The Ways of the Planets

by Martha Evans Martin

Mars will be in conjunction in November of this year (1912), and will not be visible in the evening during 1913 until toward the end of the year. The next opposition after the publication of this book will occur in January, 1914. From that time until the following autumn the planet may be seen in the evening. In 1915 Mars will not be visible in the evening sky until late in the year. After November it will be in the east in the evening, rising earlier each evening, until at opposition, early in 1916, it will rise at sunset and will be visible in the evening during the entire summer and autumn of that year, but will not be extraordinarily bright. In 1917 it will be again invisible in the evening. In 1918 it will be in opposition in the early spring, and will shine in the evening all the rest of that year. It will not be visible in the evening in 1919, but will be in opposition again in the latter part of April, 1920, and will shine in the evening all of that year and the early part of the next, when it will again disappear from evening view and will not emerge again until it is nearing a fine opposition that will take place just at the beginning of the summer of 1922. The planet will then be in the constellation Scorpio, not far from Antares, and this will afford an excellent opportunity to see these two ruddy bodies near together.

In 1924 there will be an exceptionally brilliant opposition, which will occur during the last week of August, and the planet will then be about as brilliant as it ever appears, and will be very favorably situated for observation in the constellation Pisces. We shall then see Mars in the flame-like phase of his beauty, and he will dominate the evening sky during the whole of that summer. At oppositions such as this one Mars is more favorably situated for observation from the earth than any other heavenly body except the moon.

The next oppositions will take place the last week in October, 1926, in December, 1928, January, 1931, early March, 1935, the middle of May, 1937; and then we will have two more splendidly brilliant oppositions in July, 1939, and early October, 1941, respectively.

During the years that Mars does not appear in the evening we need not be deprived of a sight of the planet if we will look for it in the morning sky. A few months after conjunction it may be seen as a morning star, rising shortly before the sun. It rises earlier each morning, and hence can be seen each morning for a longer time. After its hour of rising has reached midnight it then passes into the evening sky and rises earlier each evening until it reaches opposition.

The movement of Mars among the stars, as we see it, is generally toward the east, and we can see by looking that it changes its place among the constellations in that direction, going from Aries to Taurus, from Taurus to Gemini, and so on. On each side of opposition, however, the planet appears for a few weeks to be moving westward among the stars. This is the retrograde motion which an outer planet appears to have when we are overtaking and passing it, and which has been explained in the chapters on the movements of the planets.

SIZE, ATMOSPHERE, AND TEMPERATURE

In size Mars is one of the smallest members of our solar family. Its mass is a little more than one-ninth that of the earth, and its entire surface is only about one-third as great as ours. It is the merest trifle more dense than Mercury, but only about sixty-six one-hundredths as dense as the earth. Its force of gravity is about thirty-six one-hundredths as powerful as that of the earth. A man weighing two hundred pounds here would be relieved of about one hundred and twenty-four pounds of his weight if transported to Mars, weighing there only seventy-six pounds, which would greatly increase his efficiency if he were in other respects the same.

It would necessarily follow that Mars, having such small force of gravity, could not long retain a heavy atmosphere, even if it had set out with such a one. No molecule of gas moving at a greater speed than three and thirteen-hundredths miles a second could be held by Mars in its atmosphere, and so much as it may have had of the rarer gases which move with great rapidity must have escaped long ago. But it did not begin life with an atmosphere heavy in proportion to that which the larger planets have. We have seen, in the case of Mercury, that being one of the small planets entails many restrictions in development. Such planets not only lose their atmosphere more quickly than the larger ones, but it is less dense to begin with. The atmosphere of Mars is probably no denser than we have at the tops of our highest mountains, more than likely not even so dense as that. There is some water vapor, and there are a few clouds most of the time; but in the main the atmosphere is so clear and thin that we can without any doubt see the actual surface of the planet. It is not certain that the clouds we see are formed from water vapor, as clouds of the ordinary kind are. It has been suggested that they may be simply dust-clouds. But this is as yet not much more than a suggestion, and nothing convincing has been offered to substantiate the idea. Even dust-clouds would need currents of air to create and carry them; so, whether dust or vapor, the presence of clouds implies an atmosphere.

The famous white polar caps, which furnish so many news items to the journals, are also of uncertain origin, and their true nature can be determined only by a fuller knowledge of the atmosphere of Mars. They appear in the winter season on the planet and disappear in its summer, so there seems to be no doubt that they are dependent in some way on the temperature in the polar regions of Mars. If they are hoar-frost or snow, they are condensations of water vapor; and, in that case, when they disappear there must be sufficient heat to melt them. It has been contended that the sun’s rays fall too obliquely on the poles of Mars to melt more than a few inches of snow, but that the caps may be light snow or frost, and thus capable of being dissolved by even such oblique rays of sunlight as they receive. Also it has been suggested that the deposit resembling snow may be carbon dioxide, which condenses into a white substance at a temperature more than a hundred degrees (-109° Fahr.) lower than is necessary to produce snow and melts at a correspondingly low temperature. What the nature of the phenomenon seen at the poles of Mars is depends largely upon what the temperature is; and the temperature in turn is dependent in some measure on the density and constitution of the atmosphere, as well as the planet’s distance from the sun.