times as great as that of Earth, and it seemed logical to him that the smaller body would revolve around the larger.

A Retreat from Heliocentrism: The Dominance of Aristotle and Ptolemy

Despite Aristarchus’achievement, the heliocentric theory was tabled until Copernicus rediscovered it in the sixteenth century. This retreat from heliocentrism was partially because a moving Earth contradicted sensory experience. Part of the decline of heliocentric theory was also because of the growing authority of Aristotle’s (384-322 BC) geocentric (Earth—centered) cosmology and the accuracy of Ptolemy’s (AD c.90-c.168) astronomical tables.

Aristotle’s school in Athens, the Lyceum, was the world’s first polytechnic university, and the ideas taught there became extremely influential. His cosmological theory gained predominance in the ancient world, with lasting impact. Aristotle posited that the universe was geocentric, consisting of a system of nested spheres that revolved around Earth. The surrounding spheres contained the orbs of the moon, sun, the five known planets, and the sphere of fixed stars, all of which were contained in the primum mobile, the outermost sphere that set all the other spheres in motion.

Aristotle outlined some of his theories about the universe in his work De Caelo (On the Heavens). He believed the universe contained two distinct areas: a sublunar region and the eternal, or celestial, realm. The sublunar region of the universe was the area below the moon’s orbit, consisting of Earth surrounded by its elements of water, air, and fire. Earth, which was made of the heaviest earthly element, rested immovably in its natural place, at the center of the sublunar region. The watery sphere surrounded that of Earth, but the boundaries between the two were irregular, because the higher parts of the land projected above the oceans that surrounded our globe. The sphere of the air was next. Above it, but below the moon, was the sphere of fire, which was the lightest of the elements and the transition to the eternal realms of the planets. This sublunar realm was a realm of change and corruption, with each of the elements being constantly displaced from their spheres by the motions and influences passed down from the primum mobile and the sun.

The earthly region was in turn surrounded by the heavenly domain, beginning with the moon and extending upward to the celestial heaven, which was considered perfect and unchanging. It was composed not of the four terrestrial elements but rather of one special fifth element, the ether. Aristotle was convinced that the celestial ether was incorruptible and that it composed the unchangeable heavens. He posited that the planetary orbits were solid crystalline spheres made of the ether, to which the perfectly polished planetary bodies, also made of the ether, were firmly attached.

The ether, like earth, water, air, or fire, determined the movement of the object it composed. Aristotle believed the composition of a body decided its natural motion—the motion that it would undergo spontaneously without any external force. Natural motion could be linear or circular. In the case of the four terrestrial elements, their natural motion was either upwards or downwards from or to the center of Earth; fire and air were thought to rise naturally, as candle flames pointed upwards and air rose. Water and earth were thought to fall naturally, due to observations of rain, and gravity. Aristotle thought there was, in addition, another type of body whose natural motion was circular, nobler, and eternal. He concluded that bodies to which circular motion were natural were to be identified with the ether of which the heavenly bodies were composed.

In the second book of On the Heavens, Aristotle argued that the “shape of the heavens must be spherical” as the circle was the “primary shape in nature.” Once Aristotle had established that the shape of the planetary orbits were circular, he had to provide an explanation for the motion itself. Aristotle conceptualized God as an “unmoved mover” who functioned as an object of love and desire for the soul that animated the body of the outermost sphere, the primum mobile. The primum mobile rotated at an enormous speed every twenty-four hours and communicated this motion to the planetary orbital spheres.

The planets have their own motion opposite to that of the stellar sphere because they all have a westward diurnal motion with the stars, and all move gradually eastward among the stars until they return to approximately their original positions. In other words, the planets’ yearly orbital motion is west to east, but their daily motion is east to west. Saturn, the planet closest to the stellar sphere, had the greatest difficulty in overcoming its force and therefore required the longest time to complete its own revolution in the contrary direction. Because the moon was farthest from the primum mobile, it had the least force imposed upon it; it thus took the smallest amount of time to finish its orbit.

Aristotle thus ordered the planets according to their orbital times of revolution, known as the sidereal periods. Of the seven planets, the sun occupied the fourth position between the moon, the lowest planet, and Saturn, the highest planet. According to Aristotle, the sun’s middle position reinforced its status as the most fundamental and important of the planets. In the midst of the planets it could radiate light in all directions, essential because Aristotle believed that the generation and corruption, or becoming and passing-away, of all sublunar creatures and elements were dependent on the sun.

