ASTRONOMY. 



?6th volume of the Philosophical Trans- 

 actions, Mv. Edward Pigot gives a disser- 

 tation on the stars suspected by the astro- 

 nomers of the last century to be change- 

 able. For the greater accuracy in the in- 

 vestigation of his subject, he divides them 

 into two classes ; one containing those 

 which are undoubtedly changeable, and 

 the other those which are only suspected 

 to be so. The former contains a list of 

 12 stars, from the first to the fourth mag- 

 nitudes, including the new one which ap- 

 peared in Cassiopeia in 1572, and that in 

 Serpentarius in 1604 : the other contains 

 the names of 38 stars, of all magnitudes, 

 from the first to the seventh. He is of 

 opinion, that the celebrated new star in 

 Cassiopeia is a periodical one, and that it 

 returns once in 150 years. Keillis of the 

 same opinion ; and Mr Pigot thinks, that 

 its not being observed at the expiration of 

 each period is no argument against the 

 truth of that opinion ; " since (says he) 

 perhaps, as with most of the variables, it 

 may, at different periods, have different 

 degrees of lustre, so as sometimes only 

 to increase to the ninth magnitude ; and 

 if this should be the case, its period is 

 probably much shorter." For this reason, 

 in September, 1782, he took a plan of the 

 small stars near the place where it for- 

 merly appeared ; but in four years had ob- 

 served no alteration. The star in the 

 neck of the Whale had also been exa- 

 mined by Mr. Pigot, from the end of 1782 

 to 1786, but he never found it exceed the 

 sixth magnitude ; though Mr. Goodricke 

 had observed it on the ninth of August to 

 be of the second magnitude, and on the 

 third of September, the same year, it was 

 of the third magnitude. Mr. Pigot de- 

 duced its period from its apparent equa- 

 lity with a. smaller star in the neighbour- 

 hood, and thence found it to be 320, 328, 

 and 337 days. The most remarkable of 

 these changeable stars is that called Al- 

 gol, in the head of Medusa. It had long 

 been known to be variable ; but its period 

 was first ascertained by Mr. Goodricke, 

 of York, who began to observe it in the 

 beginning of 1783. It changes continu- 

 ally from the first to the fourth magni- 

 tude ; and the time taken up from its 

 greatest diminution to its least is found, at 

 a mean, to be 2 d O h 49m an d 3 s . During 

 four hours it gradually diminishes in lus- 

 tre, which it recovers during the succeed- 

 ing four hours ; and in the remaining part 

 of the period, it invariably preserves its 

 greatest lustre, and after the expiration 

 of the term its diminution again com- 

 mences. According to Mr. Pigot, the 



degree of brightness of this star, when at 

 its minimum, is variable in different pe- 

 riods, and he is of the same opinion with 

 regard to its brightness when at its full ; 

 butwhether these differences return re- 

 gularly or not has not been determined, 



OF ECLIPSES. 



When any one of the heavenly bodies 

 is obscured or darkened by the shadow of 

 another falling upon it, or by the interpo- 

 sition of any body, it is said to be eclipsed. 

 The eclipses of the sun and moon are 

 the most striking of any. They were 

 formerly considered as ominous, and have 

 often excited the dread and apprehension 

 of the vulgar ; but the improvement of 

 science has shewn that they have no con- 

 nection with future events ; that they de- 

 pend upon regular and invariable causes, 

 and may be calculated and foretold with 

 the greatest certainty. As the earth is 

 an opaque body, enlightened only by the 

 sun, it will cast a shadow towards that 

 side which is farthest from the sun. If 

 the sun and earth were of the same size,, 

 this shadow would be cylindrical, and 

 would extend to an infinite distance : but 

 as the sun is much larger than the earth, 

 the shadow of the latter must be conical, 

 or end in a point, (see fig. 6.) On the 

 sides of this conical shadow, there is a 

 diverging shadow, the density of which 

 decreases in proportion as it recedes from 

 the sides of the former conical shadow : 

 this is called the penumbra. As the moon 

 revolves round the earth sufficiently near 

 to pass through the shadow of the earth, 

 an eclipse must always take place when 

 these three are all in one straight line. 

 An eclipse of the moon can never happen 

 but at the time of full moon ; but, on ac- 

 count of the inclination of the moon's or- 

 bit to that of the earth, an eclipse cannot 

 take place every full moon. When the 

 moon passes entirely through the earth's 

 shadow, the eclipse it total ; but when 

 only a part of it passes through the sha- 

 dow, the eclipse is partial. The quantity 

 of the moon's disc which is eclipsed (and 

 the same thing is to be understood of that 

 of the sun in a solar eclipse) is expressed 

 by twelfth parts, called digits ; that is, the 

 disc is supposed to be divided by twelve 

 parallel lines : then, if half the disc is 

 eclipsed, the quantity of the eclipse is 

 said to be six digits. When the diame- 

 ter of the shadow through which the 

 moon must pass is greater than the dia- 

 meter of the moon, the quantity of the 

 eclipse is said to be more than 12 digits ; 



