On this principle it has gradually been determined that the sun is composed of elements similar to those which go to make up our earth. Further, the composition of the stars can be ascertained in the same manner; and we find them formed on a like pattern, though with certain elements in greater or less proportion as the case may be. It is in consequence of our thus definitely ascertaining that the stars are self-luminous, and of a sun-like character, that we are enabled to speak of them as _suns_, or to call the sun a _star_. In endeavouring to discover the elements of which the planets and satellites of our system are composed, we, however, find ourselves baffled, for the simple reason that these bodies emit no real light of their own. The light which reaches us from them, being merely reflected sunlight, gives only the ordinary solar spectrum when examined with the spectroscope. But in certain cases we find that the solar spectrum thus viewed shows traces of being weakened, or rather of suffering absorption; and it is concluded that this may be due to the sunlight having had to pass through an atmosphere on its way to and from the surface of the planet from which it is reflected to us. Since the sun is found to be composed of elements similar to those which go to make up our earth, we need not be disheartened at this failure of the spectroscope to inform us of the composition of the planets and satellites. We are justified, indeed, in assuming that more or less the same constituents run through our solar system; and that the elements of which these bodies are composed are similar to those which are found upon our earth and in the sun. The spectroscope supplies us with even more information. It tells us, indeed, whether the sun-like body which we are observing is moving away from us or towards us. A certain slight shifting of the lines towards the red or violet end of the spectrum respectively, is found to follow such movement. This method of observation is known by the name of _Doppler's Method_,[9] and by it we are enabled to confirm the evidence which the sunspots give us of the rotation of the sun; for we find thus that one edge of that body is continually approaching us, and the other edge is continually receding from us. Also, we can ascertain in the same manner that certain of the stars are moving towards us, and certain of them away from us. [9] The idea, initiated by Christian Doppler at Prague in 1842, was originally applied to sound. The approach or recession of a source from which sound is coming is invariably accompanied by alterations of pitch, as the reader has no doubt noticed when a whistling railway-engine has approached him or receded from him. It is to Sir William Huggins, however, that we are indebted for the application of the principle to spectroscopy. This he gave experimental proof of in the year 1868. CHAPTER XII THE SUN