Geocentrism

As a frum astronomer, the questions I'm asked most often are about supposed discrepancies between Torah and science. These usually focus on either the age of the universe, or geocentrism vs. heliocentrism. (The third popular subject is the Rambam's Hilchot Kiddush HaChodesh, but that is another issue.)

Usually, the latter question is posed in terms of annual geocentrism: "You, Mr. Astronomer, claim that the Earth goes around the Sun, but Torah says that it is the other way around." But what is the source for the latter statement? If you look at any pre-Copernican text, Jewish or otherwise, indeed the Earth is (roughly) at the center and everything, including the Sun in its yearly cycle on the sky, is in motion around it. But this annual geocentrism is, in a sense, a corollary of diurnal geocentrism; the idea that the sky revolves around the Earth once daily. It is only for the latter geocentrism that one finds Biblical support. There we have texts where the Sun, Moon, and stars are rising and setting and standing still in the sky. So, goes the classical argument, it must be these bodies that are moving, right? After all, if the Earth were turning, we'd feel it, wouldn't we? We would be blown away by the strong winds. So the Earth must be fixed in place, and everything else moving. Q.E. D.

One of the nice things about classical physics is that it is intuitively obvious, unlike modern physics which, when it isn't being bizarre, is simply counterintuitive. After all, a soccer ball will, pace Newton's first law, come to a stop in the grass unless someone keeps kicking it. What is unrecognized is that the force of the kicks are required to overcome the opposing force of friction from the ground. Similarly with the sky. We see the Sun rise and set, and so conclude that it is the Sun that is moving. But, is it reasonable to expect the Torah to say that "the Earth rotated eastward until the Sun was hidden by the body of the Earth" every time it wanted to refer to "sunset"? No one speaks in such a way, not even the modern, educated person, who knows that the Earth rotates daily on its axis. Indeed, chazal clearly state that the Torah speaks in the language of man. So why is it seen to be a contradiction that modern science says that it is the Earth that is rotating once daily, and not the heavens?

If we hold that diurnal geocentrism is nothing more than linguistic convention, then what of annual geocentrism? To begin with, let us recognize that the scientific viewpoint has moved on considerably from the strict heliocentrism advocated by Copernicus and charactured by the opponents of modern science. Copernicus's tables for planetary positions were not that much better than those of his Ptolomaic contemporaries. The real revolutionary was not so much Copernicus, as Kepler. By substituting ellipses for the ancient dogma of strict circular motion, Kepler was able to predict planetary positions to much greater accuracy than anyone else had ever done. Within a year or so of his death, his tables permitted the first observation of a transit of Venus across the face of the Sun. The Keplerian model, however, is not strictly heliocentric. True, the Earth's orbit surrounds the Sun, but the Sun is at one focus of the ellipse, not at the center of the ellipse. Newton put Kepler's empirical laws on a physical basis by providing the explanation of why they hold. But if you compute the orbit of two bodies under Newtonian gravity you find that they both move about their mutual center of mass, which undermines the whole notion of heliocentrism. (Indeed, under most medieval solar-system models, the Earth wasn't at the center of the Sun's orbit for much the same reason that drove Kepler to his ellipses.)

Whatever the centrism, the concept is that the object in the center is fixed, and the other object moves around it. If both are moving, then neither can be considered fixed. In any case, we must ask, "Fixed with respect to what?" Newtonian dynamics permitted the concept of absolute space and time, so that at least in theory there was a reference with which to fix things, even if the Sun wasn't nailed down to it. Under Einsteinian relativity, there is no absolute reference frame. All motion is relative, and the choice of reference frames, a convention. A reference frame that makes the required equations particularly simple, or their solution straight forward, may be preferred on those grounds, but such a reference frame is not otherwise more or less "correct" than any other. For solar system problems, the center of mass of the whole system is one such frame, but that makes the Earth's motion barycentric rather than heliocentric. All that is left is a dispute about an otherwise arbitrary choice of coordinate system.