Here is a seventh. We said just now that our globe's axis of rotation is inclined, and everybody knows that the imaginary prolongation of this axis points towards the polar star. This axis itself is not fixed. It revolves in 25,765 years, keeping its inclination of 22 to 24 degrees, so that its prolongation describes a circle of 44 to 48 degrees in diameter—according to the epoch—on the celestial sphere around the pole of the ecliptic. It is in consequence of this displacement of the pole that Vega, in twelve thousand years, will again become the polar star, as she was fourteen thousand years ago. Seventh kind of movement.
An eighth motion, due to the action of the Moon on the equatorial swelling of the Earth, that of nutation, causes the pole of the equator to describe a small ellipse in eighteen years and eight months.
A ninth, due also to the attraction of our satellite, incessantly changes the position of the globe's centre of gravity and the Earth's place in space. When the Moon is in front of us, she accelerates the speed of the globe; when she is behind, she retards us, on the contrary, like a check-rein,—a monthly complication which is added to all the others.
When the Earth passes between the Sun and Jupiter, the attraction of the latter, in spite of its distance of 155,000,000 leagues, makes it deviate by 2 m. 10 sec. from its absolute orbit. The attraction of Venus makes it deviate 1 m. 25 sec. the other way. Saturn and Mars also act upon it, but more feebly. These are exterior disturbances, which make up a tenth kind of correction to add to the motion of our celestial barque.
The whole of the planets weigh about one seven hundredth part of the weight of the Sun; the centre of gravity around which the Earth annually turns is not in the very centre of the Sun, but far from the centre, and often even outside of the solar globe. Now, absolutely speaking, the Earth does not turn around the Sun; but the two heavenly bodies, Sun and Earth, turn about their common centre of gravity. Thus the centre of our planet's annual motion is constantly changing place, and we may add this eleventh complication to the others. We might even add many others to these; but the preceding ones are enough to make the degree of lightness and delicacy of our floating island appreciated, subject, as we have seen, to all the fluctuations of celestial influences. Mathematical analysis goes very far beyond this summary statement. It has found that the Moon alone, which seems to turn so peacefully about us, has more than sixty distinct motions.
The expression is therefore not exaggerated: our planet is but the plaything of the cosmic forces which accompany it in the meadows of the sky, and it is the same with everything existing in the universe. Matter is meekly obedient to force.
Where, then, is the fixed point which we desire for our support?
Our planet, then, formerly supposed to be at the base of the universe, is in fact kept up at a distance by the Sun, which makes the Earth gravitate about it with a speed corresponding to that distance. This speed, caused by the solar mass itself, keeps our planet at the same mean distance from the central star. A lesser speed would make the weight predominate, and would lead to the Earth's falling into the Sun; a greater speed, on the contrary, would progressively and infinitely send our planet away from its life-giving focus. But at the speed resulting from gravitation, our wandering home remains suspended in permanent stability, just as the Moon is upheld in space by the force of the Earth's gravity, which makes it circulate about the Earth with the speed requisite to maintain it constantly at the same mean distance. The Earth and the Moon thus form a planetary couple in space which sustain each other in perpetual equilibrium under the supreme domination of solar attraction. If the Earth existed alone in the universe, it would be forever motionless in the void, wherever it had been placed, with no power to descend or rise or change its position in any way whatsoever; these very expressions—to rise, descend, left or right—having no absolute sense whatever. If this same Earth, while existing alone, had received any impetus whatever, had been thrown with any speed in any direction, it would have whirled away forever in a straight line in that direction, never being able to stop or to slacken its pace or change its motion. It would have been the same thing if the Moon had existed alone with it; they would both have turned about their common centre of gravity, fulfilling their destiny in the same place in space, flying together, following the direction in which they had been thrown. The Sun existing and being the centre of its system, the Earth, all the planets and their satellites, are dependent upon it, and to it their destiny is irrevocably bound.
Is the fixed point that we are seeking, the solid base which we seem to need to insure the stability of the universe, to be found in that colossal and heavy globe, the Sun?
Assuredly not, since the Sun itself is not in repose, for it is bearing us and all its system away towards the constellation of Hercules.