Most people today believe the heliocentric theory, that the earth is one of eight planets orbiting the sun. This has been the dominant cosmology for four centuries. However, there has been a geocentric movement among biblical creationists dating back at least to the 1980s. The term geocentric theory, or geocentrism, usually refers to the belief that the earth does not revolve around the sun each year but rather that the sun orbits the earth. However, there is a secondary meaning to geocentrism: that the earth also does not rotate on its axis each day. Like geocentrists of old, modern geocentrists are divided as to whether the earth rotates, but they are united in belief that the earth does not revolve. Modern geocentrists usually pursue two lines of arguments: scientific and biblical. Here I will examine both.
Before embarking on the discussion, I ought to acknowledge that the modern flat-earth movement has absorbed many geocentric arguments to the extent that flat-earthers often conflate the two, thinking that an argument for geocentric theory is an argument for the earth being flat. This is news to those who have been geocentrists for some time because they believe that the earth is spherical. I shall not discuss the flat-earth movement here; for a refutation of the flat-earth movement, please see my recent book on the subject. Thus, all flat-earthers must be geocentrists, but not all geocentrists are flat-earthers. For the purpose of my discussion here, I will use the term geocentrist to refer to geocentrists who are not flat-earthers.
We define motion as the time rate of change in position. Therefore, if an object does not change position, it is at rest and hence does not move.
We define motion as the time rate of change in position. Therefore, if an object does not change position, it is at rest and hence does not move. How do we detect motion? In a qualitative sense, this is very easy: we watch an object to see if it changes position. Or is it so easy? When riding in a vehicle, objects outside the vehicle, such as trees, change position. So, are those objects moving? Hardly. We readily recognize that it is we in the vehicle that are moving; objects outside the vehicle merely appear to be moving. But can we be certain about that? Anyone who has driven a manual transmission vehicle has very much been fooled by the following experience, as I have been several times. While stopped at a traffic signal with multiple lanes on a slope, I was momentarily distracted by something inside the car, such as tuning the radio, when in my peripheral vision I saw the car next to me begin slowly moving up the hill. My instinctive reaction was not that the other car was moving, but that my foot had slipped on the brake pedal, and I was rolling backward. Without hesitation, I stomped on the brake to avoid rolling into the car behind me. But every time I did this, a quick look around revealed that I was wrong: it was the car next to me that was moving. So, determining what is at rest and what is moving is not as easy as it first appears.
The problem is that we can’t observe or measure absolute motion. Rather, the only motion we can detect is relative motion. We generally assume that there is some absolute standard of rest against which all motion can be measured. In the case of an automobile, we usually assume that the road and the rest of the earth are at rest. But this does not mean that the earth truly is at rest. If we move with the earth, then the earth being at rest is an illusion. Still, though we tend to think that there must be some standard of rest outside of the earth, keep in mind that this is an assumption. Once one makes this assumption, then one must further assume what that standard of rest is. Some geocentrists make this point and conclude that since an absolute standard of rest must be assumed, then the question of what is moving and what is not moving is not a scientific one. I have no quarrel with this, because any system of thought must begin with certain assumptions, which are usually expressed as definitions, axioms, and postulates. Therefore, one can postulate about whether an absolute standard of rest exists and, if it does, what that standard of rest is. However, many of the same geocentrists who make this distinction often end up concluding that science proves geocentrism. But how can science prove that the earth is not moving if the question of what is moving and what is not moving is not a scientific one?
In many ancient cultures, and until four centuries ago, most people assumed that it was the sun that moved. However, most people today think that it is the earth’s rotation on its axis that accounts for what we see each day.
The sun rises every morning, moves across the sky, and sets each evening, only to repeat the same process the next day. That observation is plain enough, but is it the sun that is moving around the earth, or is it the earth that is turning each day? In many ancient cultures, and until four centuries ago, most people assumed that it was the sun that moved. However, most people today think that it is the earth’s rotation on its axis that accounts for what we see each day. Even in some ancient cultures, there were people who believed that the earth rotated daily. Either possibility can explain what we see. And since all we can observe is relative motion, in a very real sense, both possibilities are true. This is not post-modern thinking. Rather, it is an acknowledgment that motion is a far trickier thing than most people realize. Let me say it again: since all we can detect is relative motion, in a very real sense the sun moves across the sky each day, even though it is the earth’s rotation that accounts for that motion. But, again, the daily motion of the sun is not what most people think of when they think of geocentrism. Geocentrism usually refers to the question of whether each year the earth revolves around the sun or the sun orbits the earth.
