by Jules Verne
CHAPTER IV.
REPLY FROM THE OBSERVATORY OF CAMBRIDGE.
Barbicane, however, lost not one moment amidst all the enthusiasm of whichhe had become the object. His first care was to reassemble his colleaguesin the board-room of the Gun Club. There, after some discussion, it wasagreed to consult the astronomers regarding the astronomical part of theenterprize. Their reply once ascertained, they could then discuss themechanical means, and nothing should be wanting to ensure the success ofthis great experiment.
A note couched in precise terms, containing special interrogatories,was then drawn up and addressed to the Observatory of Cambridge inMassachusetts. This city, where the first University of the United Stateswas founded, is justly celebrated for its astronomical staff. There areto be found assembled all the most eminent men of science. Here is tobe seen at work that powerful telescope which enabled Bond to resolvethe nebula of Andromeda, and Clarke to discover the satellite of Sirius.This celebrated institution fully justified on all points the confidencereposed in it by the Gun Club.
So, after two days, the reply so impatiently awaited was placed in thehands of President Barbicane.
It was couched in the following terms:--
"_The Director of the Cambridge Observatory to the President of the GunClub at Baltimore._
"CAMBRIDGE, _Oct._ 7.
"On the receipt of your favour of the 6th inst., addressed to theObservatory of Cambridge in the name of the Members of the BaltimoreGun Club, our staff was immediately called together, and it was judgedexpedient 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 itssatellite?
"'3. What will be the period of transit of the projectile when endowedwith sufficient initial velocity? and, consequently, at what moment oughtit to be discharged in order that it may touch the moon at a particularpoint?
"'4. At what precise moment will the moon present herself in the mostfavourable position to be reached by the projectile?
"'5. What point in the heavens ought the cannon to be aimed at which isintended to discharge the projectile?
"'6. What place will the moon occupy in the heavens at the moment of theprojectile's departure?'
"Regarding the _first_ question, 'Is it possible to transmit a projectileup to the moon?'
"_Answer_.--Yes; provided it possess an initial velocity of 1200 yardsper second; calculations prove that to be sufficient. In proportion aswe recede from the earth the action of gravitation diminishes in theinverse ratio of the square of the distance; that is to say, _at threetimes a given distance the action is nine times less._ Consequently, theweight of a shot will decrease, and will become reduced to zero at theinstant that the attraction of the moon exactly counterpoises that ofthe earth; that is to say, at 47/52 of its passage. At that instant theprojectile 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 absolutelydemonstrated; its success must depend upon the power of the engineemployed.
"As to the _second question_, 'What is the exact distance which separatesthe earth from its satellite?'
"_Answer._--The moon does not describe a circle round the earth, butrather an _ellipse_, of which our earth occupies one of the _foci;_ theconsequence, therefore, is, that at certain times it approaches nearerto, and at others it recedes farther from, the earth; in astronomicallanguage, it is at one time in _apogee_, at another in _perigee._ Now thedifference between its greatest and its least distance is too considerableto be left out of consideration. In point of fact, in its apogee the moonis 247,552 miles, and in its perigee, 218,657 miles only distant; a factwhich makes a difference of 28,895 miles, or more than one ninth of theentire distance. The perigee distance, therefore, is that which ought toserve as the basis of all calculations.
"To the _third_ question:--
"_Answer._--If the shot should preserve continuously its initial velocityof 12,000 yards per second, it would require little more than ninehours to reach its destination; but, inasmuch as that initial velocitywill be continually decreasing, it results that, taking everything intoconsideration, it will occupy 300,000 seconds, that is 83hrs. 20m. inreaching the point where the attraction of the earth and moon will be_in equilibrio._ From this point it will fall into the moon in 50,000seconds, or 13hrs. 53m. 20sec. It will be desirable, therefore, todischarge it 97hrs. 13m. 20sec. before the arrival of the moon at thepoint aimed at.
"Regarding question _four_, 'At what precise moment will the moon presentherself in the most favourable position, &c.?'
"_Answer_.--After what has been said above, it will be necessary, firstof all, to choose the period when the moon will be in perigee, and alsothe moment when she will be crossing the zenith, which latter event willfurther diminish the entire distance by a length equal to the radius ofthe earth, i.e. 3919 miles; the result of which will be that the finalpassage remaining to be accomplished will be 214,976 miles. But althoughthe moon passes her perigee every month, she does not reach the zenithalways at _exactly the same moment._ She does not appear under these twoconditions simultaneously, except at long intervals of time. It will benecessary, therefore, to wait for the moment when her passage in perigeeshall coincide with that in the zenith. Now, by a fortunate circumstance,on the 4th December in the ensuing year the moon _will_ present these twoconditions. At midnight she will be in perigee, that is, at her shortestdistance from the earth, and at the same moment she will be crossing thezenith.
"On the _fifth_ question, 'At what point in the heavens ought the cannonto be aimed?'
"_Answer_.--The preceding remarks being admitted, the cannon ought tobe pointed to the zenith of the place. Its fire, therefore, will beperpendicular to the plane of the horizon; and the projectile will soonestpass beyond the range of the terrestrial attraction. But, in order thatthe moon should reach the zenith of a given place, it is necessary thatthe place should not exceed in latitude the declination of the luminary;in other words, it must be comprised within the degrees 0 deg. and 28 deg.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 theheavens at the moment of the projectile's departure?'
"_Answer_.--At the moment when the projectile shall be dischargedinto space, the moon, which travels daily forward 13 deg. 10' 35", willbe distant from the zenith point by four times that quantity, i.e. by52 deg. 42' 20", a space which corresponds to the path which she willdescribe during the entire journey of the projectile. But, inasmuch asit is equally necessary to take into account the deviation which therotary motion of the earth will impart to the shot, and as the shotcannot reach the moon until after a deviation equal to 16 radii of theearth, which, calculated upon the moon's orbit, are equal to about elevendegrees, it becomes necessary to add these eleven degrees to those whichexpress the retardation of the moon just mentioned: that is to say, inround numbers, about 64 degrees. Consequently, at the moment of firingthe visual radius applied to the moon will describe, with the verticalline of the place, an angle of sixty-four degrees.
"These are our answers to the questions proposed to the Observatory ofCambridge by the members of the Gun Club:--
"To sum up,--
"1st. The cannon ought to be planted in a country situated betweenbetween 0 deg. and 28 deg. of N. or S. lat.
"2ndly. It ought to be pointed directly towards the zenith of the place.
"3rdly. The projectile ought to be propelled with an initial velocity of12,000 yards per second.
"4thly. It ought to be discharged at 10hrs. 46m. 40sec. of the 1stDecember of the ensuing year.
"5thly. It will meet the moon four days after its discharge, preciselyat midnight on the 4th De
cember, at the moment of its transit across thezenith.
"The members of the Gun Club ought, therefore, without delay, to commencethe works necessary for such an experiment, and to be prepared to setto work at the moment determined upon; for, if they should suffer this4th December to go by, they will not find the moon again under the sameconditions of perigee and of zenith until eighteen years and eleven daysafterwards.
"The Staff of the Cambridge Observatory place themselves entirely attheir disposal in respect of all questions of theoretical astronomy; andherewith add their congratulations to those of all the rest of America.
"For the Astronomical Staff,
"J. M. BELFAST,
"_Director of the Observatory of Cambridge._"
