46 POPULAR SCIENCE MONTHLY. 



THE LIGHT OF THE STARS.* 



By Professor E. C. PICKERING, 



DIRECTOR OF THE HARVARD COLLEGE OBSERVATORY. 



f F 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 bright- 

 ness. 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 meas- 

 ure 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 dis- 

 coveries, a public interest which could not otherwise have been awak- 

 ened. 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 ob- 

 servations. 



One department of the new astronomy, the relative brightness of 

 the stars, is as old as, or older than, the old astronomy. An astron- 

 omer even now might do useful work in this department without any 

 instruments whatever. Hipparchus is known to have made a cata- 

 logue 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 pur- 

 pose. But he went a step further, and subdivided these classes. If 

 a star seemed bright for its class, he added the letter fi (mu), standing 

 for iacl^wv (meizon), large or bright; if the star was faint, he added 

 t (epsilon), standing for eAao-o-wv (elasson), small or faint. These 



* An address at the International Congress of Arts and Science, St. Louis, 

 September, 1904. 