The effect of the earth’s rotation is not restricted to the sun. Each night the stars appear to spin around the earth as does the sun during the day. The difference is that the two motions are slightly mismatched, with the stars taking nearly four minutes less to complete a rotation. This has the effect of the sun appearing to move with respect to the stars, taking one year for the sun to complete one circuit. We call the annual path of the sun through the stars the ecliptic. Most ancient societies assumed that it was the sun moving through the stars once a year that explained this, though there were a few exceptions, such as Aristarchus (310–230 BC). For the past four centuries, the dominant belief in the West has been that it is the earth’s orbital motion that explains the sun’s annual motion along the ecliptic. Again, either possibility explains what we see.
The situation gets trickier as we consider other celestial bodies. The moon moves through the stars once per month along a path inclined about five degrees to the ecliptic. Even in ancient times, this generally was interpreted as the moon orbiting the earth, as it is understood today (the exception would be flat-earthers). But the motion of the five naked-eye planets, Mercury, Venus, Mars, Jupiter, and Saturn, are much more difficult. These planets appear as bright stars that move along paths inclined to the ecliptic by only a few degrees. Their observed periods of motion with respect to the sun ranged from 116 days for Mercury to 780 days for Mars. We refer to these as the synodic periods of the planets. What makes planetary motion peculiar is that planets generally move west to east through the stars like the sun and moon do, but from time to time the planets halt their eastward motion and move backward, east to west, through the stars before resuming their normal eastward motion. For a long time, this retrograde motion defied a rational explanation.
The heliocentric theory explains retrograde motion in a simple and straightforward way. The planets closer to the sun move more quickly than planets farther away from the sun. Hence, the earth travels more quickly than the superior planets, planets that are farther from the sun than the earth. Moving more quickly and traveling on a smaller orbit, the earth overtakes superior planets each synodic period. From the perspective of the earth as this occurs, the superior planets appear to move backward. In the same way, as one car passes other cars, the passed cars appear to move backward. In a similar manner, inferior planets, planets orbiting closer to the sun than the earth does, go through retrograde motion as they overtake the earth each synodic period. Note, per the previous discussion, this is relative, not absolute motion, but how can we tell which is which?
There is no simple geocentric explanation for retrograde motion, but Claudius Ptolemy (AD 100–170) developed a geocentric explanation in the second century. Ptolemy had each planet move uniformly on a circle called an epicycle. The center of each planet’s epicycle, in turn, moved uniformly around the earth along a larger circle called the deferent. Both motions were counterclockwise as viewed from above the earth’s North Pole. As a planet moved along the side of its epicycle closest to the earth, the planet appeared to retrograde. By adjusting the sizes of each planet’s epicycle and deferent, as well as the speed on either, Ptolemy was able to produce a good fit to the observational data of each planet. However, Ptolemy had to add a few adjustments to make the fit perfect. First, Ptolemy added a second, smaller epicycle perpendicular to the first for each planet. This accounted for the slight inclination of planetary orbits to the ecliptic, which caused planets to bob up and down slightly with respect to the ecliptic as they moved. Second, Ptolemy had to move the earth slightly off center of each deferent and have the planets move on their epicycles at a uniform rate around a second off-center point called the equant. This was to account for the fact that planetary orbits are ellipses, not circles, as the Ptolemaic model assumed. Of course, Ptolemy didn’t know about elliptical orbits or that the orbits of the planets were inclined to the earth’s orbit around the sun. Because the earth’s orbit around the sun and the moon’s orbit around the earth are ellipses, Ptolemy had to add some small epicycles to the motions of the sun and moon.
There is no simple geocentric explanation for retrograde motion, but Claudius Ptolemy (AD 100–170) developed a geocentric explanation in the second century. Ptolemy had each planet move uniformly on a circle called an epicycle.
With these refinements, the Ptolemaic model did a good job of explaining and predicting planetary positions, so this model remained the dominant cosmology in the West for 15 centuries. However, over those centuries, small discrepancies between the predicted and observed positions of the planets arose. What to do? People found that additional small epicycles would bring predictions back into alignment with observations. But after 1500 years, the number of necessary refining epicycles exceeded a hundred. To some, the increasing complexity of the Ptolemaic model argued against it being true, causing them to cast about for other alternatives. One of those people was Nicolaus Copernicus (1473–1543). Shortly before his death, Copernicus published his work, but he had been discussing it for many years. Copernicus’ work attracted much attention. Many people think that Copernicus invented the heliocentric theory, but he didn’t, because the heliocentric model had been around since ancient times. Copernicus did, however, reintroduce the heliocentric model, and he gave arguments for the simplicity of the model compared to the Ptolemaic model. But his most important contribution was to use observational data to work out the true orbital periods (sidereal periods) and relative sizes of the planetary orbits within the heliocentric model. Apparently, no one had done this before, and this information was key in making further improvements to the heliocentric model.
