Page:Astronomy for Everybody.djvu/202

year, and near the same point of its orbit, as before. In this period the earth will have made seventeen revolutions and Mars nine.

The difference of a month or so in the interval between oppositions is due to the great eccentricity of the orbit, which is larger than that of any other major planet except Mercury. Its value is 0.093, or nearly one tenth. Hence, when in perihelion, it is nearly one tenth nearer the sun than its mean distance, and when in aphelion nearly one tenth farther. Its distance from the earth at opposition will be different by the same amount, measured in miles, and hence in a much larger proportion to the distance itself. If opposition occurs when the planet is near perihelion, the distance from earth is about forty-three million miles; but if near the aphelion, about sixty million miles. The result of this is that, at a perihelion opposition, which can occur only in September, the planet will appear more than three times as bright as at an aphelion opposition, occuring in February or March. An opposition occurred near the end of March, 1903; the next following early in May, 1905. We shall then have oppositions near the end of June, 1907, and in August, 1909, which will be quite near to perihelion.

Mars, when near opposition, is easily recognised by its brilliancy, and by the reddish colour of its light, which is very different from that of most of the stars. It is curious that a telescopic view of the planet does not give so strong an impression of red light as does the naked eye view.