In astronomical terms, the strain that is produced on a celestial object undergoing variations in gravitational attraction, variations that are cyclic in nature as it orbits, is termed tidal friction. Tidal friction is also observed in cases when one celestial object is orbiting another such as the Earth and the moon.
Tidal friction is usually seen occurring between sea bottoms and water tides, especially in parts where the sea is relatively shallow. It is also observed between parts of the planet’s or satellite’s solid crust that rotate against each other.
Tidal Friction Theory
The tidal friction theory was first developed after 1879 in mathematical terms. George Darwin, the English astronomer and son of Charles Darwin is credited with having formulated the mathematical theory of tidal friction.
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The moon’s gravitational force and the tidal force of the sun cause the tides in the oceans. The inverse square law states that a net stretching force is produced as a result of the force being greater on the nearer side as compared to the force on the far side. This is the principle behind tidal forces stretching the earth in the direction of the body that produces the tide. Tidal stretching, while more prominent in the oceans is also observed in the case of landmasses.
The character of the orbit of an orbiting body is gradually changed by the tidal forces acting on it. For example, in the case of an orbiting moon which is also undergoing rotation about an axis that is perpendicular to the plane of the orbit, the tidal force acting stretches the moon along the line that joins it with the planet. At the same time, the stretching relaxes as the diameter is rotating away from the line. The stretching is subjected to frictional resistance and in the stretching and relaxation of the deformation energy is dissipated as heat which eventually takes energy away from the system.
Tidal Friction Between the Earth and the Moon
The tidal bulge caused on the Earth’s crust and seas by the pull of the moon is prevented by tidal friction. The rotation of the earth, instead, carries out the bulge from under the moon directly. The earth spins in its orbit a total of 30 times for every revolution carried out by the moon in its orbit. There exists a mutual attraction between bulge material and the moon which increases the tendency of the moon to accelerate in its orbit. This causes the moon to move farther from the earth by about 1.2 inches or 3 centimetres every year and also slows down the daily rotation of the earth per year by a small fraction of a second.
Impacts of Tidal Friction
The moon’s rotational angular momentum is diminished by the torque exerted by gravity as the moon rotates away and eventually, its rotation rate slows down. Over a prolonged period, this braking effect brings the rotation rate of the moon to zero, relative to the connecting line. This causes the rotation period to approach the orbital period and causing the same part of the surface to face the planet constantly.
Scientists have attributed the bygone effects of the moon’s tidal friction as the main reason behind that only one of its surfaces is constantly turned towards the earth. Similarly, the tidal friction is the cause behind the same side of Mercury always faces the sun so much that one side is scorchingly hot and the other side is always cold.
The tidal force of the moon on the Earth so that tidal friction causes energy to dissipate. The Earth’s tidal deformation, in turn, causes it to rotate away from the connecting line. Additionally, the elongated shape of the earth provides an asymmetry causing it to slow down due to the braking torque. These effects, in the event of a million years from the present time, may cause the Earth‘s rotation to become 50 times longer or perhaps equal to a month’s duration of that time. The earth might also always keep the same side facing a distant moon. These conditions are, however, unlikely to be stable owing to the Sun’s tidal effects on the moon-earth system.
On this page, we have provided an insight into tidal friction and its resulting impacts. Tidal friction has been long studied by scientists to understand the mechanisms by which the rotational energy of the earth is dissipated and the origin of the moon.