ON THE FIXED STARS. 495 



changes of brightness, which are suppnsed to arise either from the temporary 

 interpositioa of opaque bodies revolving round them, or, still more probably 

 from a rotatory motion of their own, which brings at certain periodical times 

 a less luminous part of the surface into our view. Thus, the star Algol, 

 which is usually of the second magnitude, becomes, at intervals of ^ days 

 and 21 hours each, of the fourth only, and occupies 7 hours in the gradual 

 diminution and recovery of its light. A less probable conjecture respecting 

 this change of brightness was advanced by Maupertuis, who imagined that 

 the disc of the star might be greatly flattened by a rapid rotation, and its 

 edge occasionally presented to us, in consequence of the disturbances produc- 

 ed by the attraction of planets revolving round the luminary. Other irregular 

 variations may possibly be occasioned by the appearance and disappearance of 

 spots, occurring, like the spots of the sun, without any determinate order 

 or assignable cause; and many stars have in the course of ages wholly disap- 

 peared, and sometimes have been again recovered ; others have made their ap- 

 pearance for a short time, where no star had before been seen. Such a temporary 

 star was observed by Hipparchus, 120 years before our era, and the circum- 

 stance suggested to him the propriety of making an accurate catalogue of all 

 the stars, with their respective situations, which is still extant, having been 

 preserved by Ptolemy, who added 4 stars to the 1022 that it contained. In 

 1572, Cornelius Gemma discovered a new star in Cassiopeia, which was so 

 bright as to be seen in the day time, and gradually disappeared in sixteen 

 months. Kepler, in 1604, observed a new star in Serpen tarius, more bril- 

 liant than any other star or planet, and changing per{>etually into all the 

 colours of the rainbow, except when it was near the horizon ; it remained 

 visible for about a year. Many other new stars have also been observed at 

 different times. 



For describing the particular fixed stars according to their relative situa- 

 tions, it is necessary to consider them as they are visible to the inhabitants 

 of the earth. They have been divided, for the sake of convenience, into 

 parcels, making up imaginary forms, denominated constellations. This 

 division is of very remote antiquity, and though it may be useless, and 

 sometimes even inconvenient, for the purposes of minute observation, yet for 

 a general recollection of the great features of the heavens, these arbitrary 

 names and associations cannot but greatly assist the memory. It is also 