Though Aristotle’s Earth-centered cosmology explained why the planets moved, it did not explain where the planets would be at any given time. Aristotle’s universe of nested spheres and uniform circular orbits also did not provide an explanation for the retrograde or “looping” motion of the planets across the heavens. While Aristotle was adequate for popular texts and general explanations, astronomers wishing to calculate tables of planetary positions turned to Ptolemy’s Almagest, translated as his “Great Book” (AD 150).

To explain retrograde motion, Ptolemy used a geometric model involving two devices: the epicycle and the deferent. He assumed the planets move in a small circle, the epicycle, which in turn moves along a larger circle, the deferent. Hungarian-British author and philosopher Arthur Koestler (1905-1983) created an excellent analogy to visualize this motion. Imagine a ferris wheel, its center representing Earth, and its passenger carriage representing a planet like Mars. As the carriage (Mars) revolves around the center of the wheel (Earth), it demonstrates uniform circular motion. The big circular path it traces is the deferent.

Now imagine if the passenger carriage (Mars), instead of hanging down from the rim of the ferris wheel, rotates around the pivot from which it is suspended while the pivot revolves slowly with the big wheel. The carriage (Mars) is now tracing looping motions in the sky, and the circle it traces as it rotates around its pivot is called an epicycle.

By varying the size of the deferent, the speeds of the rotations of the epicycle and deferent, and by adding more epicycles, Ptolemy could, by a combination of circles, create curves that mimicked the actual path of planets across the sky. Ptolemy’s system dominated the ancient world because his naked eye observation of the stars was reasonably accurate; the tables in his Almagest were readily accessible; and his system of epicycles provided a model to predict planetary movement.

The Fall of the Roman Empire and the Medieval Revival of Aristotle

After the Western half of the Roman Empire fell in the fifth century AD, the disruption to continuous systems of government and education meant that a good deal of Greek and Roman astronomical knowledge was temporarily lost in Western Europe. Indeed, from the fifth century until the end of the ninth century, Earth and the firmament of the stars were thought by some church fathers such as St. Jerome and the monk Cosmas to be shaped like a holy tent or rectangular tabernacle. It was not until the end of the ninth century that the monk the Venerable Bede (673-735) stated that Earth was believed to be spherical again. The ascension of Pope Sylvester II (950-1003), a distinguished classical scholar, geometer, and astronomer, to the papal throne in AD 999 marked the beginning of the revival of Greek and Roman astronomical knowledge.

In the interim, classical knowledge continued to be preserved, although in a different context. Because the eastern half of the Roman Empire survived, Greek and Roman astronomy passed through Mesopotamia, Egypt, and Spain to the Muslim world. The zenith of the Arab empire was between the eighth and thirteenth centuries, the territory reaching from Spain to central Asia. Scholars such as Rhazes (al Razi, c.800) and Averroes (1126-1198) refined Ptolemy’s astronomical tables and Aristotle’s cosmological speculations. The Muslims established schools and universities in Mespotamia, Egypt, Spain, and Jerusalem, translating and preserving ancient Greek and Roman texts.

Because of the Crusades against the Arab world in the twelfth century, the ancient scientific knowledge that Muslim scholars had compiled was transferred west via southern Italy and Spain. Euclidean geometry, algebra and trigonometry, and Arabic numbers also made astronomical calculations easier and more accurate. Europeans eagerly adopted foreign instruments such as astrolabes, which contained star charts and navigational guides.

The recovery of Greek and Roman knowledge in Western Europe, as well as the rise of towns and trade routes with the Middle East in the twelfth century, led to the rise of universities in Paris, Bologna, and Oxford. Aristotle’s corpus of works that were taught at the ancient Lyceum were rediscovered and served as the basis of the university curriculum. His cosmological system therefore became predominant, and Ptolemy’s Almagest was also reinstated in astronomical studies. As a result, geocentrism defined astronomy and cosmology in the middle ages.

Nicolas Copernicus and the Rise of the Sun

Heliocentrism was thus largely forgotten, not to be revived again until the sixteenth century by the Polish astronomer Nicolas Copernicus (1473-1543) who “rediscovered” it in his study of Pythagorean mysticism, astronomy, and astrology. The date 1543, the year of the publication of Copernicus’s De Revolutionibus, or the Revolutions of the Heavenly Spheres, has traditionally been accepted as the beginning of the termination of medieval conceptions of the universe. The introduction of heliocentrism helped to shatter the distinction between the corrupt, earthly sublunar realm and the perfection of the heavens that had been established by Aristotle and popularized by Ptolemaic astronomy.

