# Popular Science Monthly/Volume 40/January 1892/Communication with the Planets

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* COMMUNICATION WITH THE PLANETS.
By M. AMÉDÉE GUILLEMIN.
STRIKING discoveries in astronomy, of a character to excite the public mind, have been rare in recent years. Those who have kept in current with the work that has been done in that science are not ready to believe that this is because progress has not been made in it. As evidence of the new work accomplished by its students, and potentially fruitful work, too, we cite the preparation of a map of the sky, accomplished by the aid of photography, which gives the exact position of the stars to the fourteenth magnitude. The co-operation of observatories certainly assures the success of this immense work, which is now in process of execution. La Nature has made known the beginnings and has kept its readers in the current of the very minute and profound preliminary studies, without which the undertaking of operations of an extreme delicacy might have been compromised. It has also made clear the importance of the results to be obtained, and of the various consequences that would necessarily accrue from them. The problems of parallax or of stellar distances, of the proper motions of the stars, of nebulae, the search for minor planets and new comets, everything relative to the constitution of sidereal systems, may, by an attentive study of the plates of the new celestial maps, receive positive solutions. A new horizon is thus opened to science. These are not sensational novelties, like the appearance of a comet with a long, nebulous tail, which attracts the attention of idlers to the sky; but the importance of astronomical observations is not measured by the noise they make in the public ear. Yet, if the prize of a hundred thousand francs, which an honorable lady has recently bequeathed to the French Academy of Sciences, should be gained by some one, the resultant emotion would be legitimate. To establish voluntary and direct communication between the earth and a ​planet, or rather between its inhabitants and the inhabitants of a planet, would be something to sharpen the curiosity of the whole world. I do not see that astronomy or mankind would gain anything by it, but what conjectures, what paradoxes, what high fancies, we should enjoy if it were carried out!
The Academy is said to be disposed to accept the legacy, by virtue of a clause like that which makes the Bréant prize an annual recompense allotted to the authors of discoveries tending to advance the solution of the problem of a cure for cholera. In the same way, the income of the capital bequeathed by Madame Guzman will work in favor of investigations relating to the constitution of the heavenly bodies. I do not think I am hazarding much when I assert that it will be a long while before the new prize is awarded, in its totality at least. But this was doubtless not the opinion of the testatrix. Without going deeply into the question—for that would require a long discussion—the probable correctness of my prediction can be shown in a few lines.
To any one well acquainted with the present knowledge possessed by astronomers concerning the physical aspect of the stars of our system, it is evident that only two of the planets are in a condition to encourage the hopes of those who believe in the possibility of interplanetary communications, to wit, the moon and Mars—the moon especially. Its small distance of 240,000 miles, the clearness of its disk, the facility with which minor features can be distinguished upon it with the telescope, the absence of all cloudiness that can conceal spots upon it, make our satellite an eminently fitting body to which to send signals from the earth. We must believe that the inhabitants of the moon have not thought of this, or the numerous observers of its disk, the industrious authors of the lunar maps, the Beers, Mädlers, Schmidts, at least, would have perceived the signals. But stop. Are there, can there be, inhabitants in the moon, where air and water are absent? If there is any point generally admitted, it is the negative of this question.
Under these conditions, it seems idle for us of the earth to trouble ourselves about means of answering the inhabitants of the moon, or of ourselves provoking signals thence; and this is a pity, for the second heavenly body to be questioned, the planet Mars, is infinitely less favorable for the establishment of an interastral telegraphy. At its most favorable oppositions, Mars is still 42,000,000 miles from us, or a hundred and sixty times farther than the moon; while the diameter of its disk is only 25″. According to Schiaparelli, the smallest objects visible on its surface under the most favorable circumstances—such as a bright spot on a dark ground, or a dark spot on a bright ground—must have a diameter equal to a fiftieth part of that of the planet, or about ​eighty-five miles. This minimum can, it is true, be reduced by using large objectives permitting stronger magnifying; but even then it is certain that luminous signals, for example, visible from the earth on Mars, must have enormous dimensions.
The inhabitants of Mars, if more advanced in astronomical knowledge than we, as one of our imaginative astronomers supposes they are, would have, in case they should desire to start an exchange of telegraphic communications with their earthly neighbors, to give their signals diameters of miles in every direction. But would they think of it? The reciprocal question to this is the one that puzzles me. The earth, during all the oppositions of Mars, is in conjunction to it. It is lost in the rays of the sun, and invisible from Mars, unless it is in transit over the sun's disk. Then it is a little black, round spot, on which we have every reason to suppose the Martian astronomers will be able to distinguish nothing. The earth will be better situated at the quadratures, but also at a much greater distance.
I stop here, not desiring to discourage absolutely the candidate for the prize of one hundred thousand francs so generously and so imprudently offered to investigators. But my conclusion, which I have sufficiently foreshadowed, is, that the problem of interplanetary communication is still far from solution; and I believe I shall never be contradicted by real astronomers. I have faith in the indefinite progress of the science, while I am convinced that there are limits to this progress; but I believe also that there is no profit in letting the imagination chase chimeras, and I am free to avow that the desired communication is such to my eyes.—Translated for The Popular Science Monthly from La Nature.
The compilation of a digest of the literature of the mathematical sciences was suggested at the American Association by Prof. Alexander S. Christie. The digest should contain everything of value hitherto done in these sciences logically arranged, with each truth or method referred to its discoverer, and the whole thoroughly indexed. Mathematicians throughout the world should be invited to engage in the preparation of the work, and the co-operation of the British Association especially should be secured.