ASTRONOMY. 
to be so. The former contains a list of 12 
stars, from the first to the fourth magni- 
tudes, 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 opi- 
nion, that the celebrated new star in Cas- 
siopeia is a periodical one, and that it re- 
turns once in 150 years. Keill is 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 onty to increase 
to the ninth magnitude; and if this 
should be the case, its period is probably 
much shorter.” For this reason, in Sep- 
tember, 1782, he took a plan of the small 
stars near the place where it formerly ap- 
peared, but in four years had observed no 
alteration. The star in the neck of the 
Whale had also been examined by Mr. Pi- 
got from the end of 1782 to 1786, but he 
never found it exceed the sixth magnitude ; 
though Mr. Goodricke had observed it on 
the 9th of August to be of the second mag- 
nitude, and on the 3d of September the 
same year it was of the third magnitude. 
Mr. Pigot deduced its period from its ap- 
parent equality with a smaller star in the 
neighbourhood, and thence found it to be 
320, 328, and 337 days. The most re- 
markable of these changeable stars is that 
called Algol, in the head of Medusa. It had 
long been known to be variable ; but its 
period was first ascertained by Mr. Good- 
ricke of York, who began to observe it in 
the beginning of 1783. It changes conti- 
nually from the first to the fourth magni- 
tude ; and the time taken up from its great- 
est diminution to its least is found, at a 
mean, to be 2 d 20 h 49 m and 3 s . During 
four hours it gradually diminishes in lustre, 
which it recovers during the succeeding 
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 commences. According 
to Mr. Pigot, the degree of brightness of this 
star when at its minimum is variable in dif- 
ferent periods, and he is of the same opinion 
with regard to its brightness when at its 
full ; but whether these differences return 
regularly 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 interposi- 
tion 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 ex- 
cited the dread and apprehension of the 
vulgar; but the improvement of science 
has shewn that they have no connection with 
future events ; that they depend upon regu- 
lar and invariable causes, and may be cal- 
culated and foretold with the greatest cer- 
tainty. As the earth is an opaque body, en- 
lightened 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 cy- 
lindrical, 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 de- 
creases in proportion as it recedes from the 
sides of the former conical shadow : this is 
called the penumbra. As the moon re- 
volves 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 account of the 
inclination of the moon’s orbit 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 is 
total ; but when only a part of it passes 
through the shadow, the eclipse is partial. 
The quantity of the moon’s disc which is 
eclipsed, (and the same thing is to be under- 
stood 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 12 parallel lines; then if half the disc is 
eclipsed, the quantity of the eclipse is said to 
be six digits. When the diameter of the sha- 
dow through which the moon must pass is 
greater than the diameter of the moon, 
the quantity of the eclipse is said to be 
more than 12 digits ; thus, if the diameter 
of the moon is to that of the shadow as four 
to five, then the eclipse is said to be 15 digits. 
The duration of a lunar eclipse is various, 
it sometimes lasts two or three hours. Tiie 
eclipses of the sun are owing to a different 
cause than , those of the moon. They are oc- 
casioned by the moon’s coming directly be- 
tween us and the sun, and therefore ob- 
structing our view of it. When the moon 
