The physical conditions of the smaller planets are much more like those with which we are familiar on earth. Owing to its greater distance from the sun, Mars is somewhat, but not enormously, colder than the earth. Its day of 24 hours 37 minutes is only slightly longer than our own, so that its surface must experience alternations of warmth by day and cold by night similar to those we find on earth. In the equatorial regions the temperature rises well above the freezing point at noon, occasionally reaching 50° Fahrenheit or even more. But even here it falls below freezing some time before sunset, and from then until well on in the next day, the climate must be very cold. The polar regions are of course colder still, the temperature of the snowcap which covers the poles being somewhere about -70° Centigrade or -94° Fahrenheit—126 degrees of frost! Venus, being nearer the sun, must have a higher average temperature than the earth. But as each of its days and nights is several days of our terrestrial time, the difference between the temperatures of day and night must be far greater than with us, so that its surface must experience great extremes of heat by day and of cold by night. The night temperature appears to be fairly uniformly equal to about -25° Centigrade or -13° Fahrenheit. At any point on the planet’s surface weeks of this bitterly cold night temperature must alternate with weeks of a roasting day temperature. Mercury is so near the sun that its average temperature is necessarily far higher than that of the earth. It reflects only a tiny fraction—about a fourteenth—of the light and heat it receives from the sun. All the rest goes to heating up its surface. A number of considerations make it likely that the planet always turns the same face to the sun, just as the moon always turns the same face to the earth. If so the unwarmed half of its surface must be intensely cold, and the warmed half intensely hot. It can be calculated that in this case the warmed hemisphere ought to have a temperature of about 357° Centigrade; if however the planet was in fairly rapid rotation, its whole surface would have a temperature of only about 170° Centigrade. Quite recently Pettit and Nicholson have measured the amount of heat received on earth from the warmed hemisphere, and find that its temperature must be about 350° Centigrade or 662° Fahrenheit, thus confirming that the planet always turns the same face to the sun. Its warm hemisphere is at a temperature which melts lead; the other hemisphere, eternally dark and unwarmed, is probably colder than anything we can imagine. [Illustration: PLATE I _Franklin-Adams Chart_ The Milky Way in the neighbourhood of the Southern Cross]