Some of the most significant refinements were those of Johannes Kepler (1571–1630) about 75 years after the publication of Copernicus’ book. Kepler attempted to fit empirical data of the five naked-eye planets to the heliocentric model. It took Kepler a long time, but he finally found success when he scrapped circular orbits for elliptical orbits, resulting in his three laws of planetary motion. The next step was Isaac Newton (1642–1727), who derived Kepler’s three laws of planetary motion from his own three laws of motion, law of gravity, and the newly invented calculus. By the time of Newton, most people in the West had become convinced of heliocentrism, and Newton’s work seemed to put last nails in the coffin of geocentrism.
The transition from geocentrism to heliocentrism in the 17th century was not easy. An early proponent of Copernicus’ model was Galileo Galilei (1564–1642). While Galileo became convinced of heliocentrism due to its relative simplicity as compared to the geocentrism, it was his telescopic observations that buried the Ptolemaic model. Though often incorrectly attributed with the invention of the telescope, Galileo was the first to use the telescope for astronomical study. He found that Venus went through a series of phases similar to lunar phases. This required that Venus orbit the sun, but the Ptolemaic model did not allow for this. In the Ptolemaic model, Venus was restricted to a lower orbit around the earth than the sun’s orbit. Furthermore, Venus was constrained to be in the same general direction of the sun. This would allow for crescent phases, but not for half-lit or nearly fully lit phases. One could achieve half-lit and nearly fully lit phases by moving Venus to a higher orbit above the sun, but that would preclude crescent phases.
Galileo also overcame another objection to the heliocentric theory. Based upon Aristotelian theory of motion, it was thought that if the earth moved around the sun, the moon would be left behind in its orbit around the earth. However, Galileo saw four satellites, or moons, orbiting Jupiter with periods between 1.8 and 16.7 days. It was clear even in the Ptolemaic model that Jupiter moved, yet its four Galilean satellites, as they are called, had no difficulty keeping up. Hence, Aristotle’s ideas about motion were incorrect, and this objection to the earth’s movement was unfounded. Galileo went on to challenge other aspects of Aristotelian thinking.
This challenge to the Aristotelian/Ptolemaic worldview did not go unnoticed, and it inevitably led to what is known as the Galileo affair.
This challenge to the Aristotelian/Ptolemaic worldview did not go unnoticed, and it inevitably led to what is known as the Galileo affair. I will not fully discuss the Galileo affair here; instead, I will summarize it. Much of the Galileo affair has been misinterpreted as religion sticking its nose where it didn’t belong. For instance, it is usually reported that theologians immediately pounced upon Galileo’s heliocentric teachings as being contrary to Scripture. However, heliocentrism initially received welcome from theologians as an interesting topic of discussion. The complaints at first came from other scientists who thought that Galileo was attempting to upend science as then known. That was true. Since these scientists, like Galileo, mostly worked within the confines of church institutions, and with the blessing of church authorities, it was legitimate to raise these concerns within the church. Being early 17th century Italy, these officials were all Roman Catholic. A trial was convened, and a decision was handed down that forbad Galileo to continue teaching the heliocentric theory as truth. In many respects, this was nothing new, because those had been the conditions already in force. Remember that this squabble wasn’t about religion: it was a scientific controversy.
The controversy lay dormant for nearly two decades, upon which Galileo resurrected it by publishing a book on the subject. But it need not have erupted as it did. Galileo had to get permission from the Pope to publish the book, which he received. The only stipulation was that Galileo include discussion of the Pope’s position (geocentrism) in the book. Rather than writing the book in Latin, which was the custom of such treatises at the time, Galileo chose to write it in Italian to gain a much wider audience. His book was a dialogue between three individuals, one a geocentrist, one a heliocentrist, and the third a neutral moderator. But as it turned out, the moderator was hardly neutral. Furthermore, the arguments put forth by the geocentrist made him look foolish. Finally, Galileo named the geocentrist Simplico, which roughly translates into English as simpleton. Since the Pope had insisted that Galileo include the Pope’s position (geocentrism) in his book, this made the Pope look foolish. The Pope was rightly furious, and all support Galileo had enjoyed rapidly evaporated. A second trial was convened, but the fix was already in. The court found Galileo guilty of insubordination and heresy. Occurring in the aftermath of the Protestant Reformation, the latter charge could have carried very serious consequences. Fortunately, the church deemed that Galileo had not challenged theological dogma, but Galileo, now advanced in age, was required to recant and spent the rest of his life in house arrest. One consequence of the Galileo affair was that the teaching of heliocentrism was officially banned by the Roman Catholic Church, a ban that was only lifted in the late 20th century. But this was a ban in name only; within a few years even Roman Catholic institutions abandoned geocentrism in favor of heliocentrism. Also, keep in mind that prior to the ban instigated by Galileo’s actions, there had been no ban on teaching heliocentrism since Copernicus had reintroduced it 75 years earlier.
