Page:Astronomy for Everybody.djvu/96

bring to a focus those rays of light to which the photographic film is most sensitive. So rapid has been the progress during the past few years that the greater part of the astronomical work of the future seems likely to be done by photography. The great advantage of the method is that when a picture either of some heavenly body or of the stars in the sky is taken, it can be studied and measured at leisure with all the care the astronomer chooses to bestow upon it, while the observation in the heavens is nearly always more or less hurried, and made difficult by the diurnal motion of the star.

Formerly the spots on the sun were investigated by watching that luminary through the telescope, recording the number of spots, and measuring their position on the solar disk. Now, at the Greenwich Observatory and elsewhere, a photograph of the sun is taken almost every day, and the position of the spots is found by measuring the photograph. Thus a study of the sun and the changes going on on its surface is kept up from year to year.

Formerly the astronomer studied the physical constitution of a comet by making a drawing of it. This was a rather uncertain process, and as a general rule no two men would quite agree in the minute details. Now the comet is photographed and a study is made upon the negative. The same remark applies to nebulae. Drawings of them are no longer made—only photographs which show a great deal more than any drawing will.