234 POPULAR SCIENCE MONTHLY. 



observers made different estimates, some calling a star of the second 

 magnitude which others would call of the first, while others would 

 designate a star of the third magnitude which others would call of the 

 second. It is therefore impossible to state with absolute numerical pre- 

 cision what number of stars should be regarded of one magnitude and 

 what of another. 



An idea of the magnitude of a star can be readily gained by the 

 casual observer. Looking at the heavens on almost any cloudless 

 evening, we may assume that the two, three or more brightest stars 

 which we see are of the first magnitude. As examples of those of the 

 second magnitude, may be taken the five brightest stars of the Dipper, 

 the Pole Star and the brighter stars of Cassiopeia. Some or all of 

 these objects can be seen on any clear night of the year in our latitude. 

 Stars of the third magnitude are so numerous that it is difficult to 

 select any one for comparison. The brightest star of the Pleiades 

 is really of this magnitude, but it does not appear so in consequence 

 of the five other stars by which it is surrounded. At a distance of 

 15° from the Pole Star, Beta Ursa Minoris is always visible, and may 

 be distinguished by being slightly redder than the Pole Star; it lies 

 between two fainter stars, the brighter of which is of the third and the 

 other of the fourth magnitude. The five readily visible but fainter 

 stars of the Pleiades are about of the fourth magnitude. Of the fifth 

 magnitude are the faintest stars which are easily visible to the naked 

 eye, while the sixth comprises those which are barely visible with 

 good eyes. 



Modern astronomers, while adhering to the general system which 

 has come down to them from ancient times, have sought to give it 

 greater definiteness. Careful study showed that the actual amount 

 of light corresponding to the different magnitudes varied nearly in 

 geometrical progression from one magnitude to another, a conclusion 

 which accords with the well-known psychological law that the intensity 

 of sensation varies by equal amounts when the exciting cause varies in 

 geometrical progression. It was found that an average star of the 

 fifth magnitude gave between two and three times as much light as an 

 average one of the sixth; one of the fourth gave between two and three 

 times as much light as one of the fifth; and so on to the second. In 

 the case of the first magnitude, the diversity is so great that it is 

 scarcely possible to fix an average ratio. Sirius, for example, is really 

 six times as bright as Altair, which is commonly taken as a standard 

 for a first magnitude star. To give precision to their estimates, modern 

 astronomers are gradually seeking to lay the subject of magnitudes on 

 an exact basis by defining a change of one unit in the magnitude as 

 corresponding to an increase of about two and one half times in the 

 amount of light. 



