# Chapter 4 Chapter 4 Reply from the Cambridge Observatory

Barbicane wasted not a single minute amidst the cheers for him.
His first act was to summon the members to the offices of the Cannon Club.There, after some discussion, it was agreed to consult astronomers about the astronomical part of the plan.As for the mechanism, we had to wait until we got a reply.In order to ensure the success of this great experiment, nothing should be neglected.
So they drew up a blunt notice, including various specialized investigations, and sent it to the Cambridge Observatory in Massachusetts.The suburbs of Cambridge, home of America's first university and known precisely for its observatory, are home to many scholars of great contribution, the plane that enabled Baldor to resolve Andromeda's meteor stream, and Clarke to discover the satellite of Sirius. The real, powerful telescope is there.This well-known institution proves that the Cannon Club has every reason to trust it.
So two days later, Chairman Barbicane received the reply that everyone was eagerly waiting for.The reply is as follows:
Letter from the Director of the Cambridge Observatory to the President of the Baltimore Cannon Club Cambridge, 7th October
After our station office received the letter sent to our station in the name of all members of your club on the 6th of this month, we immediately held a meeting.Regarding the questions raised in the letter, we believe that the answers should be as follows:
First question: "Is it possible to shoot a cannonball to the moon?"
Yes, we can fire a cannonball in both directions to the moon, provided the muzzle velocity of the cannonball reaches twelve thousand yards per second.Calculations prove that this speed is sufficient.When an object leaves the earth, its weight is inversely proportional to the square of the distance. In other words, if the distance is tripled, the weight will be reduced by nine times.The weight of the ball, therefore, decreases rapidly, and finally disappears completely when the gravitational forces of the moon and the earth are equal, that is to say, at 47/52 of the total distance.At this time, if the weightless shell passes through this point, it will fall to the moon due to the gravitational relationship of the moon alone.Theory shows that this experiment is absolutely achievable; as for whether it can be successful, it depends on your ability to use the launcher.
Second question: "What is the exact distance between the Earth and its satellites?"
The moon's orbit around the earth is not perfectly circular; it is elliptical, with the earth occupying the center of the two circles; therefore, the moon is sometimes closer to the earth.Sometimes far away, in astronomical terms, it is sometimes at perigee and sometimes at apogee.
However, the difference between the farthest distance and the shortest distance is quite large, so large that we cannot ignore it.Actually, the moon's apogee is 247,552 miles, and its perigee is 218,657 miles, a difference of 28,895 miles, or the total one-ninth of the distance.Therefore, the distance of perigee should be used as the basis for calculation.
The third question: "How long does it take for the shell to reach it, given a sufficient muzzle velocity, and when should it be fired, so that it falls at a given point on the moon?"
If the shell had maintained the initial velocity of twelve thousand yards per second which it left the earth, it would have reached its destination in only nine hours; Ten thousand seconds, that is to say, eighty-three hours and twenty minutes to reach the place where the gravitational forces of the earth and the moon are balanced, and another fifty thousand seconds, that is to say, thirteen hours, fifty-three minutes and twenty seconds, to reach the place where the gravity of the earth and the moon are balanced. to the moon.The shell should therefore be fired ninety-seven hours, 13 minutes and 20 seconds before the moon reached the point on which it was aimed.
Question 4: "When is the moon in the easiest position to hit?"
According to the above materials, we should first select the moment when the moon passes through the zenith at the same time as it reaches the perigee, so that the distance equivalent to the radius of the earth can be reduced, that is to say, the distance can be reduced by 3,919 miles; So the projected course of the shell was 214,976 miles.However, although the moon passes perigee every month, it does not necessarily pass the zenith at the same time.
It takes a long time for these two conditions to meet together.So one has to wait for the moment when it passes both perigee and zenith.As it happens, on December 4 next year, the moon will meet these two conditions: it will pass perigee at midnight, which means it will be closest to the earth at that time, and will pass through the zenith at the same time.
Question 5: "Which point in the sky should the cannon that fires the shell be aimed at?"
From the material mentioned above, it seems that the cannon should be aimed at the zenith; so that the firing line can be perpendicular to the horizontal plane, and the cannonball can escape the shackles of the earth's gravity more quickly.But for the moon to climb to the zenith, the latitude of this place must be lower than the inclination of the orbital plane of this celestial body, in other words, it must be between zero and twenty-eight degrees of south or north latitude.Anywhere else you would have to shoot at an angle, which would hinder the success of the experiment.
Question 6: "Where should the moon be in the sky when the cannonball is fired?"
When firing a cannonball, the moon, which advances 13 degrees, 10, 10, and 35 seconds every day, should be four times this degree away from the zenith, that is, 52 degrees, 42 minutes, and 20 seconds from the zenith. The time it takes for the cannonball to travel.But we should take into account the deviation of the shell caused by the rotation of the earth. The shell must go through a deviation equivalent to 16 earth radii to reach the moon. Calculated from the orbit of the moon, it is about 11 degrees, so it should be mentioned above Add these eleven degrees to the distance from the zenith of the moon to the zenith to get a total of sixty-four degrees.Therefore, when firing bubble bombs, the orientation of the moon must intersect the vertical line at an angle of sixty-four degrees.
Summary:
1. Cannons should be installed at places between 0° and 28° south latitude or north latitude.
2. The muzzle should be aimed at the zenith.
3. The shell should have a muzzle velocity of twelve thousand yards per second.
4. Shells should be fired ten minutes and twenty seconds before 11:00 p.m. on December 1 next year.
5. It will arrive at exactly midnight on December 4th, when the moon crosses the zenith, four days after its launch.
Members of your club should do without delay the various tasks required for such an enterprise, and must be ready for launch at the appointed time, for if the fourth of December is missed, eighteen years and eleven more years must pass. Only a genius can meet the same conditions that the moon passes through the perigee and zenith at the same time.
In terms of astronomical theory, our office is ready to assist you at any time, and together with the people of the whole country, I wish you success!
Director of the Cambridge Observatory
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