Why did Copernicus make the decision to support a sun-centered universe? As a student in Italy, he rediscovered the writings of Pythagoras. During the time Copernicus was at university, at the beginning of the Italian Renaissance, there was a revival and rediscovery of more Greek and Roman writings, particularly the works of Plato. The Italian Renaissance made many formerly “lost” writings available, including those that provided information about the Pythagorean brotherhood, as well as the heliocentric theories of Aristarchus.

Copernicus himself in the De Revolutionibus declared that the arguments for Earth’s movement around the sun revived ideas of the Pythagoreans, and in the sixteenth and seventeenth centuries his teaching was seen as a mathematical formalization and confirmation of Pythagorean cosmology. Copernicus realized that a heliocentric universe would solve the problems of retrograde motion without the need for epicycles. He also chose a heliocentric system for reasons that were purely aesthetic, presenting an impassioned encomium to the sun in his work: “In the center of all rests the sun. For who would place this lamp of a very beautiful temple in another or better place than this where from it can illuminate everything at the same time?”

However, many sixteenth-century astronomers had the following problems with Copernicus’ system:

  1. Copernicus postulated the orbits of the planets were circular, when in fact they would later be shown by Johannes Kepler to be elliptical. As Copernicus himself did very few astronomical observations, and in fact relied on the Ptolemaic tables, his system did not offer any improvement in accuracy in calculating planetary positions.
  2. Copernicus offered little physical explanation in the De Revolutionibus about how or why Earth and the other planets moved, as his system was conceived before Newton’s theory of gravity.
  3. If Earth indeed moved, then astronomers should have been able to detect a phenomenon called parallax with regard to the fixed stars. As Earth moves its orbit, our “point of view” should be changing with regard to the fixed stars, producing the optical illusion of a change in their position. In short, the stars should appear to moves lightly side-to-side as Earth revolves around the sun. Observable parallax thus is a proof of the rotation of Earth around the sun, but it usually is a phenomenon that can only be observed with a telescope. Since Copernicus published his findings before telescopes were invented in the early seventeenth century, parallax could not be observed. (Parallax was actually not observed using telescopic astronomy until 1838).
  4. The Copernican system was contrary to biblical accounts of a non-moving earth in the Old Testament. As many educated astronomers were members of different church orders, especially the Jesuit order, there was reluctance to embrace a theory that was contrary to religious doctrine.

The Tychonic Scheme: A Possible Solution?

Another challenge to heliocentrism came in the form of an astronomical system proposed by Danish astronomer Tycho Brahe (1546-1601; commonly referred to as Tycho rather than by his surname). Tycho had spent 20 years committed to systematic observation of planetary motion and created his own model of planetary motion. Tycho’s system was basically that of the ancient Greek Heraclides—the “halfway house” geocentric model in which the sun revolved around Earth, and all the other planets revolved around the sun.

For the Jesuits and other church authorities, Tycho’s system presented obvious advantages. It did not contradict scripture, it explained retrograde motion without the need for epicycles, it explained the absence of observed parallax as Earth did not move, and Tycho’s data was the most accurate of its day.

From Tycho to Kepler: The Elliptical Orbit

Johannes Kepler (1571-1630) was a German astronomer and, like Pythagoras before him, came to believe that mathematics reflected the fundamental structure of the universe. In contrast to Tycho, he also was a firm believer in heliocentrism and the Copernican system.

During a lecture in mathematics in which he was drawing a geometrical figure on the board, Kepler had an inspiration, a “eureka moment” in which he believed he was beginning to understand the secrets of creation. His idea was that the universe was built around certain symmetrical figures discovered by Pythagoras—the pyramid, the cube, the octahedron, the dodecahedron, and the icosahedron—which form its invisible skeleton.

Although his idea was completely false, it eventually would provide the inspiration for Kepler’s planetary laws and contribute to the birth of modern cosmology.

To verify his ideas empirically, Kepler needed the most precise data on planetary orbits available, and the keeper of that data was Tycho. Hence began a collaboration between Tycho, the empiricist and data-keeper, and Kepler, the theorist, which would bear fruit in Kepler’s planetary laws. In the midst of trying to prove the veracity of his cosmic cup of the five solids, Kepler concentrated on ratios of planetary orbits. While focusing on planetary orbits, he became interested in trying to create a model that would explain how the planets moved, yet also verify Tycho’s observational data.

As mentioned previously, Kepler was an affirmed believer in Copernicanism. However, Kepler soon realized that Copernicus’ circular orbits for the planets matched Tycho’s data very poorly. After Tycho died, Kepler inherited his post and his data, and he decided that he had to create a mathematical model explaining planetary motion.

