170 



SCENERY OF THE HEAVENS. 



same part of the heavens. The chief difficulty which lies in the way of the supposition, 

 that the temporary stars are objects moving in orbits, which display them periodically 

 to us, is, that no change of place has been observed during the whole time of apparition. 

 Besides cases of apparent stellar creation and complete extinguishment, there are a 

 number of stars whose light undergoes a periodical increase and diminution, forming the 

 class called variable. Some retire for a time into absolute invisibility, while others 

 merely suffer changes in their brightness, without being absolutely lost to view. 



The earliest observed example of this class is o Ceti, called also Stella Mira, or the 

 wonderful star, situated in the neck of the Whale. It was first particularly remarked by 

 Fabricius, August 13, 1596, when it appeared as a star of the third magnitude, but 

 before the end of the year it had retreated entirely from observation. Holward 

 remarked it again in 1637, after which it disappeared for nine months, when he again 

 saw it. The following are now its general phases, which are gone through in 331 days, 



10 hours, and 19 minutes. When at the greatest 

 brightness, it is equal to a star of the second magnitude, 

 and remains so for about a fortnight. It then decreases 



t$$i$&$$:WliP^ I during three months, passes entirely out of sight, con- 

 . neb'" '' "' ^ tinues invisible about five months, again comes into 



view, increases during three months, when it attains 

 once more its maximum lustre. It does not always, 

 however, return to the same degree of brightness, or 

 increase and diminish by the same gradations, or in- 

 variably remain invisible the same length of time. For 

 the four years between 1672 and 1676, it was never 

 seen at all, though Hevelius searched that part of the 

 heavens diligently for it. The relative position of 

 Stella Mira is shewn in the diagram. The two upper- 

 most stars are the bright companion orbs which mark 

 the head of Arie/ about 25 distant. Those to the left 

 are in the head of Cetus. 



Algol, the name of the star j3 in Perseus, in the head 

 of Medusa, is another remarkable instance of stellar 

 mutation. It varies from the second to the fourth degree 

 of magnitude. This was determined by Maraldi in 

 1694 ; but Mr. Goodricke of York in 1782, and about 

 the same time Palitzch, the Saxon farmer, first accu- 

 rately fixed the period of its changes. The star goes 

 through its variations in a remarkably short space of 

 time : it continues at its brightest two days and four- 

 teen hours. Then its splendour suddenly begins to 



diminish, and in three hours and a half it is reduced to its minimum. Its feeblest lustre 

 lasts but little more than fifteen minutes. It then begins to increase, and in three hours 

 and a half more, it is restored to its usual brightness. Its full period is therefore 2 

 days, 20 hours, and 48 minutes. The remarkable law of variation to which this star is 

 subject, suggested to Goodricke the idea of some opaque body revolving around it, which, 

 interposing between the earth and the star, cuts off a portion of its light. Algol may 

 be seen on any fine evening from August to May, as it continues above the horizon 

 twenty hours out of the twenty-four. With Algenib in the side of Perseus, and Almaak 

 in the foot of Andromeda, it forms a triangle, with the open part towards Cassiopeia. 

 X Cygni was ascertained to be variable, and its period determined by Kirch in 1686. 



