In referring to the view of the earth from Mercury it was remarked that the moon is probably visible to the naked eye.  From Venus the moon is not only visible, but conspicuous, to the naked eye, circling about the earth, and appearing at times to recede from it to a distance of about half a degree—­equal to the diameter of the full moon as we see it.  The disk of the earth is not quite four times greater in diameter than that of the moon, and nowhere else in the solar system is there an instance in which two bodies, no more widely different in size than are the moon and the earth, are closely linked together.  The moons of the other planets that possess satellites are relatively so small that they appear in the telescope as mere specks beside their primaries, but the moon is so large as compared with the earth that the two must appear, as viewed from Venus, like a double planet.  To the naked eye they may look like a very wide and brilliant double star, probably of contrasted colors, the moon being silvery white and the earth, perhaps, now of a golden or reddish tinge and now green or blue, according to the part of its surface turned toward Venus, and according, also, to the season that chances to be reigning over that part.

Such a spectacle could not fail to be of absorbing interest, and we can not admit the possibility of intelligent inhabitants on Venus without supposing them to watch the motions of the moon and the earth with the utmost intentness.  The passage of the moon behind and in front of the earth, and its eclipses when it goes into the earth’s shadow, could be seen without the aid of telescopes, while, with such instruments, these phenomena would possess the highest scientific interest and importance.

Because the earth has a satellite so easily observable, the astronomers of Venus could not remain ignorant of the exact mass of our planet, and in that respect they would outstrip us in the race for knowledge, since, on account of the lack of a satellite attending Venus, we have been able to do no more than make an approximate estimate of her mass.

With telescopes, too, in the case of a solar eclipse occurring at the time of the earth’s opposition, they could see the black spot formed by the shadow of the moon, where the end of its cone moved across the earth like the point of an invisible pencil, and could watch it traversing continents and oceans, or thrown out in bold contrast upon the white background of a great area of clouds.  Indeed, the phenomena which our globe and its satellite present to Venus must be so varied and wonderful that one might well wish to visit that planet merely for the sake of beholding them.

Thus far we have found so much of brilliant promise in the earth’s twin sister that I almost hesitate to approach another phase of the subject which may tend to weaken the faith of some readers in the habitability of Venus.  It may have been observed that heretofore nothing has been said as to the planet’s rotation period, but, without specifically mentioning it, I have tacitly assumed the correctness of the generally accepted period of about twenty-four hours, determined by De Vico and other observers.  This period, closely accordant with the earth’s, is, as far as it goes, another argument for the habitability of Venus.