ASTRONOMY. 
happens to be in conjunction with the sun, 
or between the sun and the earth, viz. at the 
time of the new moons, the shadow of the 
moon falls upon the surface of the earth ; 
hence, properly speaking, such eclipses 
should be called eclipses of the earth. But 
the whole disc of the earth cannot be in- 
volved in the shadow of the moon, because 
the moon is much smaller than the earth, and 
the shadow of the moon is conical. Thus, 
in Plate III. fig. 1, the rays of the sun, S, 
being intercepted by the moon, L, form the 
conical shadow C D G, which falling upon 
the surface of the earth, entirely deprives 
that portion of it upon which it falls of the 
sun’s light, and of course the inhabitants of 
that part of the earth will have a total eclipse 
of the sun. Beyond the dense conical sha- 
dow C D G there is a diverging half sha- 
dow, or penumbra CDEF, which is occa- 
sioned by the moon’s intercepting only a 
part of the sun’s rays from those places 
w hich fall within this penumbral cone, and 
are out of the dense shadow'. Thus from the 
part of the earth Z the portion Y Y B of the 
sun only can be seen ; consequently the in- 
habitants of that part will have a partial 
eclipse. As the moon is not always at the 
same distance from the earth, it sometimes 
happens that the conical dense shadow does 
not reach the earth, as in fig. 2, and only the 
penumbral shadow falls upon it, the eclipse 
consequently is partial to every part of the 
earth. Those who are at the centre of the 
penumbra will lose sight of the centre of the 
sun by the interposition of the moon’s body, 
which subtending a smaller angle than the 
sun, will not entirely cover its surface, so 
that there will be a ring of light all round. 
The eclipse is then said to be annular. The 
satellites, or moons, are often eclipsed by the 
planets to which they belong. The eclipses 
of Jupiter’s moons, as we have already ob- 
served, are very useful in ascertaining the 
longitude. When any of the planetary 
bodies disappear by another coming before 
it, it is called an occultation. The occul- 
tations of the fixed stars by the moon are 
of great importance also in determining the 
longitudes of places. 
OF THE TIDES. 
The ebbing and flowing of the sea was 
first shewn by Kepler to be owing to the 
moon’s attraction, and Newton demon- 
strated it upon the principles of gravitation. 
The attraction of the moon cannot alter the 
shape of the solid of the globe: but it lias a 
considerable effect upon the fluid part, which 
YOL. I. 
it causes to assume a spheroidal figure, the 
longest axis being in the direction of the 
moon. It is therefore the highest tide at 
that place perpendicularly under the moon, 
or where the moon crosses the meridian. 
The sun also has some action upon the wa- 
ters, though its attraction, on account of its 
distance, is not so strong as that of the moon. 
When the action of the sun and moon con- 
spire together the tide rises higher, and pro- 
duces what are called spring tides. On the 
contrary, when they counteract each other, 
they produce neap tides. The ocean, it is 
well known, covers more than one-half of 
the globe ; and this large body of water is 
found to be in continual motion, ebbing and 
flowing alternately without the least inter- 
mission. What connection these motions 
have with the moon we shall see as w'e pro- 
ceed ; but at present it will be sufficient to 
observe, that they always follow a certain 
general rule. For instance, if the tide be 
now at high-water mark in any port or har- 
bour which lies open to the ocean, it will 
presently subside, and flow regularly back 
for about six hours, when it will be found at 
low-water-mark. After this, it will again 
gradually advance for six hours, and then 
return back in the same time to its former 
situation ; rising and falling alternately twice 
a day, or in the space of about twenty-four 
hours. And by observing the tides conti- 
nually at the same place, they will always 
be found to follow the same rule ; the time 
of high water upon the day of every new 
moon being nearly at the same hour, and 
three quarters of an hour later every suc- 
ceeding day. Let M (fig. 3.) represent the 
moon, O the centre of ths earth, and A, B, 
C, &c. different points upon its surface, and 
let us suppose the earth to be entirely co- 
vered by the ocean. Then, because it is 
the property of a fluid for its parts to yield, 
and obey any force impressed upon them, it 
is clear that the moon M, acting upon the 
surface of the sea at the points A, B, C, &c. 
will elevate the waters in those parts, and 
draw them towards her, by her attractive 
power. But the point A being nearer to 
the moon than the point C, the attraction at 
A will be greater than at C ; and because 
the points B and D are at equal distances 
from the moon, the attraction at those points 
will also be equal; and so at any other in- 
termediate points the attractive force will 
be different, according to their different 
distances from the moon. 
From this example then, it is sutficiently 
evident, that the attractive force of the 
F. e 
