# What is synchronous rotation and tidal coupling?

Synchronous rotation is a concept used in astronomy to describe the characteristic that occurs when a celestial body rotates on itself and in orbit around another celestial body, and the rotation speed is the same as the orbital translation speed.

Synchronous rotation has a very peculiar effect: the synchronously rotating celestial body keeps the same hemisphere pointing towards its companion around which it is orbiting. The best-known example is undoubtedly the synchronous rotation of the Moon around the Earth.

## Synchronous rotation and tidal coupling

The synchronous rotation between two celestial bodies occurs as a consequence of the so-called tidal coupling. While there are no tides on other planets, at least not ocean tides as we know them on Earth, synchronous rotation similarly occurs on many natural satellites in the Solar System.

Generally, satellites are much smaller in size than the planets they orbit and only satellites achieve synchronous rotation, but in theory, if the mass difference between the two bodies and the distance between them is small, both should achieve synchronous rotation. each other; this can be observed in some binary star systems.

In the specific case of the Moon, both the rotation on its axis and the translation around the Earth have a duration of 27.33 days (although the duration of the lunar cycle is 29.53 days). Having the same duration, no matter when we look at the Moon, we will always see the same face.

The unseen other side, or hidden side, was first observed in 1959 on the Soviet mission Luna 3 or E-2A (in Russian Луна-3)

### Mechanism

Consider two co-orbiting objects, A and B, such that A is the larger body and generates synchronous rotation in body B. For this to happen, body A has to modify the rotation that body B would have in gravitational equilibrium on its own. .

First, the force of gravity on A causes the shape of body B to distort. Body B is stretched in the direction of gravity, causing it to lose its spherical shape. Deformation creates a gravitational gradient, greater on the deformation axis. This effect can be smaller or larger depending on the mass of B and the mass and force of gravity of A.

At the beginning of the formation of both bodies, body B would rotate much faster on itself, and over time the gravity of A would slow down B’s rotation.

The angular momentum of the entire system AB is maintained during coupling, so the reduction in rotational velocity at B is accompanied by an equal magnitude increase in translational velocity until they become equal and one hemisphere of B is locked facing to A. A torque would be created.

The effect of B’s ​​gravity on A would also produce the coupling of A’s rotation and translation, but since B’s mass is much smaller, this effect would take many years to complete. In fact, using atomic clocks it was possible to estimate that the Moon slows down the Earth’s rotation by 15 microseconds each year; each passing year has days 15 microseconds shorter.

At that speed, the Earth’s docking with the Moon will not be complete until the Sun is extinguished.