This effect has been used to measure the speed of the sun's motion among the stars. The displacement of spectrum lines, when properly analysed, shows that the solar system is moving at a speed of about 12 miles per second among the stars.
Another application of the Doppler effect to the sun is the verification of the speed of rotation by observing the rates of approach of the east limb and recession of the west limb. The values obtained agree very well with those indicated by the spots, and the observations can be extended beyond the belts to which the spots are confined. The same spectra can be used for a further purpose ; namely, to determine what lines in the solar spectrum are produced by absorption in the earth's atmosphere. Our atmos phere, though not luminous, is able, in great thickness, to absorb certain lines (known as telluric lines) proper to its constituents, which are mixed with the true solar lines in the ordinary Fraun hofer spectrum. Such lines, however, occupy identical positions in the spectra of the two limbs and are therefore readily distin guished from the displaced solar lines.
One of the most difficult and important applications of the Doppler effect is to the detection of atmospheric currents in the sun itself. Solar storms not infrequently occur, of incomparably greater fury than the hurricanes and tornadoes of the earth, and the violent motions of the solar gases are revealed by distortions and displacements of the spectrum lines. Eruptive prominences are often the results of such storms. But there are more syste matic movements also which have been brought to light in the same way, and of these perhaps the most interesting are the move ments occurring near sunspots. The researches of John Evershed and Charles Edward St. John have shown that in the lower atmos pheric levels gases move upwards and outwards from a spot, while in the higher levels the movements are inwards and down wards, as if a spot were a sort of whirlpool into which the high level gases are drawn. Indeed, there is definite evidence of high level circulatory movements round the axes of spots, which point strongly to the same conclusion.