IF an intelligent observer should see the stars for the first time, two of their properties would impress him as subjects for careful study; first, their relative positions, and secondly, their relative brightness. From the first of these has arisen the astronomy of position, or astrometry. This is sometimes called the old astronomy, since until within the last twenty years the astronomers of the world, with few exceptions, devoted their attention almost entirely to it. To the measure of the light should be added the study of the color of the stars (still in its infancy), and the study of their composition by means of the spectroscope. In this way a young giant has been reared, which has almost dwarfed its older brothers. The science of astrophysics, or the new astronomy, has thus been developed, which during the last few years has rejuvenated the science and given to it, by its brilliant discoveries, a public interest which could not otherwise have been awakened. The application to stellar astronomy, of the daguerreotype in 1850, of the photograph in 1857, and of the dry plate in 1882, has opened new fields in almost every department of this science. In some, as in stellar spectroscopy, it has almost completely replaced visual observations.
One department of the new astronomy, the relative brightness of the stars, is as old as, or older than, the old astronomy. An astronomer even now might do useful work in this department without any instruments whatever. Hipparchus is known to have made a catalogue of the stars about 150 B. C. Ptolemy, in 138 A. D., issued that great work, the Almagest, which for fourteen hundred years constituted the principal and almost the sole authority in astronomy. It contained a catalogue of 1,028 stars, perhaps based on that of Hipparchus. Ptolemy used a scale of stellar magnitudes which has continued in use to the present day. He called the brightest stars in the sky, the first magnitude, the faintest visible to the naked eye, the sixth. More strictly, he used the first six letters of the Greek alphabet for this purpose. But he went a step further, and subdivided these classes. If a star seemed bright for its class, he added the letter μ (mu), standing for μειζων (meizon), large or bright; if the star was faint, he added ε (epsilon), standing for ελασσων (elasson), small or faint. These
- An address at the International Congress of Arts and Science, St. Louis, September, 1904.