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PLANETS AND THEIR SATELLITES

scopic observations have not yet been made with sufficient precision to teach us the exact time of revolution. Recent discoveries as to the nature of the atmosphere of Venus make it almost certain that all the observers who supposed that they saw markings on the planet were mistaken.

Atmosphere of Venus

It is now well established that Venus is surrounded by an atmosphere which is probably denser than that of the earth. This was shown in a remarkable and interest-

Fig. 31.—Effect of the Atmosphere of Venus during the Transit of 1882.

ing way during the transit of Venus over the sun's disk in 1882, which was observed by the writer at the Cape of Good Hope. When the planet was a little more than halfway on the disk, its outer edge appeared illuminated, as shown on the figure. This illumination, however, did not commence at the middle point of the arc, as it

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