404 A Short History of Astronomy [CH. xm. 



capable of precise expression, and agreeing roughly, at 

 any rate as far as naked-eye stars are concerned, with the 

 current usages ; while at the Cape he measured carefully 

 the light of a large number of bright stars and classified 

 them on this principle. According to the scale now gener- 

 ally adopted, first suggested in 1856 by Norman Robert 

 Pogson (1829-1891), the ligit of a star of any magnitude 

 bears a fixed ratio (which is taken to be 2 '5 12...) to that 

 of a star of the next magnitude. The number is so chosen 

 that a star of the sixth magnitude thus defined is 100 

 times fainter than one of the first magnitude.* Stars of 

 intermediate brightness have magnitudes expressed by 

 fractions which can be at once calculated (according to 

 a simple mathematical rule) when the ratio of the light 

 received from the star to that received from a standard star 

 has been observed. t 



Most of the great star catalogues ( 280) have included 

 estimates of the magnitudes of stars. The most extensive 

 and accurate series of measurements of star brightness have 

 been those executed at Harvard and at Oxford under the 

 superintendence of Professor E. C. Pickering and the late 

 Professor Pritchard respectively. Both catalogues deal with 

 stars visible to the naked eye ; the Harvard catalogue 

 (published in 1884) comprises 4,260 stars between the 

 North Pole and 30 southern declination, and the Urano- 

 metria Nova Oxoniensis (1885), as it is called, only goes 

 10 south of the equatorand includes 2,784 stars. Portions 

 of more extensive catalogues dealing with fainter stars, in 

 progress at Harvard and at Potsdam, have also been 

 published. 



* I.e. 2-512... is chosen as being the number the logarithm of which 

 is -4, so that (2-512. ..) 5/2 = 10. 



f If L be the ratio of the light received from a star to that received 

 from a standard first magnitude star, such as Aldebaran or Altair, 

 then its magnitude m is given by the formula 



m- 1 

 \ b , whencem-i = - $ log L. 



, 



-5I2/ \ioo 2 



A star brighter than Aldebaran has a magnitude less than I, while 

 the magnitude of Sinus, which is about nine times as bright as 

 Aldebaran, is a negative quantity, 1-4, according to the Harvard 

 photometry. 



