We usually think of gravity as a simple, large-scale force. There is nothing intricate about gravity. But I wonder if gravity can be more intricate than it gets credit for.
There is something about the earth and Mars that really gets my attention.
Orbits in the Solar System operate much like an arrangement of gears, except that they operate by gravity rather than by being in direct contact.
Let's start with the earth's orbit around the sun. The orbit is elliptical, with the earth being closest to the sun in January and furthest in June. The northern hemisphere of the earth is heavier than the southern hemisphere because of the mass of the continents, as most of the earth's land area is in it's northern hemisphere.
The earth is tilted on it's axis, relative to the plane of it's orbit around the sun, at an angle of 23 1/2 degrees. The earth's heavier northern hemisphere tilts toward the sun when the earth is furthest from the sun. This provides a kind of mechanical balance to the earth's orbit. The earth's mass, relative to it's orbit around the sun, is more consistent than if the heavier hemisphere tilted toward the sun when the earth is closest to the sun.
Next is the question of why the earth has to be tilted on it's rotational axis, relative to the plane of it's orbit around the sun, at all. We know that the earth's continents have drifted northward. This changed the earth's center of gravity although not, relatively speaking, by much.
But when the earth is in orbit around the sun, the line between the center of mass of the earth and of the sun must remain constant. It cannot just be changed by tectonic movement of the earth's continents, which is driven by the centrifugal force of the earth's rotation.
The way that this line is kept constant is by the earth tilting on it's axis by the 23 1/2 degrees. This is what gives us seasons, summer when our hemisphere is pointing toward the sun and winter when it is facing away.
The Solar System formed when a large star that preceded the sun exploded in a supernova. Only the largest stars explode in a supernova. Some of the matter that was scattered across space fell back together by gravity to form the sun and planets. We know that the sun is such a second-generation star because it already contains heavy elements that are beyond it's current stage in the fusion process.
The heavier matter that did not fall in to become part of the sun fell into groupings to become the planets. Eventually one geometric plane became predominant and this is the general plane in which the planets orbit today, sometimes referred to as the Ecliptic.
But after this plane of the rotation of the planets formed, it took time for all of the matter in orbit around the sun in other geometric planes to consolidate into the predominant orbital plane, the Ecliptic, by the gravity of the planets.
But if we can see how the earth had to tilt on it's axis, as the continents moved technically, so that the line between the centers of gravity of the earth and sun could be maintained, then doesn't it make sense that the center line of the masses of the planets around the sun would also have to be maintained?
What I find to be so interesting about the earth and Mars is that earth's northern hemisphere is heavier, due to the mass of the continents, while Mars' southern hemisphere is heavier, because it's average elevation is 1-3 km higher than it's northern hemisphere.
Presuming that the upper layers of Mars is made of rock that is of roughly the same density as that of the upper layers of rock on earth, the difference in mass between Mars' lighter northern hemisphere and heavier southern hemisphere is just about exactly the same as the difference between earth's heavier northern hemisphere and lighter southern hemisphere.
This is just so interesting. Could it be that it had to turn out like this so that the center line of the mass of the planets in orbit around the sun had to be maintained just as the earth tilted on it's rotational axis when it's continents moved tectonically northward so that the line between the centers of gravity of the earth and the sun would remain constant?
Both the continents on earth and the higher in elevation, and thus heavier, southern hemisphere of Mars are both the results of matter that had been orbiting the sun colliding with the planets. Having the earth's northern hemisphere heavier, while Mars' southern hemisphere is heavier by an approximately equal amount, kept the center line of the mass of the planets in orbit around the sun from changing.
In ways that mean gravity is capable of operating with greater intricacy than we usually give it credit for, the earth's continents moved tectonically northward, driven by the spin of the earth, so that the difference in mass between earth's northern and southern hemispheres was equal, but opposite, to the corresponding difference on Mars.
This maintained the center line of the masses of the planets in orbit around the sun, which could not just be changed from within, without input from outside. But it would have changed the line between the centers of mass of the sun and the earth. To avoid this, the earth tilted on it's axis by the 23 1/2 degrees.
To achieve further balance of mass, the earth's heavier northern hemisphere leans toward the sun when the earth is furthest from the sun, and the lighter southern hemisphere leans toward the sun when the earth is closest to the sun.
The reason that Mars' southern hemisphere is heavier than it's northern hemisphere is also due to balance. A planet's axis of rotation is defined as north-south. The rotation of a spherical body has to be balanced. If one hemisphere of the planet is significantly heavier than the other, it has to be either the northern or southern hemisphere, in order to keep the rotation balanced. It cannot be the eastern or western hemisphere, as this will unbalance the rotation. The planet will shift so that the heavier hemisphere is either the northern or southern.
Another way we can see how important balance is in the Solar System is Kepler's Law that a line between the center of a planet and the center of the sun must sweep over equal areas of space in equal periods of time.
The planetary orbits are elliptical, rather than circular. It has to be this way because planets formed by smaller orbiting pieces collecting together by gravity. The orbits of the pieces that were closer to the sun, as well as those that were further, have to still be maintained and the way this is done is for the planet to be in an elliptical orbit. But the planet moves faster in orbit when it is closer to the sun so that balance is maintained as the line between the centers of the planet and the sun sweep over equal areas of space in equal periods of time.
So we can see how important balance is in the operation of the Solar System. The way it seems is that gravity is capable of more intricacy than is generally supposed in maintaining that balance.
The rotation of the earth caused it's continents to move tectonically northward in a way that counterbalanced the difference in mass between Mars' lighter northern and heavier southern hemispheres by making the earth's northern hemisphere heavier by just about exactly the same amount that Mars' southern hemisphere is heavier.
This maintains the center line of the orbital plane of the planets around the sun. But it couldn't possibly have come about by the slight direct gravity between the earth and Mars. The gravitational influence would have had to go through the sun.
This can only mean that gravity, the simple attractive force that dominates the universe on a large scale, must be capable of far greater intricacy than it is generally given credit for.
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