264 
POPULAR SCIENCE REVIEW. 
many respects the most remarkable eclipse that has taken 
place within historical times, or that will take place for a 
thousand and more years.* 
A dissertation upon the theory of eclipses would be very 
much out of place in these pages, even if space could be spared 
for it. But there are one or two points not treated of satis- 
factorily in most astronomical works, which require to be briefly 
discussed, in order that the reader may be able to understand 
the importance of such an eclipse as that of August 17th next. 
Fig. 1 (Plate XXIX.) illustrates the general theory of eclipses 
both of the sun and moon. The cone, e c e', is the shadow of 
the earth ; so that c e, c e' produced would touch the sun's globe. 
In reality, the angle e c e' is very much smaller than it is repre- 
sented in the figure : it is, in fact, about half a degree. The 
distance ec is variable, being as great as 870,300 miles when 
the earth is in aphelion, and as small as 843,300 miles when 
the earth is in perihelion. The moon travels round the earth in 
an orbit w^hose mean radius is 238,770 miles. Thus it is seen 
that the shadow of the earth extends nearly four times as far 
from the earth as the moon's orbit. Now it is obvious that, if 
the moon when in opposition — that is, towards m m — passes 
wholly or in part within the cone e c e', she will be totally or 
partially eclipsed. And if the moon, when in conjunction — that 
is, towards m' m ' — falls wholly or in part within the cone 
m' c m', she must throw a shadow upon the earth ; or, in other 
words, there will be an eclipse of the sun. 
Now, opposite m m, the cone is considerably smaller than it 
is opposite m' m'. The moon, therefore, in travelling round 
the earth — in a plane inclined to e c at a small angle — passes 
more frequently within the part of the cone opposite m'm' 
than within the part opposite mm; in other words, there are 
more solar than lunar eclipses.f But when the moon is eclipsed, 
it is clear, from Fig. 1, that she can be seen from a whole hemi- 
sphere. On the contrary, the shadow of the moon covers a com- 
* It will perhaps come one day to he recorded among the coincidences 
which the history of science has presented, that the invention of spectro- 
scopic analysis should have preceded, by only nine j^ears, the occurrence of 
this great eclipse. 
t If, however, the passage of any part of the moon through the penumbra, 
— represented by the shaded space between the lines e m, e' 7ii — were looked 
on as a partial eclipse of the moon, lunar eclipses would he slightly more 
numerous than solar ones. For it is easily shown that m m slightly exceeds 
m' m', since c m', c m', m e, and m e', all touch the sun’s globe ; hut the.two 
former, proceeding from a more distant intersecting point, are inclined at a 
smaller angle than the two latter. However, astronomers take no notice of 
any lunar eclipses in which the moon’s body does not pass wholly, or in part, 
within the true umbra. 
