The sun is a variable star with a twofold variation. One is of long period during the eleven-year sun-spot cycle with a range of from three to five per cent. The other is a short irregular variation with a period of a few days, weeks or months and a range of from three to ten per cent. Sun-spots are solar cyclones and appear black only by contrast with their hotter and brighter surroundings. They come in eleven-year cycles (approximately) with periods of maximum and minimum appearance. The brightness and blue color of the sky is due to the scattering of sunlight by the molecules of oxygen and nitrogen in the earth's upper atmosphere. If there were no atmosphere the skies would appear black except in the direction of the heavenly bodies, which would be visible by day as well as by night. The solar corona is the rare outer envelope of the sun and it is visible only during a total eclipse of the sun. It is partly of an electrical nature and it varies in form during the sun-spot cycle. It often extends to a distance of several solar diameters on either side of the sun. The warmth and the habitability of the earth's surface is due to the presence of water-vapor and carbon-dioxide in the atmosphere. Without these substances in the atmosphere life on the earth's surface would be impossible. Half of the earth's atmosphere and all clouds lie within seven miles of the earth's surface, and at high elevations above the earth the temperature is many degrees below zero. The temperature of space approaches the absolute zero of -459° F. The only planets in the solar system with the exception of the earth that might possibly support life are Venus and Mars. Stars shine by their own light but planets shine only by reflected light from the sun. * * * * * If the earth were represented by a six-inch school globe the sun would be on the same scale a globe fifty-four feet in diameter. Mercury would be a small ball two and a third inches in diameter. Venus would be another six-inch globe. Mars would be a ball about the size of a baseball, three and a fifth inches in diameter. The moon would be about the size of a golf ball, one and a half inches in diameter. The largest asteroids would be the size of marbles. Average-sized asteroids would be the size of shot and the smallest would be merely grains of sand. Jupiter would be a huge globe standing as tall as a man five feet six inches in height. Saturn would be a smaller globe four and a half feet in diameter and its ring system would extend to a distance of five and a half feet on either side of the globe. Uranus would be represented by a globe almost exactly two feet in diameter and Neptune would be a slightly larger globe with a diameter of two feet two and a half inches. The satellites of the outer planets would range in size from tennis and golf balls for the largest, to marbles for the smaller and grains of sand for the smallest.