Afterward, Newton showed how it happens that the planets obey these laws, but as his part of the work had no particular reference to Mars, I say no more about it in this place.
Here, in Fig. 3, are the real paths of Mars and the Earth, and also of Venus and Mercury. No loops, you see, in any of them, simply because we have set the sun in the middle. Set the earth in the middle, and each planet would have its own set of loops, each set enormously complicated, and all three sets mixed together in the most confusing way. It is well to remember this when you see, as in many books of astronomy, the old theory illustrated with a set of circles looking almost as neat and compact as the set truly representing the modern theory. For the idea is suggested by this simple picture of the old theory that the theory itself was simple, whereas it had become so confusing that not merely young learners, but the most profound mathematicians, were baffled when they tried to unravel the motions of the planets.
I think the figure pretty well explains itself. All I need mention is, that while the shape and position of each path is correctly shown, the size of the sun at center is immensely exaggerated. A mere pin point, but shining with star-like splendor, would properly represent him. As for the figures of the earth and Mars, they are still more tremendously out of proportion. The cross-breadth of the lines representing these planets' tracks is many times greater than the breadth of either planet on the scale of the chart.
On September 5 the earth and Mars came to the position shown at E and M. You observe that they could not be much nearer. It is indeed very seldom that Mars is so well placed for observation. His illuminated face was turned toward the dark or night half of the earth, so that he shone brightly[page 29] in the sky at midnight, and can be well studied with the telescope.
When Galileo turned toward Mars the telescope with which he had discovered the moons of Jupiter, the crescent form of Venus, and many other wonders in the heavens, he was altogether disappointed. His telescope was indeed too small to show any features of interest in Mars, though the planet of war is much nearer to us than Jupiter. Mars is but a small world. The diameter of the planet is about 4,400 miles, that of our earth being nearly 8,000. Jupiter, though much farther away, has an immense diameter of more than 80,000 miles to make up, and much more than make up, for the effect of distance. With his noble system of moons he appears a remarkable object even with a small telescope, while Mars shows no feature of interest even with telescopes of considerable size.
It was not, then, till very powerful telescopes had been constructed that astronomers learned what we now know about Mars.[4]
It is found that his surface is divided into land and water, like the surface of our own earth. But his seas and oceans are not nearly so large compared with his continents and lands. You know that on our own earth the water covers so much larger a surface than the land that the great continents are in reality islands. Europe, Asia and Africa together form one great island; North and South America another, not quite so large; then come Australia, Greenland, Madagascar, and so forth; all the lands being islands, larger or smaller. On the other hand, except the Caspian Sea and the Sea of Aral, there are no large seas entirely land-bound. In the case of Mars a very different state of things prevails, as you will see from the three accompanying pictures (hitherto unpublished), drawn by the famous English observer, Dawes (called the Eagle-eyed). The third and best was drawn with a telescope constructed by your famous optician, Alvan Clark, of Cambridge, Massachusetts. The dark parts are the seas, the light parts being land, or in some cases cloud or snow. But in these pictures most of the lighter portions represent land; for they have been seen often so shaped, whereas clouds, of course, would change in shape.
The planet Mars, like our earth, turns on its axis, so that it has day and night as we have. The length of its day is not very different from that of our own day. Our earth turns once on its axis in —— but before reading on, try to complete this sentence for yourself. Every one knows that the earth's turning on its axis produces day and night, and nine persons out of ten, if asked how long the earth takes in turning round her axis, will answer, 24 hours; and if asked how many times she turns on her axis in a year, will say 365 times, or if disposed to be very exact, "about 365-1/4 times." But neither answer is correct. The earth turns on her axis about 366-1/4 times in each year, and each turning occupies 23 hours 56 minutes and 4 seconds and 1 tenth of a second. We, taking the ordinary day as the time of a turning or rotation, lose count of one rotation each year. It is necessary to mention this, in order that when I tell you how long[page 30] the day of Mars is, you may be able correctly to compare it with our own day. Mars, then, turns on his axis in 24 hours 37 minutes 22 seconds and 7 tenth-parts of a second. So that Mars requires 41 minutes 18 seconds and 6-tenths of a second longer to turn his small body once round than our earth requires to turn round her much larger body. The common day of Mars is, however, only about 39 minutes longer than our common day.
Mars has a long year, taking no less than 687 of our days to complete his circuit round the sun, so that his year lasts only about one month and a half less than two of ours.