Kepler began his investigation with the orbit of Mars, which has the most eccentric orbit and exhibits the most retrograde motion. Kepler rejected a circular orbit for Mars, much to his dismay, and then proposed a series of different curves, including an oval. Finally, almost by accident, he found that an elliptical orbit worked beautifully with Tycho’s data.

What is an ellipse? Basically, it is an elongated circle. Mathematically, an ellipse is the locus of points for which the sum of the distances from the foci (two fixed points) has a constant value.

Utilizing the rest of Tycho’s observations, Kepler found that the rest of the planets, including Earth, also moved in ellipses. His initial response to his discovery however, was not one of uniform delight. Why? An elliptical orbit deviated from the geometrical perfection of the circle, and because Kepler at heart believed that God was a divine geometer, ellipses seemed a little messy for the heavens.

Despite Kepler’s misgivings, ellipses worked for all the planetary orbits he tested, correlating where planets were in their orbits, including at aphelion (when they were farthest from the sun) and at perihelion (when they were closest to the sun.) Therefore, Kepler formulated his first planetary law: Planets move in elliptical orbits.

Kepler’s first law established where the planets would be, but not when they would be in a particular position. Ptolemy thought that the planets moved in their epicycles and deferents at constant angular speeds. However, Kepler found that Ptolemy’s conception of constant speeds did not correlate with his newly discovered elliptical orbits. The planets sped up and slowed down in different parts of their orbits.

At perihelion, the point where the planet was closest to the sun, the planet moved the fastest. The planet moved the slowest at aphelion. The ratio between the planetary speeds at perihelion and aphelion was in proportion to the ratio of solar distances.

His observations of planetary positions using Tycho’s data led Kepler to the realization that it was the sun itself that exerts some type of force on the planets. Although Kepler’s conception of this force was limited to the idea that the sun somehow pushes (as if with an invisible broomstick) the planets around, it was an early idea of gravity. Kepler’s ideas of gravitational attraction would later influence the Italian astronomer and physicist Galileo Galilei (1564-1642) who proved why objects on a spinning Earth would not fly off its surface. Kepler’s planetary laws also were used by Sir Isaac Newton to formulate his own law of gravity in 1687.

In 1609, in his publication The Starry Messenger, Galileo also observed with a telescope of his own design that Jupiter had its own moons, and that Venus had phases, indicating it revolved around the sun. Both of his observations provided support for a moving Earth. Although the pope officially denounced Copernicus’ system in 1616, and Galileo spent nine years under house arrest accused by the Catholic Church of heresy for supporting heliocentrism, by the end of the seventeenth century, heliocentrism was largely accepted by the scientific community in Western Europe. A sun-centered universe had become part of modern astronomy.

Modern Cultural Connections

Ancient cosmological thought continues to influence and shape cultural and religious belief across the globe. Some non-Western religious groups or indigenous peoples have long-developed scriptural or mythological cosmologies. Despite the universal scientific acceptance of the heliocentric model, some continue to reject it. Some Western religious groups who favor a scripture-based understanding of the universe actively promote a geocentric model.

Though the Catholic Church had long accepted the heliocentric model of the solar system, in 1992, Pope John Paul II (1920-2005) made an apology for the church’s treatment of Galileo and his work that bolstered the Copernican model.

Primary Source Connection

The Copernican Revolution changed Western cosmology. The universal scientific consensus is that Earth orbits the sun—a fact backed up by the modern laws of physics and substantial observational evidence. However, in the United States today about one-fifth of residents think the sun goes around Earth. The following article puts that statistic in its social context, citing lack of consistent science education as a predominant factor.

(Cornelia Dean. New York Times. August 30, 2005)

CHICAGO—When Jon D. Miller looks out across America, which he can almost do from his 18th-floor office at Northwestern University Medical School in Chicago, he sees a landscape of haves and have-nots—in terms not of money, but of knowledge.

Dr. Miller, 63, a political scientist who directs the Center for Biomedical Communications at the medical school, studies how much Americans know about science and what they think about it. His findings are not encouraging.

While scientific literacy has doubled over the past two decades, only 20 to 25 percent of Americans are “scientifically savvy and alert,” he said in an interview. Most of the rest “don’t have a clue.” At a time when science permeates debates on everything from global warming to stem cell research, he said, people’s inability to understand basic scientific concepts undermines their ability to take part in the democratic process.

Over the last three decades, Dr. Miller has regularly surveyed his fellow citizens for clients as diverse as the National Science Foundation, European government agencies and the Lance Armstrong Foundation. People who track Americans’ attitudes toward science routinely cite his deep knowledge and long track record.

