Year: 1874

VIC give poloW letter from Mr. Henry NJ Park
hurst, addressed to the editor Of the HERALD.
concerning the coming total lunar eclipse. 1t was
Mr.. Parknurst who conceived and delineated the
siagrams published in Thursdays HERALD The
sequel VIII be found interesting. not only on &C.
count OF the prominence OF the writer and the
amount Of scientific information 1t conveys, but
also on account Of the plain, simple and popular
manner IN which its subject matter is wrought up


mr.. Henry M. Parkhurst's Letter.
To THE EDITOR OF THE HERALD:-


SiR-It is safe to predict that the quiet OF the
next Sabbath morning will not be disturbed by an
alarmed populace beating upon pans and Kettles
or blowing tin horns to trighten away the dragon
devouring the moon and yet even the appear
Of new comet inspires less terror In the minds or
barbarous tribes than a lunar eclipse. When
science explains the causes OF these comparatively
unusual appearances in the heavens they are
viewed no longer with dismay but with intelligent
interest.


The fact that lunar eclipse never occurs except
IT fui moon naturally led to the conjecture that
1t is caused by the moon passing through the
shadow OF the earth. In the HERALD OF Thursday,
october %, there is given diagram OF the moons
path through the eartn's shadow, Illustrating the
different phases OF the eclipse. 1t may be inter.
estlng to learn how astronomers know that this is
anything more than conjecture. How DO they
know that It represents the actual fact


THE TRANSIT OF VENUS.


At the present moment the astronomers OF the
world are gathering in selected stations in the
Eastern Hemisphere and the Southern Ocean to
observe the transit OF Venus That planet now
shining SO brilliantly in our evening sky, and very
nearly Of the size OF the earth, will pass directly
between us and the sun, necessarily casting its
shadow toward u.s. Beng so much smaller than
~hn rufn0
passing across Its lace. Now, suppose that
In the midst OF the transit we should start, with
the speed OF light, and travel directly toward the
planet Venus. As we approach it 1t would ap.
parently grow larger, covering more and more
Of the sun, until finally, when we have
come within somewhat less than million miles or
IT, II would appear exactly large enough to cover
the entire face OF the sun. We should then be ex-
actiy at the end Of the shadow. Beyond this point
We could see the sun around 1t; within this polnt
IL would entirely eclipse the sun. At the end OF
the shadow the light which strikes the eye
from one side OF the sun just grazes that side OF
Venus, while the light which strikes the eye
from the opposite side OF the sun just
grazes the opposite side OF the planet. AS light
moves in straight lines we can represent this
upon paper. We draw a circle to represent the
sun. At the proper distance We draw q smaller
circle to represent Venus. Then we draw straight
lines Just tonching the edges Of the sun and Venus
and the point where those lines meet will repre-
sent the position OF the eye at the end OF the
shadow.


THE DIAGRAM.


Suppose that an inhabltanr ofsome outer planet
~ the moment the earth passed between that
planet and the sun. should approach in the same
way, he would reach the extremity or the earth's
shadow at q distance from the earth between
three and four times the distance OF the moon. In
the above diagram the sun Is represented upon
the lett, and to the right the earth, with the
moons orbit around lt and with Its black shadow
extending to the extreme right. Our traveller,
instead Of approaching nearer, now moves q little
toward side, and ands that the edge of the sun
becomes visible; but as most of the
sun is still covered by the earth its light ts much
dimmed. Passing iurther In the same direction
the sun comes more and more into view,
until At last he reaches point where the edges OF
the earth and sun just touch each other, and
there Is no obscuration whatever To represent
his position at this moment We draw line from
one edge OF the sun, just touching the opposite
edge OF the earth Anywhere between this line
and the former there Is some 0bscnrat aton, called
the penumbra. gradually increasing as we ap-
pr0ach the centre. The shadow Is conical. coming
TO point, but the penumbra, beginning with the
size OF the earth, continually expands IS we recede
from the earth and covers all points In space from
which the earth would appear ON the SUMS disk.
During the transit OF Venus, then, the earth will
be within its penumr.a.


1t is proper to say here that the diagram does
not represent the true relative sizes and distances
OF the sun, earth and moon, and TO explain the
reason. II had adopted q scale OF 10000, 900
miles to the inch the diagram would have been Of
the same length as above, but upon this scale the
sun would have been only one tenth OF an inch In
diameter, the earth only one -hundredth OF an inch
and the moon only one f0ur-hundredth OF an inch.
IL Is manifest that IL was impossible TO preserve
the true proportion, and l have therefore aimed
only TO represent the manner in which the eclipse
1s produced.


Our traveller, upon passing within the earth's
shadow, sees the sun cclipsed by the earth. In
precisely the same way the Inhabitants OF the
earth see the sun eclipsed BY the moon when II is
between rs and the sun. This is represented also
upon the diagram, where the moon is represented
to the left OF the earth with It's shadow coming
nearly to a point upon the earths surface. Per-
sons within that little circular shadow, less than
TWO hundred miles in diameter, will see total
eclipse OF the sun. Outside OF that shadow, and
within the penumbra, which Is not shown in the
diagram, the sun appears partially eciipsed


SOLAR AND LUNAR EcLIPsEs.


But large part OF the Inhabitants OF the earth,
20 whom the sun is visible and TO whom the moon
would also oe visible II 1t shone By It's own light,
do not see any eclipse, because they are outside
OF the penumbra, and the moon does not come
between them and the sun. But when the moon
moves around IN It's orbit to the opposite side
OF the earth and passes within tho earths
shadow II ceases TO receive light rom the sun,
and Of course becomes nearly invisible to all the
inhabitants OF that side OF the EARTH When .
total eclipse or tile sun occurs therefore but lew
witness IT. but when q total eclipse of the moon
occurs 1t is visible to a whole hemisphere. There
are more eclipses OF the sun than OF the moon,
the reason Of which will presently explain, and
yet eclipses OF the moon are much more frequently
seen In any particular place.


II the moons orbit were exactly in the same
plane with the eartn's orbit there would be total
eclipse OF every full moon. But In fact IL IL so
much inclined that the moon usually passes above