The first fact that we have to remember with regard to the planet Mars is the smallness of its apparent size. To the eye it is nearly a star--a point of light without visible surface. It is almost twice the size of the Moon in actual diameter, but as its mean distance from the Earth is 600 times that of the Moon, its mean apparent diameter is 300 times smaller. We cannot, however, watch Mars in all parts of its orbit; it is best placed for observation, and, therefore, most observed, when in opposition, and oppositions may be favourable or unfavourable. At the most favourable opposition, Mars is 140 times as distant as the Moon; at the least favourable, 260 times; so that on such occasions its apparent size varies from 1/70th of the diameter of the Moon to 1/130th. But a telescope with a magnifying power of 70 could never, under the most perfect conditions, show Mars, even in the closest opposition, as well as the Moon is seen with the naked eye, for the practical magnifying power of a telescope is never as great as the theoretical. In practice, a child's spy-glass magnifying some six diameters will show the full Moon to better advantage than Mars has ever been seen, even in our most powerful telescopes. The small apparent size of the planet explains how it was that Galileo does not seem to have been able to detect any markings upon it. In 1659, Huyghens laid the foundation stone of areography by observing some dark spots, and determining from their apparent movements that the planet had a rotation on its axis, which it accomplished in about the same time as the Earth. Small and rough as are the drawings that Huyghens made, the identification of one or two of his spots is unmistakable. Seven years later, in 1666, both Cassini and Hooke made a number of sketches, and those by Hooke have been repeatedly used in modern determinations of the rotation period of the planet. The next great advance was made by Sir William Herschel, who, during the oppositions of 1777, 1779, 1781, and 1783, determined the inclination of the axis of Mars to the plane of its orbit, measured its polar and equatorial diameters, and ascertained the amount of the polar flattening. He paid also special attention to two bright white spots upon the planet, and he showed that these formed round the planet's poles and increased in size as the winter of each several hemisphere drew on and diminished again with the advance of summer, behaving therefore as do the snow caps of our own polar regions.