190 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 193 3 



it must be remembered that these applications no longer require 

 astronomical research, nor do they now occupy more than a very 

 small fraction of the time of an astronomical observatory. Whether 

 an astronomer is engaged in determining the positions or measuring 

 the radial motions or distances of the stars, whether he is examining 

 their apparent brightness or investigating their orbital motions, 

 whether he is analj^zing the constitution and physical conditions of 

 the stars and nebulae by means of the spectroscope, or whether he 

 is only applying mathematical analysis to the investigation of the 

 dynamical conditions in the galaxy, he is, by every one of these 

 methods and many others, hoping to add something of value toward 

 the solution of the fundamental astronomical problem and to aid 

 in building up a true conception of the structure of the galaxy. 



A mere glance at the sky on a clear moonless night shows a lumi- 

 nous belt, the INIilky Way, hence the name " galaxy ", stretching 

 across the sky, while observations at different times of the year and 

 in both hemispheres show that this Milky Way belt goes clear 

 around the sky, practically in a great circle. The concentration 

 of the stars in the Milky Way seems to indicate unmistakably to 

 us that they must extend to greater distances in that direction but, 

 until comparatively recent times, the prepossession that the earth 

 must be the center of the universe tended to the assumption of a 

 spherical distribution. That the stellar system must have a rela- 

 tively flattened form was first pointed out by Wright, only about 

 200 years ago. This idea of the flattened shape of the galaxy was 

 further elaborated by the elder Herschel, who by means of his star- 

 gaging about the end of the eighteenth century stated that the shape 

 of the stellar system approximated that of a grindstone. 



Our conceptions of the dimensions of the stellar system have 

 grown with increasing knowledge of the distances of the stars. As 

 the distance or parallax of any star was not measured until 1838 

 and as by the end of the nineteenth century less than 100 stellar 

 parallaxes had been measured, with many of them very uncertainly 

 determined, it is obvious that the ideas of the dimensions of the 

 galaxy prevalent at the beginning of the present century were based 

 on meager observational data and were little better than guesses. 

 It was not until 1905 that Newcomb and Seeliger, who were the first 

 to apply analytical methods to the problem, arrived at an estimate 

 of 7,000 light years for the diameter of the galaxy. 



I hope I may be pardoned at this stage for interpolating some 

 explanatory matter with illustrations about the astronomer's yard- 

 stick, the light-year. While I believe that everyone knows what a 

 light-year means, I am sure that very few have any adequate con- 

 ception how great a distance it represents and, therefore, I venture 

 to repeat the well-known fact that a light-year, the distance that light 



