THE GKEAT ECLIPSE OE AUGUST 17, 1868. 
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If the earth were not rotating, the moon’s apparent path would 
be approximately straight during the transit; in fact, referred to 
the earth’s disc, it is so. But the region actually traversed by 
the moon does not, in either figure of Plate XXX., appear with 
straight edges. This is due to the earth’s rotation, which brings 
regions within the path of the shadow, which would not other- 
wise be eclipsed. 
We see also that the length of the region actually traversed 
by the black shadow, which would be a semi-circumference of the 
earth (in the case of an eclipse so nearly central) if there were 
no rotation, is diminished considerably through the effects of the 
earth’s axial rotation. . 
There is an oval in each figure, and each oval is divided by 
a curved line into two halves. The oval in fig. II. contains all 
those regions over which the sun is eclipsed totally or partially 
at rising. We see from fig. L, that the line of country extending 
from the Black Sea across Africa which has just reached the 
visible, that is, the illuminated half of the earth’s disc is in 
shadow ; in other words the sun is partially eclipsed at rising. 
The upper and lower intersections of the shadow’s outline with 
the circular boundary of the disc, mark two points at which the 
eclipse ends at the moment of sunrise, and these two points lie 
on the left-hand curve of the divided oval in fig. II. This half 
of the oval’s boundary contains all such points. The other half 
contains all points at which the eclipse begins at sunrise. The 
dividing line contains all points at which the middle of the 
eclipse occurs at sunrise. 
The oval in fig. I. contains all those regions over which the 
sun is eclipsed wholly or partially at sunset. The right-hand 
half contains all points at which the eclipse begins at sunset, 
the other contains all points at which the eclipse ends at sunset ; 
and the dividing line contains all points at which the middle of 
the eclipse occurs at sunset. In this case as in the former, one 
figure illustrates the other; we see, for instance, that a part of the 
region within the oval of fig. L, is on the edge of the disc and 
within the shadow of fig. II. ; that is, is partially eclipsed at 
sunset. The points of intersection of the shadow’s outline with 
the edge of the disc in fig. II. indicate, of course, points at 
which the eclipse begins at sunset. 
The region traversed by the eclipse could hardly have been 
better suited than it actually is, for the purposes of observation. 
Had it occupied any other part of the tropics, as the Pacific 
Ocean, the South American Continent, or Africa, it would not 
have been easy to supply skilled European observers in suffi- 
cient number, nor instruments of adequate power. If it had 
fallen much farther north or south, again (still lying within the 
tropics), the difficulties of observation would have been largely 
increased. 
