Year: 1869

THE NATURE or AN ECLIPSE.


AS seen by the accompanying illustration
(Figure y, an eclipse of the sun is caused by the
shadow of the moon falling on certain parts of
the earth-like the passing shadow of u wind
dr(ted cloud on q river-and hiding the face, or
disc, of the sun at those districts. In exact Ian
guage it is the earth not the sun, thatis ecpsed;
and should the much-loOked for Man in the
Moon take celestial observation on the coming
Saturday, he will see the great sun shining with
all its dazzling splendor, while part of our big
globe will be enveloped in unusual darkness.
And this is how our light will be taken away
from us.


How Solar Eclipscs Occur.


The earth and moon are two spherical and
opaque bodies and the halves of both are con-
stantly Illuminated by the sun, while the other
halves are in the shade. While the moons da-
meter is 2160 miles and the earth's 1926 the suns
diameter is S52, 5S4 miles. Thus, as above re-
presented each of the two former throws behind
itself, of course in the direction opposite from
the sun, shadow conical in its form, and deter
mined as to its magnitude shape, etc, by inua-
riable mathematical principles.


This cone of shade extends over all the parts
of space from which the rays of sunlight are





body. 1t has two degrees of intensity. The
inner portion s, is called the shadow or umbra,
and if within its bounds, a spectator beholds q
total eclipse. The outer section, p, on either
side is called penumbra, and the section of the
earth over which it extends experiences par
tia eclipse. The changing from the penumbra
to the umbra is quite insensible, owing to the
gradual blendings of the shadngs; but at the
outer eXtremities the eclipse is but slight


The moon and the earth carry with them in
their revolutions these cones of shades, and it is
by the projecting of these partial or total
shadows upon each other that the phenomena of
eclipses are caused.


The calculation has been made and verified
that the greatest length of the moons shadow
is 1913 semi-dameters of the earth, at the same
time that we know that the least distance of the
moon from the earth is 55'95 semi-diamcters.
1t is thus plain that when n con junction of the
sun and moon happens at time when the
length of the shadow and the distance of the
moon from the earth are identical, or nearly So
with the foregoing values, the moons shadow
extends beyond the earth.


Fig. 1 explains, by its central bodies, the
reason of the approaching observation, and
shows that an eclipse of the sun can only hap
pen at the time of new moon. for then alone
occurs the conjunction of her interpostion be-
tween the earth and the sun, with the projection
of her cone of shade upon the earth. The lower
figure of the moon exhibits the phase of its
eclipse by the projection of the earths shadow
directly upon it at the time of full moon.


In brief, eclipses may be defined as short in
terrupton in the passage OF light to the earth,
which is either real, as in the case of the intru-
sion of the moon between the earth and the sun,
| pha, fha rth itSal(
passes between the sun and the moon. The
former comprises all phases of solar eclpses
and the latter the lunar eclipses. Solar eclpses
| always begin on the west side of the sun and
terminate on the east side; that is, the moon
moves over the sun from west to east. The
whole duration OF a total eclipse is about two
| hours, and that of the total phase two or three
minutes more or less. There is no premoniton
of a solar eclipse, the moon (whose dark side is
towards us) being invisible in the intensely bril-
liant beams of the adjacent sun, until her eastern
limb Cor rim) actually touches his western limb
Different Kinds of Solar EcIIpses.
| There are three classes of eclipses of the sun
| 1t is total when the dark disc of the moon en-
| trely covers the sun; partial, when only por-
ton of one side of the sun is covered; and
aanular, when the moon is at her furthest dis
| tance from the earth, so that her cone of sha.
dow blots out only the centre of the sun, leaving
the ripg-lke border unobscured. The visibility
or invisibility of the eclipse at any point de-
pends wholly upon the fact whether the cone
like shadow is projected upon that particular
surface of the earth or not.


Theory and observation alike prove that in
the period of eighteen years and eleven days
in which the moon passes to the same position
in reference to the sun and its nodes. e., one
of the two points in which the orbit of the
satellite intersects that of its primary-therC
are in general seventy eclpses, of which forty
one are of the sun and kyenty-nine of the moon
There cannot be more than seven eclipses il
one year, nor less than two; and if the latter
they must both be eclipses of the sun. Eclipse
of the sun can only be seen at one zone of th
earth, and that very narrow one; for th
dimensions of the shadow of the moon arr
small as compared with the earth. Out of tn
forty-ene solar eclipses which may occur il
eighteen years, there are in general twenty-eigh
of those which may become central according
to ciIcumstances-that is, either total or annu
lar 1t results from Du Sejour's calculation
that the greatest possib duration of an eclips
of the sun cannot be more than hours 20 min
A sec. at the equator, or hours 20 min. 80 sec
for the latitude of Paris In total eclipses tn
greatest possible duration of total obscuratio
NAV only be min. SO sec. at the equator and
min. 10 sec. at the latitude of Paris. Such com
binatons are, of course, very rare.