AIRY'S FAILURE RECONSIDERED Walter van der Kamp As regards the science of astronomy: at the beginning of the eighteenth century the heliocentric theory was generally considered self- evident. Only an empirical demonstration that would validate the theoretical accomplishments of Kepler (1571-1630) and Newton (1642- 1727) was lacking. Standard astronomical history still holds and teaches that it was James Bradly (1692-1762) who found “the first experimental proof that the earth has a yearly motion, and that Copernicus was right.” Careful analysis of the relevant data, however, shows that this is not true at all, but that on the contrary, Bradley's so-called ”aberration of star light” gave the first experimental disproof of the heliocentric hypothesis. To recall to mind the bare facts: in December 1725 James Bradley and Samuel Molyneux began a prolonged observation of the star Gamma Draconis, which passed almost vertically overhead at their location. For their observations they fixed a telescope to a chimney stack of the Molyneux house in the hope of detecting the eagerly sought-after parallax, thus proving at long last the Copernican theory. Substantially correct, but simply stated, the matter is this: if indeed the earth moves in its orbit around the sun, then the place from which we observe the stars will be continually changing. Therefore, we shall see a nearby star as moving in a small circle against a background of more distant stars (see Figure 1). What is more: looking at such a nearby star A from point M in March, and point S in September, and knowing the distance SM to be about 3x108 km, we can by means of triangulation obtain the distance to A. Now Bradley found that Gamma Draconis indeed does describe a small circle with a radius of 20.5 seconds of arc (20”.5). The problem facing him was how to explain this phenomenon. Did it indeed result from the earth's revolution about the sun, and hence relative to the array of fixed stars? That is, did it show the parallax he had hoped to find or was the motion caused by the sun and stars circling with respect to an earth “at rest?” Bradley was forced to opt for the first alternative, but then had to reject it; for Gamma Draconis did not circle against the backdrop of stars, but all the stars joined in the motion which would imply that they were all at the same distance from earth. In other words, to accept the phenomenon as a parallax would mean re-introducing the discarded medieval concept of a Stellatum, a gigantic shell of stars centered on the sun which revolves about us. Since this was considered to be impossible, another interpretation of the observational facts had to be found. The circlets were decidedly not offering parallaxes, but what, then, did cause them? After pondering the problem for a time, so the story goes, Bradley invented the correct interpretation in 1728 during a sailing trip on the Thames. In doing so, he thought he had solidly established the truth of the Copernican-Newtonian synthesis by means of what he called the ”aberration of starlight.” Raindrops and stovepipes The common misconstruction often given is that of a man walking in the rain with a straight stovepipe. “If the stovepipe is held vertically,” thus a well-known textbook, “and if the raindrops are assumed to fall vertically, they will fall through the length of the pipe only if the man is standing still. If he walks forward, he must tilt the pipe slightly forward, so that drops entering the top will fall out of the bottom without being swept up by the approaching inside wall of the pipe…. Similarly, because of the earth's orbital motion, if starlight is to pass through the length of a telescope, the telescope must be tilted slightly forward in the direction of the earth's orbital motion.” The speed of light is about 10,000 times that of the earth. Hence the angle through which the telescope will have to be tilted forward, if Bradley's explanation fits the facts, will be 20”.5. That tallies with the angle he observed, and thereby we are forced to conclude that Copernicus has the right sow by the ear! But are we? Well, not totally! Logically considered this conclusion uses the invalid theoretical syllogism, the modus ponendo ponens. If situation P is the case, we agree, then we shall observe the phenomenon Q. Now, indeed, we observe Q. Does it therefore follow that P is the factual state of affairs? By no means necessarily, for Q may be caused by a variety of other circumstances. As one of my textbooks of logic remarks: “We shall have frequent occasions to call the reader's attention to this fallacy. It is sometimes committed by eminent men of science, who fail to distinguish between necessary and probable inferences, or who disregard the distinction between demonstrating a proposition and verifying it.” The long and the short of it is that this heliocentric corollary does not bind us until it has been duly verified. That verification, logically unbeatable, was suggested by...