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FIGURE 2.13. Drawing of Mars by Cassini showing polar caps. (From C. Flammarion, "La Planète Mars," Vol. 1.)

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FIGURE 2.13. Drawing of Mars by Cassini showing polar caps. (From C. Flammarion, "La Planète Mars," Vol. 1.)

FIGURE 2.14. Drawing by Huygens showing south polar cap. (From C. Flammarion, "La

FIGURE 2.14. Drawing by Huygens showing south polar cap. (From C. Flammarion, "La

and during subsequent opportunities, were published in 1781 and 1784 by William Herschel, the German-born musician turned telescope builder and astronomer, who was then private astronomer to George III of England. Among other things, he determined the angle between the axis of rotation of Mars and the perpendicular to the plane of its orbit around the Sun. Herschel found that this angle, about 30 degrees, was not very different from the corresponding angle of inclination of Earth's axis.

This angle of inclination determines the seasons on Earth (ch. Ill) ; hence, Herschel argued, Mars should also have four seasons. Furthermore, he thought that the planet "has a considerable but moderate atmosphere." Because the length of the day is also close to that on Earth, Herschel concluded that the "inhabitants [of Mars] probably enjoy a situation similar to our own." Incidentally, in the 17th and 18th centuries it was commonly accepted that Mars (and other planets) were inhabited.

Like several of his predecessors, Herschel considered the dark areas on the planet's surface to be oceans or seas, just as the dark areas on the Moon were regarded as being bodies of water. The lighter colored regions were believed to be dry land. It is now known, however, that such open bodies of liquid water cannot be present on Mars, and that both dark and bright areas are essentially dry land. The color differences are probably caused by the nature of the surface materials.

The bright polar caps were assumed by Herschel to consist of snow and ice, like those of Earth. In this respect it is possible, but by no means certain, as will be pointed out in chapter VI, that he may have been right. Furthermore, he argued that the caps were probably not thick because they decrease considerably in size and almost disappear, as a result of melting and evaporation, during the Martian summer in each hemisphere.

One of Herschel's interesting observations, which he may have interpreted incorrectly, was that there were changes apparently occurring on the surface of Mars. "Besides the permanent spots," he wrote, "I have often noticed occasional changes in partial bright belts; and also once on a darkish one, in a pretty high latitude." Herschel attributed these changes to "clouds and vapors floating in the atmosphere of the planet." It is now

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