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Galileo Galilei, The Sidereal Messenger

In this passage from The Sidereal Messenger (1610), Galileo Galilei recounts his experiments to build a telescope and his observations of the moon. By discovering the irregularity of the moon’s surface, Galileo disproved a central tenet of medieval cosmography: that the heavens were composed of perfect, unblemished spheres essentially different from the base matter of earth.

About ten months ago a report reached my ears that a Dutchman had constructed a telescope, by the aid of which visible objects, although at a great distance from the eye of the observer, were seen distinctly as if near. . . . A few days after, I received confirmation of the report in a letter written from Paris . . . , which finally determined me to give myself up first to inquire into the principle of the telescope, and then to consider the means by which I might compass [achieve] the invention of a similar instrument, which a little while after I succeeded in doing, through deep study of the theory of refraction; and I prepared a tube, at first of lead, in the ends of which I fitted two glass lenses, both plane on one side, but on the other side one spherically convex, and the other concave. . . . At length, by sparing neither labour nor expense, I succeeded in constructing for myself an instrument so superior that objects seen through it appear magnified nearly a thousand times, and more than thirty times nearer than if viewed by the natural powers of sight alone. . . .

Let me speak first of the surface of the moon, which is turned towards us. For the sake of being understood more easily, I distinguish two parts in it, which I call respectively the brighter and the darker. The brighter part seems to surround and pervade the whole hemisphere, but the darker part, like a sort of cloud, discolours the moon’s surface and makes it appear covered with spots. Now these spots . . . are plain to every one, and every age has seen them, wherefore I shall call them great or ancient spots, to distinguish them from other spots, smaller in size, but so thickly scattered that they sprinkle the whole surface of the moon, but especially the brighter portion of it. These spots have never been observed by any one before me, and from my observations of them, often repeated, I have been led to that opinion which I have expressed, namely, that I feel sure that the surface of the moon is not perfectly smooth, free from inequalities and exactly spherical, as a large school of philosophers considers with regard to the moon and the other heavenly bodies, but that, on the contrary, it is full of inequalities, uneven, full of hollows and protuberances, just like the surface of the earth itself, which is varied everywhere by lofty mountains and deep valleys.

EVALUATE THE EVIDENCE

Source: Galileo Galilei, The Sidereal Messenger (London: Rivingtons, 1880), pp. 10–11, 14–15.