THEORETICAL ASTRONOMY. 101 
be exactly determined, as the earth’s shadow fallmg on the moon 
(pl. 6, fig. 18), like any other shadow, is not bounded by sharp outlines, but 
fades off into light, and in every eclipse there is also distinguished a full 
shadow or umbra, and a half shadow or penumbra. From the preceding 
it is clear that an eclipse of the moon is not merely an apparent, but a real 
occurrence. Consequently, the inhabitants of all those portions of the earth 
in which the moon shines at the time of the eclipse, see every particular of 
the eclipse in the same manner and at the same time, even if the local times 
of the places should be different. 
Sometimes a black disk is seen to pass gradually from right to left before 
the sun. But as this phenomenon is not seen alike at all places above 
whose horizon the sun may be at the time, since in some countries the sun 
is covered more or less than in others, or not at all, it is evident that this 
phenomenon cannot be the result of an actual obscuration of the sun. It 
is rather produced by the moon (pl. 6, fig. 18, and pl. 14, fig. 56), since it 
only happens at the time of new moon. It has been observed that these 
uncommon phenomena, called eclipses of the sun, occur only when the new 
moon happens to be in or near one of its nodes. By its intervention 
between the earth and sun, it then hides the view of the latter entirely or 
partly from the former. The phenomenon exhibited, is precisely as if the 
sun in a cloudless sky were covered for a time by a little black cloud pass- 
ing over his disk. As the shadow of the cloud moves along in the plain 
beneath, in the direction in which the cloud is driven by the wind, and con- 
ceals from the observer whom it overtakes the sight of the sun, while others 
out of the bounds of the shadow still see that luminary; just so the shadow 
of the moon moves along over the earth’s surface from west to east in the 
direction of the moon’s motion around the earth, conceals from the coun- 
tries traversed by it the view of the sun, and produces the phenomenon of 
eclipse. All regions not thus traversed see the sun as usual. 
Eclipses of the sun are distinguished into partial, total, central, and annu- 
lar. An eclipse is partial, when the moon covers the sun only in part; 
total, when this covering is complete.. When the moon’s disk is apparently 
smaller than that of the sun and stands directly before him, the eclipse is 
annular. A total or annular eclipse is central when the centres of the sun 
and moon’s disk coincide. For the entire earth’s surface, a partial eclipse 
can last about 7 hours; one, partial and total, 4 hours and 48 minutes; 
but for a given place on the earth’s surface, a total eclipse cannot continue 
more than 4} hours at the very utmost. The calculation of the separate 
circumstances of an eclipse of the sun becomes, therefore, more difficult 
and circumstantial than those of the moon, since, as before mentioned, the 
former are not actual occurrences like the latter, but only apparent pheno- 
mena, whose shape, size, and duration depend upon the place of the observer 
on the earth’s surface. 
An eclipse of the sun cannot take place when the new moon is more than 
1° 35’ distant north or south of the ecliptic: 1° 24’ is the minimum distance 
that the moon can pass near the sun without causing an eclipse. A north 
or south latitude of the moon less than 1° 24’ at time of new moon always 
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