Keep in mind that the Galileo affair was a scientific squabble, not a battle between the Bible and science. Most of the refutation to Galileo came not from Scripture, but from Aristotle and Ptolemy.
Keep in mind that the Galileo affair was a scientific squabble, not a battle between the Bible and science. Most of the refutation to Galileo came not from Scripture, but from Aristotle and Ptolemy. Biblical references, such as Joshua 10:12–14, played a much smaller role, and they were interpreted in terms of geocentrism. And the affair might have had a very different outcome had Galileo behaved himself. Galileo’s insulting approach alienated those who had been on his side. Unfortunately, these facets of the Galileo affair are hardly discussed.
Geocentrists often respond that the Copernican model had more epicycles than the Ptolemaic model. This is technically true, but hardly significant. While Copernicus was able to eliminate the large epicycles required by the Ptolemaic model to explain retrograde motion, he still was stuck in the Aristotelian concept that motions of heavenly bodies must exhibit perfect, circular motion. Given that planetary orbits are ellipses with non-uniform motion, Copernicus used the available fix of small epicycles to match observations. Also keep in mind that the original Ptolemaic model contained relatively few epicycles. But by the time of Copernicus, the discrepancies were so large as to require many more epicycles, so the comparison between the original Ptolemaic model and the Copernican model is hardly fair. At any rate, Kepler’s refinement to Copernicus’ model eliminated the need of any epicycles.
Since the 17th century, it is doubtful that anyone has believed the Ptolemaic model. A few years prior to the Galileo affair, Tycho Brahe (1546–1601) introduced a compromise cosmology. In the Tychonic model, the other planets orbit the sun, but the sun in turn orbits the earth each year. The other planets are carried along with the sun as it orbits the earth. The Tychonic model amounts to a coordinate transformation from the sun being the center to the earth being the center. The Tychonic model is the preferred model of modern geocentrists.
While the heliocentric model became the dominant cosmology by the second half of the 17th century, this acceptance came without any direct proof. The phases of Venus disproved the Ptolemaic model and amount to evidence for the heliocentric model. But modern proponents of geocentric theory are keen to point out that the phases of Venus do not disprove the Tychonic model. Since the Tychonic model was published prior to the discovery that Venus exhibited phases, the observation that Venus has phases amounts to evidence for the Tychonic model. However, as we shall see, the Tychonic model did not predict other observations. It’s not that the Tychonic model cannot be altered to accommodate these things. But it is a fact that the Tychonic model didn’t predict these things and hence must be amended to account for them.
The first direct evidence for the heliocentric model came in 1727 when James Bradley (1693–1762) discovered aberration of starlight. Imagine standing in rain with no wind blowing. Holding the umbrella vertically will maximize protection against the falling rain. But what happens when one walks in the rain? To maintain proper protection from the rain, one must tilt the umbrella in the direction of travel. How much the umbrella must be tilted depends upon the walking speed and how fast the rain is falling. In similar manner, as the earth orbits the sun, one must tilt a telescope slightly in the direction of the motion of the earth. The amount of tilt depends upon the earth’s orbital speed and the speed of light. The tilt is about 20 arcseconds (this is the apparent diameter of a dime at 660 feet).
Stellar parallax is the apparent shift in position that stars undergo as we view them from different sides of the earth’s orbit. There are similarities between stellar parallax and aberration of starlight.
