Page:Astronomy for Everybody.djvu/302

The cause of meteors was unknown until after the beginning of the nineteenth century. It is now, however, well made out. Besides the known objects of the solar system—planets, satellites, and comets—there are, coursing through space, and revolving around the sun, countless millions of particles, or minute collections of matter, too small to be seen with the most powerful telescope. Quite likely the greater number of these objects are scarcely larger than pebbles, or even grains of sand. The earth, in its course around the sun, is continually encountering them. One in the line of motion of the earth may have a velocity amounting to many miles a second; perhaps ten, twenty, thirty, or even forty. Meeting the atmosphere with this immense velocity causes the body to be immediately heated to so high a temperature that its substance dissolves away with a brilliant effusion of light no matter how solid it may be. What we see is the course of a particle thus burning away as it darts through the rare regions of the upper atmosphere.

Of course, a meteor will appear brighter and last longer the larger and solider it is. Sometimes it is so large and solid that it comes within a few miles of the earth before being finally melted and dissolved away. Then, the people in the region over which it is passing, see a remarkably bright meteor. In such a case it frequently happens that in a few minutes after the meteor has passed a loud explosion, like the firing of a cannon, is heard coming from the region through which it passed.