The Wilson effect
We present two short extracts about Alexander Wilson's observation of sun spots, the first from a late 19th century book, the second from a 21st century book.
1. Extract from: Agnes Mary Clerke, A popular history of astronomy during the nineteenth century (1887), 68-69.
... in November, 1769, a spot of extraordinary size engaged the attention of Alexander Wilson, professor of astronomy in the University of Glasgow. He watched it day by day, and to good purpose. As the great globe slowly revolved, carrying the spot towards its western edge, he was struck with the gradual contraction and final disappearance of the penumbra on the side next the centre of the disc; and when on the 6th of December the same spot re-emerged on the eastern limb, he perceived, as he had anticipated, that the shady zone was now deficient on the opposite side, and resumed its original completeness as it returned to a central position. In other spots subsequently examined by him, similar perspective effects were visible, and he proved in 1774, by strict geometrical reasoning, that they could only arise in vast photospheric excavations. It was not, indeed, the first time that such a view had been suggested. Father Scheiner's later observations plainly foreshadowed it; a conjecture to the same effect was emitted by Leonard Rost of Nuremburg early in the eighteenth century; both by La Hire in 1703 and by J Cassini in 1719 spots had been seen as notches on the solar limb; while in 1770 Pastor Schülen of Essingen, from the careful study of phenomena similar to those noted by Wilson, concluded their depressed nature. Modern observations, nevertheless, prove those phenomena to be by no means universally present.
Wilson's general theory of the sun was avowedly tentative. It took the modest form of an interrogatory. "Is it not reasonable to think," he asks, "that the great and stupendous body of the sun is made up of two kinds of matter, very different in their qualities; that by far the greater part is solid and dark, and that this immense and dark globe is encompassed with a thin covering of that resplendent substance from which the sun would seem to derive the whole of his vivifying heat and energy?" He further suggests that the excavations or spots may be occasioned "by the working of some sort of elastic vapour which is generated within the dark globe," and that the luminous matter, being in some degree fluid, and being acted upon by gravity, tends to flow down and cover the nucleus.
2. Extract from: J M Vaquero and M Vázquez, The Wilson Effect, The Sun Recorded Through History (Springer, Dordrecht-Heidelberg-London-New York, 2009), 153-154
Alexander Wilson (1714-1786) was a professor of practical astronomy at the University of Glasgow. He used a Gregorian telescope, with a 26 inch focus, which magnified 112 times. On 23 November 1769 he observed that :
... the umbra appeared much contracted on that part which lay towards the centre of the disc, while the other parts of it remained nearly of their former dimensions.He noticed that it was the opposite of what was expected from a pure effect of perspective. Fortunately the sunspot was long-lived and reappeared with nearly the same size on the eastern limb, observing the same effect but now on the eastern side of the penumbra.
He then suggested that the central part or nucleus was beneath the level of the Sun's spherical surface. In his own words, it was a vast excavation in the luminous matter of the Sun.
He proudly remarked that except for a few conclusions concerning the rotation of the Sun around its axis, and the inclination of its axis to the plane of the ecliptic, all that had been investigated before him on sunspots was a matter of conjecture. His sunspot model consisted of a vast cavernous opening, having the nucleus (umbra) at the bottom, and the penumbra forming its sloping sides.
In the French Memoires of 1770, Joseph Lalande (1732-1807) criticised the hypothesis of Wilson arguing that only a few spots, the roundish ones, behave the way Wilson proposed. He presented an alternative sunspot model:
... that the spots as phenomena arise from dark bodies like rocks, which by an alternate flux and reflux of the liquid igneous matter of the Sun, sometimes raise their heads above the general surface. That part of the opaque rock, which at any time thus stands above, gives the appearance of the umbra, whilst those parts, which in each lie only a little under the igneous matter, appear to us as the surrounding penumbra.Wilson  reacted rather vigorously criticising the adequacy of the old sunspot observations of J Picard and P La Hire used by Lalande for this specific purpose and stressing the need to use optical arguments. He remarked:
No doubts ought to arise of the spots being themselves what direct observation declares them, namely, excavations in the sun, as actually demonstrated by competent observations.With this aim in mind Wilson modelled a globe and the spots upon it according their proper dimensions. Then put it in a wooden frame and:
... viewed it afar off when set upon a stand, whilst the globe was turned slowly round, and subtended an angle at the telescope equal to the apparent diameter of the Sun. By an object glass micrometer I then took the distances from the limb when the farthest penumbra of different spots vanished, as also the distances of the nuclei just when disappearing.In summary, a modern experimental procedure of simulation of a phenomenon under study.
- A Wilson, Observations on the solar spots, Philosophical Transactions of the Royal Society of London 64 (1774), 1-30.
- A Wilson, An Answer to the Objections Stated by M De la Lande, in the Memoirs of the French Academy for the Year 1776, against the Solar Spots Being Excavations in the Luminous Matter of the Sun, Together with a Short Examination of the Views Entertained by Him upon That Subject, Philosophical Transactions of the Royal Society of London 73 (1783), 144-168.
Last Updated January 2021