[AS.] The body in the heavens that gives light and heat, and round which the earth and planets revolve. (See Solar System.) It is about 92 1/2 million miles distant from the earth, and its diameter is about 860,000 miles. It revolves on its own axis once in 251/2ys. Its luminous surface is called the photosphere, above which is an envelope largely of hydrogen, called the chromosphere, visible through the spectroscope, or at the time of a total solar eclipse. Above the chromosphere, and extending for millions of miles, are rays of light called the corona. Dark spots appear on the sun's disc, and consist commonly of a black central portion with a surrounding border of lighter shade. These change in their size from points to spaces 50,000 miles in diameter. It has been established that a maximum and a minimum number of sun spots occur in periods of 11 years. The sun is not a fixed body, round which the earth and other planets circulate, but it, with all the planets, has a motion through space. In regard to physical structure, the sun is believed to be a mass of incandescent (glowing hot) gases, the temperature of which is so high that none of the chemical elements entering into its composition can exist in any other than the gaseous state. The quantity of heat given off by the sun is enormous. There are several theories as to its origin, the most probable being that it is due to a gradual shrinkage of the sun's mass which reduces its power of containing heat.

SOLAR CORONA.

SOLAR CORONA.

SUN SPOTS.

SUN SPOTS.

Sun 174