Year: 1869

BY t. H sAFFORD, (DTRRCToR OF THE


DEARBORN OBSERVATORY, CHICAeO).


FRoN 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 at 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 APP
parently 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 it's full does not pass
W
pass over or under it, and so escape an
eclipse entirely.


Before going any further, it may be well
to state that q 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 rarest and q third kind, annular,
neither partial nor total, strictly speaking
The reason of this third kind we must
think about for a 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 off; 80 that, as a
pane of glass in my room appears nearly
as large as great building at half mile
distance, so the sun and moon appear rela
lively of the same size. But if go nearer
the window, shall see the pane larger
proportionably than the building and rice
oerSG If 60 away from the window. If,
then, we place the moon at such dis
lance from us, and in such place, that il
will appear just to cover the whole sun,
and then approach nearer, the moon will





does, and total eclipse will take place
On the other hand, if we go further from
the moon, il will grow Ito us) smaller, and
will not cover the whole sun, but leave a
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 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
placer 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 great deaf of
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 so
with such accuracy that we can not for
many yea rs fail by one minute of time in
predicting when any eclipse will take
place and when even this degree of &C
| curacy is reached, corrections will be made
from the results of the daily and nightly
observations now making in all civilized
countries. But it is comparatively easy to
predict when an eclipse will take place
with some approximation to the truth,
and by two considerations:


First, there are two days in every year
near which a new or full moon is likely To
/ bring an eclipse. For this year, these
dates are February 9Th and July Both and
so we find by the more refined calculation,
Il 81 - nnls IS
| January 9th and July 231, both these
dates being full moons; and of the sun
| February 11th and August 7th, both these
dates being new moons.


| Again, every echpEg is followed by q
somewhat similar eclipse-Visible, how
ever, in very different part of the eartE
at an interval of eighteen years, ter


days and q fraction; 80 that the eclipse


which took place in the forenoon of July
28th, 1851, will be followed by one in tn,


afternoon of August 7th, 1869. But thii


eclipse of 1851 was only partial in thu


United States, and total in a small part o


North America, and througha small belt O


country in Europe; that of 1869 will b.


total only in narrow belt in NOrtherI


Asia and North America belt which


passes through Illinois, as will be men


TONED by-And.by.


Between these two eclipses, July, 1851


and August, 1889, there have been oh


served about seventy others and it i


calculated that of these seventy, forty-on


have been of the sun aud twenty nine O


the moon and each one of them, excep


perhaps some of the smaller ones, wi


have its corresponding eclipse in abou


eighteen years and ten days after its owl


date.


Some one will here say that eclipses O


the moon are not rarer than those of tn


sun; but. on the contrary, much CON


mcner. On looking one moment at tn


subject, we see the fallacy of thii


Echpses of the moon can be seen every
where in that half of the earth for whic


the moon 10 up at the time of the Occur
renee. but eclipses OF the sun are only Wis


ble through AmAller space Ofcountry. Tn


great eclipse Of this year has its centre i


Alaska: 1t does not extend much south {
. the equator, nor q great way into Asia
E is in some degree visible over the whOl
4 | OF North America, and a little way int


the Atlantic Oc san and covers about on,


dfth OF the eArth's surface with the Vi
D rious boundary hnes of It's ViSibility.
u And when We come TO look ut Il
y extent OF total eclipse, we see that il e.