Behold us, then, at work. You are perfectly tranquil as to the result; for you are persuaded beforehand that the sun _must_ be farther from us in the cold season than in the hot. You regard this as a self-evident truth, like an axiom of Euclid's. But Nature is a great magician; she contrives the most dramatic surprises for the mind which takes the trouble to interrogate her in all simplicity and without dogmatic pretensions. What a _coup-de-théâtre_ it was for the observer who first established experimentally that the apparent diameter of the sun is greater in winter than in summer--that we are _nearer_ the sun in the cold season, than in the hot! * * * * * On more closely examining a result apparently so paradoxical, man discovered that the angle which subtracts the sun, as seen from the earth,--the _visual angle_ which gives the sun's apparent diameter,--varies necessarily throughout the year. Thus, the semi-diameter, or radius, which on the 24th of June equals 15' 45", will, a month later, have increased one second (15' 46"); on the 2d of August will equal 15' 47"; on the 2d September, 15' 53", and so on. We put the exact measurements before the reader in a tabulated form:-- LENGTH OF THE SUN'S RADIUS. On January 21, 16' 16" " February 25, 16' 10" " March 31, 16' 1" " April 30, 15' 53" " May 30, 15' 47" " June 24, minimum, 15' 45" " July 24, 15' 46" " August 3, 15' 47" " September 2, 15' 53" " October 2, 16' 1" " November 6, 16' 10" " December 21, maximum, 16' 17" We do not trouble the reader with the fractions of a second, which indicate the quantity of the apparent increase of the radius from the end of June to the end of December, and its apparent decrease from the beginning of January to the end of June. * * * * * A glance at the above figures shows that the mean of the apparent diameters, all measured at the moment of the sun's passing the meridian, is about half a degree, or 30'; and that--which is sufficiently curious--720 of these mean suns, set one against another, would be required to fill up the contour of a great circle of the celestial sphere. Is it this fact which suggested the idea of dividing the circle into 720/2 = 360°? * * * * *