The second direct evidence for the heliocentric model came more than a century later when Friedrich Bessel (1784–1846) measured the parallax of the star 61 Cygni. Stellar parallax is the apparent shift in position that stars undergo as we view them from different sides of the earth’s orbit. There are similarities between stellar parallax and aberration of starlight. For a given star, the displacement throughout the year of both parallax and aberration of starlight are ellipses, with the shapes and orientations of the ellipses depending on the celestial coordinates of the star defined by the ecliptic. For stars at the ecliptic poles, the ellipses have zero eccentricity, while the ellipses for stars on the ecliptic collapse to lines. However, there are two differences. First, the two ellipses are at right angles to one another. The shift due to aberration is in the direction of the earth’s orbit, while the shift due to parallax is toward the sun. Second, the amplitude of the stellar aberration is 20 arcseconds for all stars, but the amplitude of stellar parallax depends upon how far a star is. The amount of stellar parallax is inversely proportional to distance. Consequently, nearby stars have the greatest parallax, and distant stars have the smallest parallax. The star Proxima Centauri has the greatest parallax – 0.77 arcseconds. Notice that this is only 4% of the amplitude of aberration of starlight, which is why it was more than a century after the discovery of aberration of starlight that the first parallax measurement was made. The parallaxes of other stars soon followed Bessel’s first measurement. Today we have quality measurements of the parallaxes of millions of stars.
In 1887, Herman Carl Vogel (1841–1907) and Julius Scheiner (1858–1913) were the first to measure annual periodic Doppler motion of stars. Doppler motion is the spectra of stars containing absorption lines due to various elements at specific, measurable wavelengths. If there is relative motion between the earth and the star, there will be a shift in the observed wavelength of absorption lines. Let λ0 be the wavelength when there is no relative motion (determined in the lab) and let λ be the observed wavelength. Then the relative velocity, v, will be
where c is the velocity of light. When relative velocity is toward the observer (earth), λ – λ0 will be negative, so the velocity will be negative. The observed amplitude of annual periodic Doppler motion of a star is 30 km/s (the orbital speed of the earth) times the cosine of ecliptic latitude of the star. The time variation of the Doppler shift goes as the sine of the angle between the difference in ecliptic longitude of the star and the sun. This is exactly what is expected if the earth orbits the sun once a year on an orbit with a 150 million kilometers radius.
A similar phenomenon occurs when considering precise timing of stellar events, such as the times of minimum light of eclipsing binaries (something I have done for more than four decades). The amplitude is a little more than eight minutes (the light-travel-time radius of the earth’s orbit) multiplied by the cosine of ecliptic latitude. The time variation goes as the cosine of the angle between the difference in ecliptic longitude of the star and the sun.
Historically, the earth’s rotation has been less controversial than the earth’s revolution. Prior to wide acceptance of the heliocentric theory, there were many people who didn’t think that the earth orbited the sun but believed that the earth rotated each day (even some geocentrists today think that the earth rotates).
Historically, the earth’s rotation has been less controversial than the earth’s revolution. Prior to wide acceptance of the heliocentric theory, there were many people who didn’t think that the earth orbited the sun but believed that the earth rotated each day (even some geocentrists today think that the earth rotates). It is almost inconceivable that one could believe that the earth orbits the sun but also that the earth does not rotate on its axis, so most people readily accepted the earth’s rotation once they came to believe the heliocentric theory. Consequently, the earth’s rotation was widely accepted long before there was direct evidence for it. In 1851, Léon Foucault (1819–1868) provided the first direct evidence for the earth’s rotation. Foucault constructed a simple pendulum consisting of a large mass suspended by a long wire. The support at the top of the wire was free to spin in a horizontal plane. When the pendulum was set in motion swinging in a vertical plane, there were no external torques, so the plane of swing remained fixed. However, the earth’s rotation carried the earth around the plane of swing. Hence, to an observer on the earth, the plane of oscillation precessed clockwise in the Northern Hemisphere (counterclockwise in the Southern Hemisphere). The period of precession was the sidereal day (23hr 56m 04s) divided by the sine of the latitude. Foucault consistently found results matching the prediction of the earth’s rotation. This experiment has been successfully conducted many times since. Though there is other indirect evidence for the earth’s rotation, the Foucault pendulum remains the best direct evidence for it.
In 1852, a year after he demonstrated his pendulum, Foucault invented and named the gyroscope. There had been similar devices previously, but Foucault’s design is how we know the gyroscope today. Foucault’s motivation was to test whether a spinning gyroscope also appears to precess as the earth rotates. Indeed, a spinning gyroscope does this, though, because of so many other effects, much more care must be taken than with the Foucault pendulum.
Given the abundant evidence that the earth is both rotating and revolving, how is that some people today believe in the geocentric model? In a subsequent article I will discuss the rise of the modern geocentric movement.