We then started upon our return home, and on the way I drew M’Allister’s attention to the smaller size of the sun as we saw it now as compared with the size it appeared to us when on the earth. I told him that Mars was then about 131,000,000 miles from the sun, so the sun’s apparent diameter was only about 22-1/4 minutes.
On the earth that day the sun’s apparent diameter would be about 32 minutes. So to the Martians the sun only appeared about two-thirds the size it appeared to the people on the earth.
When, on 13th August this year, Mars was at its “perihelion,” or nearest point to the sun, the latter was 129,500,000 miles distant, and would appear rather more than 22-1/2 minutes in diameter.
At the opposite point of its orbit, where it will be in “aphelion,” or farthest from the sun, the sun will only appear about 19 minutes in diameter.
I then explained that, although the sun is so distant, Mars receives a very much larger percentage of the total heat and light available than we do on the earth, because of the thinness and generally cloudless condition of the atmosphere. It is estimated that our atmosphere and clouds shut out nearly 50 per cent. of the light and heat which would otherwise reach us in the course of the year. On the other hand, their “blanketing” effect considerably lessens the amount of heat radiated into space; thus, by keeping in the heat we have received, compensating to some extent for the original loss in quantity.
But, owing to its thin clear atmosphere, Mars receives nearly 99 per cent. of the total amount of heat and light proceeding to it from the sun; so that, although the sun is more distant from the planet, the warmth on Mars does not compare so unfavourably with the warmth on the earth as many have imagined it to do.
M’Allister replied that “He had expected to find it very cold indeed upon Mars in consequence of its distance from the sun, but was surprised to find it so warm,” and added, “what you have now told me, Professor, explains why this is so, and I can only say that at present I find the climate a delightful one—pleasantly warm, yet bracing and invigorating. Even in the tropical regions, although it is hot, it is not the oppressive and enervating heat that I have experienced in the tropics on our own world.”
He then remarked that “He knew the planets all moved through space and had read that some of the stars did too, and he would like to know whether our sun had any motion in space?”
“Yes,” I replied; “as the result of a long series of observations and calculations it has been determined that the sun is moving through space and carrying with it all the planets in our system. Its rate of movement is not known with certainty, but it is estimated at about 1,000,000 miles a day. Whether it is moving in a straight line or in a vast orbit around some far distant sun is also an open question, and it may take centuries to arrive at a definite result. This motion of our sun, rapid though it is, is very slow compared with the motion of some of the stars. One that appears only a small star to us, but which is probably a sun enormously larger than ours, is moving through space at a rate which cannot be less than 200 miles a second; and unless that movement is direct across our line of sight its rate must be still more rapid. Yet it is so enormously distant that, in 500 years, it would only appear to have moved over a space of one degree on the sky! It is calculated that Arcturus moves still more rapidly.
“The movements of several other stars have been calculated; but the distance of the stars is so enormously great that the majority appear to have no movement at all, though probably not one of the heavenly bodies is at rest.
“It is estimated that the light of the nearest star we know of takes at least four years to reach the earth, yet light travels at the rate of 186,000 miles a second. We know of others whose light takes centuries to reach us, and, with regard to most of the stars, the light we see probably left them thousands of years ago.
“It is only when a star is so near to us that the earth’s revolution in its orbit is sufficient to cause a change in the apparent position of the star which can be measured with our instruments that any calculation can be made to determine its distance from us. In nearly all cases where the distance has been calculated, the change in position is so minute and difficult to measure accurately, that the results obtained can only be regarded as very rough approximations to the real distances.
“The universe is infinite in extent, and the human mind is quite unable to conceive what is really implied in the distances of the planets belonging to our own solar system; yet they are as nothing when compared with the distances of the fixed stars, either from the earth or from each other. We equally fail to realise the immense numbers of the stars. The camera, it is estimated, shows at least one hundred millions in the heavens; and our great telescopes can penetrate through inconceivable distances of space and render visible millions which the smaller instruments fail to reveal. Every increase of instrumental power, however, carries us still farther, and reveals more and more stars in deeper depths of the illimitable abysses of space.
“In these matters there is no finality, for though with telescopic aid:
‘World after world, sun after sun, star after star are past, Yet systems round in myriads rise more glorious than the last: The wondrous universe of God still limitless is found, For endless are its distances, and none its depths can sound!’”
CHAPTER XXI
OUR FIRST VIEW OF THE EARTH FROM MARS—A MARTIAN COURTSHIP
Within a few days we had our first glimpse of the earth from Mars. It appeared only as a very thin but bright crescent of light, as the lighted portion was less than one-twelfth part of the whole diameter of the disc, and it was only visible for a very short time.
Owing to the clear and thin atmosphere of Mars there is very little scintillation of the stars, and the crescent form of the earth at such periods as the present can plainly be discerned without the aid of a glass. To the Martians this is more readily seen than by us, as their eyes, being larger than ours, have a much greater light grasp.
For the same reason all the stars shine much brighter than they do in our skies, and many of the smaller ones which can be seen from Mars with the unaided eye, would here require a low power-glass to render them visible to us. The fact that Saturn has a ring is quite apparent to the Martian eye.
Day by day after this we saw the lighted area extending upon the earth, just the same as on the earth Venus can be seen with a telescope gradually to pass from the crescent phase to the gibbous form, and ultimately become full. Our earth is a morning and evening star to Mars the same as Venus is to the earth, according to its position with regard to the sun.
Whilst we were looking at the earth, I asked Merna “Whether he had ever seen the earth transit the sun as we occasionally see Venus or Mercury do so?”
He answered that “He carefully observed the last transit, which occurred on a date equivalent to our 8th May 1905, and was very interested in watching the earth pass, as a small black spot, across the sun’s disc. The moon did not commence to cross until 6 hours and 7 minutes later, by which time the earth had passed over three- quarters of the sun’s diameter. The earth was 8 hours and 42 minutes in transit, and the moon, which crossed a little lower down, was 8 hours and 31 minutes in crossing.”
“That must have been an interesting sight,” said John, “and I should like to have the opportunity of watching a similar transit.”
“I am afraid you never will,” said Merna, “for the transits only occur at long intervals. The previous transits occurred in November 1879, November 1800, May 1700, and May 1621. There will not be another until May 1984, and the next after that will not occur until November 2084.”
“I am sorry to hear that,” remarked John, “for even if I stayed here, I should not be likely to live long enough to see the next transit. Possibly you may do so, Merna; you are so much younger than I am.”
“Yes,” Merna replied, “it is not unlikely that I may see another such transit, for the average length of our lives on Mars is about equal to one hundred and thirty of your years, so that leaves me an ample margin of time.”
I then went on to remark that as another result of the thinness of the Martian atmosphere twilight is much shorter than on the earth, the light being less diffused when the sun is below the horizon, and refraction also considerably less than we experience.
In this connection, I mentioned to M’Allister that we can often see the sun and the moon apparently above the earth’s horizon when they are, in fact, below it. This is caused by the refractive power of our dense atmosphere, which has the effect of making both the sun and the moon appear a little higher up than they really are.
“That is something new to me, Professor,” exclaimed M’Allister; “and I cannot say I quite understand how