“I think we should pay attention to him,” said Eugenie Scott, director of the National Center for Science Education, who cites Dr. Miller’s work in her efforts to advance the cause of evolution in the classroom. “We ignore public understanding of science at our peril.”

Rolf F. Lehming, who directs the science foundation’s surveys on understanding of science, calls him “absolutely authoritative.”

Dr. Miller’s data reveal some yawning gaps in basic knowledge. American adults in general do not understand what molecules are (other than that they are really small). Fewer than a third can identify DNA as a key to heredity. Only about 10 percent know what radiation is. One adult American in five thinks the Sun revolves around the Earth, an idea science had abandoned by the 17th century.

At one time, this kind of ignorance may not have meant much for the nation’s public life. Dr. Miller, who has delved into 18th-century records of New England town meetings, said that back then, it was enough “if you knew where the bridge should be built, if you knew where the fence should be built.”

“Even if you could not read and write, and most New England residents could not read or write,” he went on, “you could still be a pretty effective citizen.”

No more. “Acid rain, nuclear power, infectious diseases—the world is a little different,” he said.

It was the nuclear power issue that first got him interested in public knowledge of science, when he was a graduate student in the 1960’s. “The issue then was nuclear power,” he said. “I used to play tennis with some engineers who were very pro-nuclear, and I was dating a person who was very anti-nuclear. I started doing some reading and discovered that if you don’t know a little science it was hard to follow these debates. A lot of journalism would not make sense to you.”

Devising good tests to measure scientific knowledge is not simple. Questions about values and attitudes can be asked again and again over the years because they will be understood the same way by everyone who hears them; for example, Dr. Miller’s surveys regularly ask people whether they agree that science and technology make life change too fast (for years, about half of Americans have answered yes) or whether Americans depend too much on science and not enough on faith (ditto).

But assessing actual knowledge, over time, “is something of an art,” he said. He varies his questions, as topics come and go in the news, but devises the surveys so overall results can be compared from survey to survey, just as SAT scores can be compared even though questions on the test change.

For example, he said, in the era of nuclear tests he asked people whether they knew about strontium 90, a component of fallout. Today, he asks about topics like the workings of DNA in the cell because “if you don’t know what a cell is, you can’t make sense of stem cell research.”

Dr. Miller, who was raised in Portsmouth, Ohio, when it was a dying steel town, attributes much of the nation’s collective scientific ignorance to poor education, particularly in high schools. Many colleges require every student to take some science, but most Americans do not graduate from college. And science education in high school can be spotty, he said.

“Our best university graduates are world-class by any definition,” he said. “But the second half of our high school population—it’s an embarrassment. We have left behind a lot of people.”

He had firsthand experience with local school issues in the 1980’s, when he was a young father living in DeKalb, Ill., and teaching at Northern Illinois University. The local school board was considering closing his children’s school, and he attended some board meetings to get an idea of members’ reasoning. It turned out they were spending far more time on issues like the cost of football tickets than they were on the budget and other classroom matters. “It was shocking,” he said.

So he and some like-minded people ran successfully for the board and, once in office, tried to raise taxes to provide more money for the classroom. They initiated three referendums; all failed. Eventually, he gave up, and his family moved away.

“This country cannot finance good school systems on property taxes,” he said. “We don’t get the best people for teaching because we pay so little. For people in the sciences particularly, if you have some skill, the job market is so good that teaching is not competitive.”

Dr. Miller was recruited to Northwestern Medical School in 1999 by administrators who knew of his work and wanted him to study attitudes and knowledge of science in light of the huge changes expected from the genomic revolution.

He also has financing—and wears a yellow plastic bracelet—from the Lance Armstrong Foundation, for a project to research people’s knowledge of clinical trials. Many research organizations want to know what encourages people to participate in a trial and what discourages them. But Dr. Miller said, “It’s more interesting to ask if they know what a clinical trial is, do they know what a placebo is.”

The National Science Foundation is recasting its survey operations, so Dr. Miller is continuing surveys for other clients. One involves following people over time, tracing their knowledge and beliefs about science from childhood to adulthood, to track the way advantages and disadvantages in education are compounded over time and to test his theory that people don’t wait until they are adults to start forming opinions about the world.

Lately, people who advocate the teaching of evolution have been citing Dr. Miller’s ideas on what factors are correlated with adherence to creationism and rejection of Darwinian theories. In general, he says, these fundamentalist views are most common among people who are not well educated and who “work in jobs that are evaporating fast with competition around the world.”

But not everyone is happy when he says things like that. Every time he goes on the radio to talk about his findings, he said, “I get people sending me cards saying they will pray for me a lot.”