and determined shape" of the spots "as well as remarkable colour, show them to be permanent and fastened to the body of the planet". The spots which chiefly attracted his attention were the white polar caps. On 17th April, 1777, he noted, "There are two remarkable bright spots on Mars". These spots had been noticed by Maraldi early in the century, but Herschel was the first to investigate their nature and to chronicle their periodical variation in size. "I may well be permitted to surmise," he wrote in 1784, "... that the bright polar spots are owing to the vivid reflection of light from frozen regions; and that the reduction of these spots is to be ascribed to their being exposed to the Sun. In the year 1781, the south polar spot was extremely large, which we might well expect, since that pole had but lately been involved in a whole twelve months' darkness and absence of the Sun: but in 1783 I found it considerably smaller than before." Herschel's sagacious surmise has been abundantly confirmed by all subsequent observers. In addition, he determined the rotation period with considerable accuracy. In 1781 he announced it to be 24 hours, 39 minutes, 21.67 seconds; he also ascertained the axial inclination and equatorial diameter. The general conclusion which he reached as a result of his observations was that "the analogy between Mars and the Earth is, perhaps, by far the greatest in the whole solar system. Their diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, not very different." The planet, he concluded, "has a considerable but moderate atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to ours".
The planet Jupiter did not claim so much of Herschel's attention as either Mars or Saturn. Nevertheless, his short study of the giant world marked an epoch. In his paper of 1781 on the Rotation of the Planets round their Axes," he put forward as a suggested explanation