The earth and Mars approach each other every 780 days, this being the synodical period of Mars.
But because of the ellipticity of the two orbits and the difference in the direction in which their longer radii are turned, the closeness of Mars and the earth is not the same at every opposition.
At each seventh approach, which occurs every fifteen years, when Mars passes through that part of its orbit which is closest to the sun, and the earth simultaneously passes the segment of its orbit which is farthest from the sun, the conjunction of the two planets is especially close and is
"the favorable opposition." These occasions are eagerly awaited by astronomers, for no celestial body,
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with the exception of the moon, is more readily observable than Mars when at "favorable opposition."
The distance between Mars and the earth at the oppositions varies from 61,000,000 miles to 35,500,000 miles ("favorable opposition"); the distance at various times during the period of fifteen years varies greatly, from 248,600,000 to 35,500,000 miles.
Two cosmic disturbances recorded by Hebrew tradition—one on the day when Hezekiah's father, Ahaz, was entombed; the other, when Sennacherib's army invaded Palestine—were separated by a period of fourteen or fifteen years, if the figure in II Kings 18 : 13 refers to the invasion which ended in the disaster. A seemingly arbitrary period of fifteen years of grace, mentioned in Isaiah 38 : 5 and in II Kings 20 : 6, may also have had some relation to the periodicity of the catastrophes. The years -776, -747, -717 or -702, and -687 apparently were years of favorable oppositions of Mars, when perturbations, a regular phenomenon in oppositions, reached catastrophic dimensions.
If, because of other reasons, contact between Mars and the earth in the past is admitted, the combined shape of the orbits, with points of nearest approach being reached at present every fifteen years, could be regarded as a vestige of a contact or series of contacts at similar intervals in the past between the two planets then revolving on curved orbits that were closer to each other.
Mars bears a striking resemblance to the earth in the inclination of its axis of rotation to the plane of its orbit and in the period of its diurnal rotation. Whereas the equator of the earth is inclined 23% degrees to the plane of the ecliptic, the equator of Mars is inclined 24 degrees to the plane of its orbit, a similarity unequaled among other planets in the solar system. The mean time of axial rotation of the earth is 23 hours, 56 minutes, 4 seconds, that of Mars 24 hours, 37 minutes, 23 seconds. No other two planets are so alike in the duration of their day, conceding that no conclusive data are available for the length of the day on Venus.
Is it possible that the axis of rotation and the velocity of rotation of Mars, stabilized and supported in their present position and rate
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by certain forces, were influenced originally by the earth at the time of contact? Mars, being small as compared with the earth, influenced to a lesser degree the rotation of the earth and the position of its poles.
The surface of Mars is crisscrossed with a network of "canals." Their discoverer, Schiaparelli, assumed that geological forces were a factor in their formation; on the other hand, he was "very robin-bobin
careful not to combat this supposition, which includes nothing impossible," of the presence of intelligent beings on Mars who could have built these canals.
Percival Lowell spent his life in a crusade to convince fellow scholars and other contemporaries that intelligent human beings live on Mars and that the canals are their work. From his observatory in Flagstaff, Arizona, he believed he discovered water on Mars. He interpreted the polar caps as ice masses; because of the dearth of water, the intelligent beings dug the canals to bring water to desert areas.1
In the early years of the twentieth century, plans were devised to communicate by light signals with the hypothetical men on Mars; according to one plan a series of light-sending stations was to be built into a geometric figure on the planes of Siberia. The figure was to represent the Pythagorean theorem of the relation of the three sides of a right-angle triangle. If there are intelligent beings on Mars, some writers argued, they should be able to notice and interpret the signals; if they are not intelligent enough to notice the signals and understand their meaning, we should not be so eager to communicate with them. The experiment was not carried out.
The contacts of Mars with other planets larger than itself and more powerful make it highly improbable that any higher forms of life, if they previously existed there, survived on Mars. It is, rather, a dead planet; every higher form of life, of whatever kind it might have been, most probably had its Last Day. Their work could not survive either. The "canals" on Mars appear to be a result of the play of geo-ip. Lowell, Mars (3rd ed., 1897); idem, Mars and Its Canals (1906).
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logical forces that answered with rifts and cracks the outer forces acting in collisions.
The Atmosphere of Mars
The atmosphere of Mars is invisible. If there are any living creatures on that planet, and if they are endowed with organs of sight, they see a black sky, not a blue one as we do.
The atmosphere of Mars was the object of many investigations which produced conflicting and apparently unsatisfactory results. This gaseous envelope is transparent, permitting clear observation on the contours of the planet. Mars' seasonal polar caps are products of distillation: a polar cap disappears when summer arrives in its hemisphere and reappears in winter. It is not known whether these caps are composed of carbon dioxide or of ice, whether they are clouds floating over polar regions or layers of coagulated masses.
The general question as to the presence of water vapor in the atmosphere of Mars was answered in the affirmative by one group of observers (Lowell Observatory), and in the negative by another group (Lick Observatory). At present it is regarded as almost certain that there is on Mars only a low absolute content of water vapor, about one-twentieth of that in the atmosphere of the earth. This is the view supported by results announced by astronomers at Mount Wilson Observatory.
The observations concerning oxygen in the atmosphere of Mars are somewhat inconclusive; it is generally supposed that oxygen on Mars, if there is any, is less than 0.1 per cent of the oxygen content in the atmosphere of the earth per unit of surface area.1
The difficulty of a spectral analysis of the atmosphere of the planets lies in the fact that their light is the reflected light of the sun, and consequently it has in it the spectral picture of the atmosphere of the sun (emission lines of the spectrum), and also in the fact that the atmosphere of the earth, through which this reflected light travels, impresses its own characteristic spectral lines (of absorption) on the
1 W. S. Adams and T. Dunham, Contributions from the Mount Wilson Observatory, No. 488
(1934).
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light reflected from the planets. The conclusion drawn and communicated to the general public is that "Mars' spectrum is practically that of reflected sunlight only" (E. Doolittle). This would suggest that there is no atmosphere on Mars or that it is very tenuous. However, there is a change in the distribution of light through the spectrum as compared with the light that arrives directly from the sun. The presence of an atmosphere on Mars can be proved by another set of robin-bobin
observations, which indicate that it extends to about sixty miles above the surface of the planet.