UNIT II SECTION 5 THE EARTH RECEIVES MOST OF ITS FROM THE SUN ENERGY Introduction. If man wanted to find some object in the universe to worship, the sun would probably be selected, because it is the source of our light and heat, and because our food, clothing, and housing could never have been formed without its valuable rays. Perhaps it would be better to select some other star, for as stars go, the sun is only middle-class. However, although there are much larger stars a thousand times brighter, this particular star would still be selected because it is our star. The Sun Is a Giant Sphere of Hot Gas. The sun is a giant sphere of glowing gas, more than one million times the volume of the earth. Its diameter is about 864,000 miles. The average distance from the sun to the earth is 93,000,000 miles. The temperature at the surface is about 6000° C, while it is estimated to be 22,000,000° C. at the center. Experiments with high temperatures indicate that every terrestrial substance thus far examined would be in the form of a vapor at the temperature of the interior of the sun. This conclusion is confirmed by the solar spectra, which show at least 64 of the 92 known or suspected elements to be present in vapor form in the sun. It is probable that the other elements are present in too small proportions to give spectroscopic evidence. The heavier gases of the sun are surrounded by a layer of light, hot gases and vapors about 5000 to 10,000 miles in depth, which is called the chromosphere. Outside of the chromosphere is the corona, seen only during eclipses, which consists of hot, highly rarefied gases that extend from the chromosphere a distance nearly equal to that of the sun's diameter. One theory accounts for the corona by assuming that the pressure of the sun's radiation pushes the gases out into the space surrounding the sun. From this fiery layer great lurid tongues of incandescent hydrogen shoot out, sometimes to a distance of 500,000 86
miles. THE SUN 87 One such prominence, as these tongues are called, was observed to shoot out of the chromosphere at the rate of 60,000 miles per hour. Although the volume of the sun is more than a million times that of the earth, its relative mass is much less, because the density ^ of the earth is about four times that of the sun. Stinspots Are the Cooler Portions of the Sun's Surface. The surface of the sun is mottled with dark spots, ranging from 500 to 150,000 miles in diameter. These sunspots represent relatively cool areas, whose temperature is about 4000° C. as compared with the 6000° C. which is the temperature of the hottest outer portions. It is thought that the sunspots are areas of lowered temperature resulting from tremendous solar cyclones, which increase to a maximum and then for some unknown reason decrease periodically every eleven years. Studies of annual tree rings show that they vary in a similar elevenyear cycle. When these solar disturbances are at their maximum, magnetic disturbances occur on the earth that interfere seriously with telegraphic, telephonic, and radio communication. The aurorae are also more brilliant at such Fig. 19. Solar disk with many spots of unusual size, November 30, 1929. (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.) times. These disturbances on the earth may be due to increased streams of electrons and ions or greater ultraviolet radiations coming from the sun, as a result of changing solar conditions. George E. Hale proved that sunSpots behave like huge magnets. The direction of the whirl of the sunspots in each succeeding cycle is reversed, thus reversing the magnetic fields. A great magnetic storm occurred in April, 1938, in which energy was expended at the rate of two billion kilowatts for a two-hour interval, • Density is the mass of a unit volume of a substance. For the present you may consider that mass means about what is commonly meant by weight. A more exact usage of these terms will be given later. Density may therefore be considered to be weight of a unit volume.