We know it moves but can you prove it?

Over at Built on Facts Matt Springer is continuing his perusal of Neal Stephenson’s Quicksilver and chose to comment on the episode of Robert Hooke’s attempt to measure stellar parallax, if you don’t know what this is go read Matt’s post as he explains it well with diagrams. However his, and I presume he is paraphrasing Stephenson, historical explanation for Hooke’s reason for this attempt is somewhat wrong. He writes: One nearly throwaway vignette involves Robert Hooke attempting to empirically resolve a debate concerning the nature of stars, roughly, are they all sort of pasted on a crystal dome not so far beyond the edge of the soar system or are they independent sources of light scattered throughout deep space very far beyond the planets? Hooke’s adversary argues against the latter possibility (though not we take it for granted) in what’s pretty much the following way:

In fact Hooke was trying to measure parallax in order to provide the much-needed empirical proof of Copernicus’ hypothesis that the earth orbits the sun and not as believed vice versa. If Copernicus was right then stars viewed at opposite ends of the earths annual orbit should, and in fact do, display a measurable parallax. This was obvious to Copernicus and all of the astronomers who followed him and the attempts to measure that parallax in the 250 or so years following the publication of De revolutionibus form a fascinating chapter in the history of astronomy.

The first to attempt to detect stellar parallax was naturally Tycho Brahe the greatest of the naked eye, pre-telescopic observational astronomers but what Tycho couldn’t know is that stellar parallax is so incredibly small, because the stars are so incredibly far away, that his attempts were doomed to failure before he had even started. New hope appeared with the introduction of the telescope into astronomy in 1609 however although Galileo discussed an interesting approach to parallax measurement using double stars he himself never tried it. The first to make a serious attempt at stellar parallax measurement was in fact Robert Hooke. Now what on paper seems like a fairly simple exercise is in fact incredibly difficult because of a whole collection of complicated factors. One of the biggest problems is atmospheric refractions, that is the fact that the light rays emitted by the stars get bent by the earths atmosphere and the amount to which they get bent, or refracted, varies with the angle above the horizon with which the star is viewed. Even knowing this does not really help because in Hooke’s day and age nobody actually knew how much the rays were bent. However if one views the stars using a so-called zenith instrument, which is one where the observer view the heaven directly perpendicular to the earths surface, i.e. directly overhead, then there is no refraction at all. This is exactly what Hooke tried to do constructing a zenith telescope by cutting a hole in his roof into which he placed an objective lens under which he constructed a wooden tube down through the floors of his house to a pivoted eyepiece under which he laid on his back. This eyepiece was placed into the perpendicular with a plumb line. Using this instrument Hooke could measure the position of Gamma Draconis the “zenith star” over London in terms of how far it was from the true zenith. If over the period of one year it changed its position relative to Hooke’s zenith then Hooke would have succeeded in detecting parallax. In reality Hooke’s whole set up was far too ramshackle and Heath Robinson that he would ever have achieved the accuracy necessary to detect stellar parallax and in fact he in typical manner, he spent his whole life springing from one unfinished project to another, gave up his attempt after only four observation however claiming that he had in fact detected stellar parallax. His critics where not slow in pointing out that he had made far too few observation to be able to claim anything at all.

In the following century James Bradley repeated Hooke’s experiment but this time on a much more professional basis. Using two specially constructed high precision telescopes attached to the chimneys of his own and Samuel Molyneux’ houses he observed Gamma Draconis several times a night every clear night for a year in 1725. Bradley failed to detect any parallax but he did detect a consistent change over the year in the position of the star; in fact it described a small ellipse in the heavens over this period. Bradley knew that this was not parallax because the changes in position were in the wrong directions, when according to the parallax theory the star should have moved south it moved north and vice versa. Bradley could not believe his own results and spent another year meticulously repeating his observations; the result was the same. What Bradley had in fact discovered was stellar aberration that is a distortion of the light rays coming from a star caused by the not inconsiderable speed of the earth in its annual orbit. The ellipse that he had observed was in fact an image of the earths orbit around the sun. Here we have the refutation of a myth that is commonly perpetrated in most accounts of the history of the heliocentric hypothesis. It is almost always claimed that the first empirical proof of the earth’s annual journey around the sun was made in 1838 with the actual discovery of stellar parallax by Bessel. However Bradley had already delivered the necessary empirical proof through his discovery of stellar aberration more than a century earlier.

Bradley never did succeed in measuring stellar parallax although he did make several other important discoveries and in the process raised the level of accuracy of astronomical observation to new height. Numerous other attempts were made to measure stellar parallax in the 18th and early nineteenth centuries and although several observers claimed to have achieved what had become the prime goal in observational astronomy all of their claims were rejected as unfounded by the astronomical community. As I have already written above the laurels for the first discovery of stellar parallax went to Bessel in 1838 but even this is not strictly true. The first person to actually measure stellar parallax was the Scottish astronomer Thomas Henderson who measured the parallax of Alpha Centauri in South Africa in 1833 so why does Bessel get the acknowledgement. The answer is quite simple Bessel was the first to publish. Altogether three astronomers Henderson, the German Bessel and the Dane Struve all succeeded in measuring stellar parallax within a period of less than five years. Struve had announced preliminary results of his own attempts at detecting the parallax of Vega to his friend and rival Bessel in 1837 who was thus inspired to make his own attempt on Cygni 61. Bessel who had the better equipment won the race and published first followed latter by Struve but what of Henderson. As already mentioned Henderson had made his measurements in 1833 so why didn’t he publish. As noted earlier the detection of parallax in made complex by a whole series of factors and when the observer has obtained his raw observation data he then has to calculate away all the other factors such as refraction, aberration, nutation etc. that cause apparent movement in the star observed. Henderson brought his raw data back from South Africa to Scotland in 1833 and it was some time before he was able to make the necessary calculations. Having done so he did not trust his own figures as his observations had been made with an inferior instrument. Rather than rushing into print and suffering the fate of all the earlier discoverers of stellar parallax he waited until his former assistant in South Africa could repeat his observation with a better instrument. By the time he had got the new figures and performed the necessary calculations Bessel had already published. Henderson published his results 171 years ago today on 9th January 1839.

All of the above is taken from Alan Hirshfeld’s excellent book Parallax: The Race to Measure the Cosmos which I recommend to anybody who wants to read a thrilling account of the trails and tribulations of empirical science at its best.


Filed under History of Astronomy, History of Optics, History of science, Myths of Science

7 responses to “We know it moves but can you prove it?

  1. Pingback: Giant’s Shoulders « The Renaissance Mathematicus

  2. Tsk tsk, and mentioning Bradley without mentioning Samuel Molyneux? It seems that the main reason Bradley gets the credit for discovering the aberration of light is because Molyneux died too soon. Although Bradley is the one who actually realized what their data represented, I suspect that if Monlyneux had survived to that point, they’d both be listed as the discoveries.

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