Year: 1869

BY T. II. sAFFORD, (DIRECTOR OF THE


DEARBORN OBSERVATORY, CHICAGO).


FROM time TO time it is noticed that the
sun and moon are partially or totally
eclipsed that is to say, in the suns case,
a portion or the whole of that luminary,
as we usually see it, is hidden by round,
dark body, now well known to be the
moon and in case the moon is eclipsed,
dark shadow appears to cross its disc.


We know by observation that when the
moon is eclipsed it is always full moon !;
that the earth is interposed between the
sun and the moon, and that it is the earth's
shadow which produces the eclipse. Again,
when the sun is eclipsed it is always new
moon and we always see the slight lunar
crescent called the new moon day or two
afterwards.


But not ct every full or new moon does
an eclipse take place only at certain
seasons of this kind, when the sun, earth
and moon are unusually near the same
right line for if the sun and moon be ap-
psrently in conjunction, that is, in the
same region of the Heavens, but at the
same time one appears so far above the
other that they escape appearing to touch,
there will be no eclipse And in the same
way, if the moon at its full does not pass
exactly through the earth's shadow, it may
pass over or under it, and so escape an
eclipse entirely.


Before going any further, it may be well
to etate that so-called total eclipse of the
moon does not cause the moon to disap-
pear entirely but that even then she still
shines with dusky light. More of this
further on.


There are three kinds of eclipses of the
sun-partial, the commOnest kind total,
the arest and third kind, annular,
neither partial nor total, strictly speaking
The reason of this third kind we must
think about for moment.


We all know very we'll that the sun and
moon appear to US very much of the same
size but they are very different in mag
nitude The sun Is much larger, and
about as much further of; so that, Is a
pane of glass in my room appears nearly
as large a. q great building at half mile
distance, 80 the sun and moon appear rela-
tively of the same size But if 90 nearer
t'Ne window, shall see the pane larger
nrop ortiOnsbly than the building and gic
eersa il 90 away from the window. If,
then. we ,,lace the moon at such q dis
tance from us, and in such place, that il
will anpear lust tn cover the whole sun,
and then approach nearer, the moon will
appear to grow larger faster than the sun
does, and total eclipse will take place.
On the other hand, if we go further from
the moon, it viii grow Ito us) smaller, and
will not cover the whole sun, but leave
ring of light outside And just the same
thing happens in nature, when the centres
of the sun and moon appear Just in the
same place to us; that is, when the eclipse
is central" as astronomers call it, The
moon is in this case sometimes so near as
entirely to cover the sun, sometimes so far
as to leave a ring of light around itself
the eclipse in the first case is called total
in the second case annular-or, as the Ger
mans say, ringformed.


| When do the eclipses of the sun Ia ke
placef that is, how can we predict them
To do this thoroughly and with extreme
accuracy, requires the tables of the suns
and moons motions, and q great deal oi
calculation. Large volumes are devoted
TO the purpose of telling exactly at what
point in the heavens the sun and moon
will be at any future instant, and do SC
with such accuracy that we can not fol
many years fail by one minute of time in
predicting when any eclipse will take
place and when even this degree of ac
curacy Is reached, corrections will be made
from the results of the daily and nightly
observations now making in oil civilized