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REPLY FROM THE OBSERVATORY OF CAMBRIDGE
Barbicane, however, lost not one moment amid all the enthusiasm of which
he had become the object. His first care was to reassemble his colleagues in
the board-room of the Gun Club. There, after some discussion, it was agreed to
consult the astronomers regarding the astronomical part of the enterprise.
Their reply once ascertained, they could then discuss the mechanical means, and
nothing should be wanting to ensure the success of this great experiment.
A note couched in precise terms, containing special interrogatories, was
then drawn up and addressed to the Observatory of Cambridge in Massachusetts.
This city, where the first university of the United States was founded, is
justly celebrated for its astronomical staff. There are to be found assembled
all the most eminent men of science. Here is to be seen at work that powerful
telescope which enabled Bond to resolve the nebula of Andromeda, and Clarke to
discover the satellite of Sirius. This celebrated institution fully justified
on all points the confidence reposed in it by the Gun Club. So, after two days,
the reply so impatiently awaited was placed in the hands of President Barbicane.
It was couched in the following terms:
The Director of the Cambridge Observatory to the President
of
the Gun Club at Baltimore.
CAMBRIDGE,
October 7.
On the receipt of your favor of the 6th instant, addressed to
the Observatory of Cambridge in the name of the members of the
Baltimore Gun Club, our staff was immediately called together,
and it was judged expedient to reply as follows:
The questions which have been proposed to it are these—
“1. Is it possible to transmit a projectile up to the moon?
“2. What is the exact distance which separates the earth from its
satellite?
“3. What will be the period of transit of the projectile when
endowed with sufficient initial velocity? and, consequently, at what moment
ought it to be discharged in order that it may touch the moon at a particular
point?
“4. At what precise moment will the moon present herself in the
most favorable position to be reached by the projectile?
“5. What point in the heavens ought the cannon to be aimed at which
is intended to discharge the projectile?
“6. What place will the moon occupy in the heavens at the moment
of the projectile’s departure?”
Regarding the first question, “Is it possible to transmit a
projectile up to the moon?”
Answer.— Yes; provided it possess an initial velocity of 1,200
yards per second; calculations prove that to be sufficient. In proportion as we
recede from the earth the action of gravitation diminishes in the inverse ratio
of the square of the distance; that is to say, at three times a given distance
the action is nine times less. Consequently, the weight of a shot will
decrease, and will become reduced to zero at the instant that the attraction of
the moon exactly counterpoises that of the earth; that is to say at 4752 of
its passage. At that instant the projectile will have no weight whatever; and,
if it passes that point, it will fall into the moon by the sole effect of the
lunar attraction. The theoretical possibility of the experiment is therefore
absolutely demonstrated; its success must depend upon the power of the engine
employed.
As to the second question, “What is the exact distance which
separates the earth from its satellite?”
Answer.— The moon does not describe a circle round the earth, but
rather an ellipse, of which our earth occupies one of the foci; the
consequence, therefore, is, that at certain times it approaches nearer to, and
at others it recedes farther from, the earth; in astronomical language, it is
at one time in apogee, at another in perigee. Now the difference between its
greatest and its least distance is too considerable to be left out of
consideration. In point of fact, in its apogee the moon is 247,552 miles, and
in its perigee, 218,657 miles only distant; a fact which makes a difference of
28,895 miles, or more than one-ninth of the entire distance. The perigee
distance, therefore, is that which ought to serve as the basis of all
calculations.
To the third question.
Answer.— If the shot should preserve continuously its initial
velocity of 12,000 yards per second, it would require little more than nine
hours to reach its destination; but, inasmuch as that initial velocity will be
continually decreasing, it will occupy 300,000 seconds, that is 83hrs. 20m. in
reaching the point where the attraction of the earth and moon will be in
equilibrio. From this point it will fall into the moon in 50,000 seconds, or
13hrs. 53m. 20sec. It will be desirable, therefore, to discharge it 97hrs. 13m.
20sec. before the arrival of the moon at the point aimed at.
Regarding question four, “At what precise moment will the moon
present herself in the most favorable position, etc.?”
Answer.— After what has been said above, it will be necessary,
first of all, to choose the period when the moon will be in perigee, and also
the moment when she will be crossing the zenith, which latter event will
further diminish the entire distance by a length equal to the radius of the
earth, i. e. 3,919 miles; the result of which will be that the final passage
remaining to be accomplished will be 214,976 miles. But although the moon
passes her perigee every month, she does not reach the zenith always at exactly
the same moment. She does not appear under these two conditions simultaneously,
except at long intervals of time. It will be necessary, therefore, to wait for
the moment when her passage in perigee shall coincide with that in the zenith.
Now, by a fortunate circumstance, on the 4th of December in the ensuing year
the moon will present these two conditions. At midnight she will be in perigee,
that is, at her shortest distance from the earth, and at the same moment she
will be crossing the zenith.
On the fifth question, “At what point in the heavens ought the
cannon to be aimed?”
Answer.— The preceding remarks being admitted, the cannon ought to
be pointed to the zenith of the place. Its fire, therefore, will be
perpendicular to the plane of the horizon; and the projectile will soonest pass
beyond the range of the terrestrial attraction. But, in order that the moon
should reach the zenith of a given place, it is necessary that the place should
not exceed in latitude the declination of the luminary; in other words, it must
be comprised within the degrees 0@ and 28@ of lat. N. or S. In every other spot the
fire must necessarily be oblique, which would seriously militate against the
success of the experiment.
As to the sixth question, “What place will the moon occupy in the
heavens at the moment of the projectile’s departure?”
Answer.— At the moment when the projectile shall be discharged
into space, the moon, which travels daily forward 13@
10’ 35’’, will be distant from the zenith point by four times
that quantity, i. e. by 52@ 41’
20’’, a space which corresponds to the path which she will describe
during the entire journey of the projectile. But, inasmuch as it is equally
necessary to take into account the deviation which the rotary motion of the
earth will impart to the shot, and as the shot cannot reach the moon until
after a deviation equal to 16 radii of the earth, which, calculated upon the
moon’s orbit, are equal to about eleven degrees, it becomes necessary to
add these eleven degrees to those which express the retardation of the moon
just mentioned: that is to say, in round numbers, about sixty-four degrees.
Consequently, at the moment of firing the visual radius applied to the moon
will describe, with the vertical line of the place, an angle of sixty-four
degrees.
These are our answers to the questions proposed to the Observatory of
Cambridge by the members of the Gun Club:
To sum up—
1st. The cannon ought to be planted in a country situated between 0@ and 28@
of N. or S. lat.
2nd. It ought to be pointed directly toward the zenith of the place.
3rd. The projectile ought to be propelled with an initial velocity of
12,000 yards per second.
4th. It ought to be discharged at 10hrs. 46m. 40sec. of the 1st of
December of the ensuing year.
5th. It will meet the moon four days after its discharge, precisely at
midnight on the 4th of December, at the moment of its transit across the
zenith.
The members of the Gun Club ought, therefore, without delay, to commence
the works necessary for such an experiment, and to be prepared to set to work
at the moment determined upon; for, if they should suffer this 4th of December
to go by, they will not find the moon again under the same conditions of
perigee and of zenith until eighteen years and eleven days afterward.
The staff of the Cambridge Observatory place themselves entirely at
their disposal in respect of all questions of theoretical astronomy; and
herewith add their congratulations to those of all the rest of America.
For
the Astronomical Staff,
J. M. BELFAST,
Director of the Observatory of Cambridge.
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