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!'"