The New Student's Reference Work/Sun, The

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Sun, The, our nearest star and the central body of the solar system.  For the inhabitants of the earth no other body in the heavens compares for an instant with the sun in interest and vital importance.  For not only are the orbits of the earth and the other planets determined by the sun, but all forms of life on the earth immediately depend upon heat and light from the sun.

The distance of the sun from the earth is 92,897,000 miles, a stretch which baffles the imagination.  Professor Young illustrates this quantity by saying that if a child were provided with an arm long enough to reach the sun and thus burn his hand, he would die of old age before he learned that he was burnt, assuming, as Helmholtz found, that sensations travel along the nerves at the rate of about 40 feet per second.  Even light itself, traveling with the stupendous speed of 186,000 miles per second, requires 499 seconds for the trip from the sun to the earth.

The diameter of the sun is approximately 866,500 miles, a distance nearly twice as great as the diameter of the moon’s orbit, while the mass is 1,332,000 times that of the earth, making its density about 1½ times that of water and the attraction of gravitation at the surface of the sun 27 times as great as at the surface of the earth.  The weight of an ordinary man at the solar surface would, therefore, be such that he could not stand erect with his present strength of muscles.

The period of rotation of the sun was first determined after Galileo had discovered spots on the solar surface; and this rotation takes place in a very remarkable manner, for Carrington found that at the sun’s equator spots travel once around the sun in 25 days, while at latitude 30° the period is 26½ days, and at latitude 40° is no less than 27 days.  Hence the sun rotates most rapidly near the equator and diminishes constantly in angular speed as one approaches the poles.  This fact has never been satisfactorily explained.

As to the constitution of the sun and as to the nature of sun-spots there are almost as many “theories” as there are men.  What is reasonably certain, however, is that practically all of the elements which have been found in the earth’s crust are to be found in the sun, making it highly probable that at some time the earth split off from the sun in the manner suggested by the nebular hypothesis.  Concerning the interior of the sun practically nothing is known; for its temperature is outside the range of our experiments.

The white surface of the central body of the sun is called the photosphere.  The spectroscope shows that this photosphere is surrounded by a layer of incandescent gases which we call the chromosphere.  Certain parts of this chromosphere appear at times to be heaped up into brilliant clouds thousands and even hundreds of thousands of miles high; these are called prominences.  Beyond the prominences and the chromosphere is a halo of light seen only at times of eclipse.  It is called the corona; but beyond the mere fact that it is an appendage of the sun little is known about it.  The fact, however, that its spectrum contains bright lines would seem to indicate that it consists partly, at least, of a self-luminous gas.

The best account of solar phenomena is to be found in Professor Young’s delightful volume, The Sun, published in the International Scientific Series.  See Astronomy and Planets.