of the common people, where a practised observer was sure there was no star visible an hour or two before, was he to conclude that it had flared up as if it were on fire, and that it would go out as the fire died down? Or, if he saw a star brightening, paling, going out, and brightening again every three or four days, or weeks or months, or every three or four years, was he to infer that dark bodies of vast size were thrusting themselves between that distant sun and our eyes, eclipsing it, in fact; or that immense reaches of unlighted space, or dark regions on its surface, were turned for a time towards us, as it revolved on its axis? Dark spots on a sunny star's surface and a rotation more or less rapid were the causes accepted by Herschel from previous astronomers for this change of brightness in what are called changing or variable stars. He examined seven that were then known. Their periods were 3, 5, 6, 7, 331, 394, and 497 days.[1] He felt, however, that his views were discredited by the sudden bound from 7 days to 331. Unless a star were found bridging the gulf between these two, he would not have had confidence to give his theory to the world. But the star α Herculis seemed to him to bridge the gap, and satisfy the theory. Its period was found to be about 60 days. These and other changes on the face of the heavens, known for many years and registered in books, formed Herschel's prelude to the work he had set his heart on, The Construction of the Heavens. That they are a building, a wonderful temple consecrated to Almighty Power and Wisdom, he never doubted. To discover the plan
- ↑ Phil. Trans. (1796), pp. 455-56. Professor Holden gives the numbers as 3, 5, 6, 7, 334, 404, and 494 (p. 133).
