Does the earth move? That simple question has been the subject of more than 2,000 years of philosophical, theological, and scientific dispute. The Ptolemaic theory of planetary movement asserted that the earth is fixed and immovable at the center of the universe, as the bible says:
Yea, the world is established; it shall never be moved.Psalm 93:1, Revised Standard Version
Yea, the world is established, it shall never be moved.Psalm 96:10, RSV
Thou didst set the earth on its foundations, so that it should never be shaken.Psalm 104:5, RSV
A Greek astronomer by the name of Aristarchus described a heliocentric (sun-centered) model of the universe in the early 200s BCE. He first estimated the size and distance of the moon and then used those results to estimate the size and distance of the sun. His calculations told him that the sun is about seven times larger than the earth. He made these estimates without the benefit of telescopes or spacecraft or even a value of pi. But from his findings he rightly concluded that it would be ridiculous to assume that the sun revolves around the earth since the sun is much larger than the earth.
This is a very interesting conclusion as it implies that Aristarchus was assuming that if one body of mass is orbiting another, then the larger mass must be the one that is stationary and the smaller mass must be the one that is moving. Newton’s universal law of gravitation made that view explicit, but the reasoning of Aristarchus shows that it was implicit even in the earliest thinking about the motion of the planets. Aristarchus might not have been able to express his thoughts about this dynamic in words, but he must have been imagining that the larger body exerts a force on the smaller body. It is the concept of a centrally directed force that was the key to Kepler’s insights about planetary motion, and this concept was clearly nascent in the thinking of Aristarchus.
The radius of the sun is actually about 109 times larger than that of the earth, not seven times. Aristarchus’s calculations were wrong– chiefly because it is very difficult to measure the angle of incidence of light on the surface of the moon with nothing more than the human eye. But Aristarchus was right to conclude that it must be the sun that is stationary, not the earth.
If the earth moves then it should be possible to see nearby stars shift in relative position as compared to more distant stars as the earth moves through its orbit. This phenomenon is known as parallax.
Parallax of the stars was not observed by ancient astronomers. But that’s not surprising, given that the stars are vastly farther away from earth than ancient astronomers imagined. Parallax can in fact be observed with present day ground based telescopes. But no such technology was available to ancient astronomers. As a result the geocentric (earth-centered) model gained broad acceptance.
There were other arguments against the motion of the earth too. If the earth were moving through space wouldn’t we feel a wind? But we don’t feel a wind. Therefore we must not be moving. If the earth were spinning on an axis we would be feeling a constant dizziness. And we don’t feel dizzy. So we must not be moving. All of these various observations led people to believe that the earth is immobile.
The flat earth model has generally been associated with the hypothesis of an immovable earth. One can imagine a flat disk-shaped earth that moves, and one can imagine a spherical earth that is fixed and immobile. So it isn’t necessarily the case that if the earth is flat it must be immobile, or that if it is spherical it must move.
The heliocentric model puts the sun at the center of our solar system, and the earth and all of the other planets in orbit around the sun. That, in fact, is why it is called a “solar” system. Is there any evidence for the heliocentric model?
Absolutely, there is. In fact the motion of the earth has been measured.
In 1838 F.W. Bessel reported that the star 61 Cygni has a parallax of 0.314 arcseconds. That puts the star at a distance of 3.2 parsecs from earth, or roughly 10.4 light years away. So Bessel proved that parallax is real and that it can indeed be used to measure the distances to the stars.
The Hipparcos satellite was launched by the European Space Agency in the 1990s to measure the positions and parallaxes of about 118,000 stars with very high precision. The satellite orbited earth, and so it was able to take advantage of the diameter of the earth’s orbit for its observation of parallax.
In the 1720s James Bradley and Samuel Molyneux made the first successful measurement of stellar aberration, a phenomenon that causes stars to appear to be in a different location to an observer on earth than it would to an observer on the sun. Their efforts resulted in a calculated velocity of the earth as just short of 30 kilometers per second.
The earth does in fact move. We have had proof of that fact for at least 2300 years. The earth revolves around the sun with a period of one year. Because the earth is about 93 million miles from the sun the earth’s orbital velocity is about 66,660 miles per hour, or about 18 miles per second. That equates to about 30 kilometers per second.
But the earth’s movement is more complicated than that. Our solar system revolves around the center of the galaxy. The solar system is about 28,000 light years from the center of the galaxy and it rotates about the galactic center once about every 230 million years. That works out to about 230 kilometers per second, or roughly 7.6 times as fast as the earth is orbiting around the sun.
And in addition our galaxy is moving. The universe is expanding, and our galaxy is moving in the direction of the galaxy Andromeda. Beyond that, our galaxy is one of several in a cluster of galaxies known as the Local Group that is moving collectively in the direction of Virgo at a rate of about 410 kilometers per second. The earth moves, our solar system moves, our galaxy moves, and in fact everything in the universe is moving.