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 12,000 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 47/52 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._