How does gravity move the earth

The Influence of Gravity on Earth Movement

Publication: 1.9.2000
Origin: La main à la pâte, Paris

The linear movement of a body is changed by the action of a gravitational force; it is then neither straight nor uniform. If we limit ourselves to two mutually attractive bodies and no other forces are acting, we can distinguish three cases: fall and impact, deflection, entry into orbits. Let us examine the three cases under the simplifying assumption that one of the bodies (A) does not move (for this it must be held by sufficiently large forces, which is not the case for any body in space, or its mass must be that large in proportion to the mass of body B that it moves negligibly little).

a) Impact or collision

If the body B has a practically vanishing initial speed, or is moving exactly towards A, A will accelerate its movement; it will move faster and faster towards A until it touches the surfaces, i.e. until the impact with all its consequences (in space, an elastic collision, such as between two billiard balls, is rare!). Something like this happens every day with numerous meteorites that are captured by the earth's gravitational field.

If the size of the meteorites is between that of a speck of dust and a few millimeters, a falling star is created when passing through the atmosphere, meteorites a few centimeters in size get a place in the museum, and meteorites that are over 10 meters high make a sizable crater of several kilometers in diameter, as can be seen on the surface of the moon and also in some places in the earth's crust, where erosion by wind and rain has not made them disappear (Meteor Crater in Arizona - approx. 1.2 km, Manicouagan / Québec - approx . 70 km). A meteorite of a few kilometers (about 10!), After all, would release such colossal energy on impact that fires of unimaginable proportions would break out all over the world and a large part of the developed life forms would disappear from the globe for a long time. This is one of the hypotheses some have used to explain the sudden disappearance of the great dinosaurs.

Website with satellite images of meteorite craters and asteroid impacts worldwide:

Animation of the collision of two bodies
The animation as a sequence of images

b) distraction

If body B does not move in direction A and its speed is very high in relation to the changes in speed due to the attraction by A, this attraction will only lead to a bending of the path leading to a hyperload that is more or less open, depending on the initial conditions. B is distracted and, far from A, returns to straight and uniform motion.

Animation of the deflection of a body B by a body A.
The animation as a sequence of images

c) turning into an orbit (satellite formation)

If the speed of B is not great enough to free the body from the attraction of A (as in case b), but also not so small that it moves towards body A and finally collides with it (as in Case a), B will enter an orbit around A, which is always an ellipse. An ellipse is a geometric shape, a flattened circle, for the construction of which one can, for example, start from two centers, the so-called focal points of the ellipse. When B moves in an orbit around A, A occupies one of the two focal points. The shape of the ellipse (length of the two axes and eccentricity) is fixed once and for all, if you know the location and speed (direction and size) of B at a given moment, you can calculate the whole curve.

Turning into orbit

If the body A is not held, both bodies carry out a movement of the same kind. So earth and sun describe an orbit around their center of gravity. That is only a few km from the center of the sun (the earth's mass is 6 × 1024 kg, that of the sun at 2 × 1030 kg, the sun weighs about 330,000 times more than the earth). The orbit of the sun, if only the earth existed, would be an ellipse with a major axis of a few km, which is negligible in relation to the extent of the sun (solar radius = 696,000 km, i.e. 109 times more than the earth's radius). Incidentally, the presence of other, weighty planets and thousands of small bodies in the solar system makes the sun's small "vibrations" much more complicated, but they remain negligibly small.

The solar system consists, as it is defined, of the set of bodies "captured" by the gravitational field of the sun, which move around them in elliptical orbits. Some of them are also "captured" by bodies that are larger than themselves and orbit them. Then one speaks of Satellites. This applies to the moon and - to expand the meaning of the name - to the "moons" of other planets.

d) Summary

The earth orbits the sun in an elliptical orbit. The ellipse that describes the earth is almost a circle (the eccentricity is almost zero) because its perihelion (the smallest distance from the sun) is approximately 149 million km, while the aphelion (the greatest distance between the earth and the sun) 153 million km. For most considerations, this small difference can be neglected. We will therefore see the earth's orbit as a perfect circle that will be traversed in 365 days. If we depict the earth's orbit on a piece of paper, this difference in line thickness disappears.

Last update: 3.1.2018

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