Tidal locking

The bound rotation (torsion ) is a term used in astronomy and describes a phenomenon between two mutually orbiting celestial bodies: the intrinsic rotation of the ( lower-mass usually ) a celestial body is not here, as usual, regardless of the period of revolution around the other heavenly bodies, but coupled with this period. The bound rotation often occurs with moons that orbit relatively close to a planet, such as the Earth's moon.


In most cases of bound rotation, the period of rotation of the celestial body is the same as its circulating time. This turns the smaller ( rotationally bound ) celestial bodies the ever more massive the same page to. So he turns during a revolution with the same sense of rotation exactly once around its axis of rotation. The most common example of this is the Earth-Moon system: The rotational set- moon addresses the earth always the same face towards. In contrast, the planet Mercury has a synchronous rotation, in which he, during his 88 -day orbit around the sun exactly rotates 1.5 times; it shows the sun so not always the same page.

From co-rotation occurs when rotation and circulation of both celestial bodies are each matched to one another. This includes the system Pluto - Charon, in which both turn always the same side.

Since the rotational set- Earth's Moon always turns the same side, but the moon's orbit is not an exact circle, can be observed particularly well the Librationseffekt: from the moon back narrow border areas are in the course of a month to see.


The cause of bound rotation is the braking or acceleration of the original rotation of a small celestial body by tidal friction, which causes the circled him central body. This braking or acceleration is decaying until the satellite rotation of the small celestial body of its orbit rotation is approximated in a stable relationship.

By gravity of the other party, a tidal force is exerted which deforms the celestial body into an elongated shape in the connecting line between the two bodies. On the other celestial body facing side creates a tidal mountain, just on the opposite side.

As long as the heavenly bodies is not rotationally attached to the elongated body is rotated out of the connecting line. Characterized is a torque acting opposite the direction of rotation and in the direction of the connecting line. Obtained Torque

  • A slowing down of the rotation, if the rotational period is smaller than the orbital period


  • Provided that the period of rotation is an acceleration of the rotation greater than the orbital period

This mechanism acts until the occurrence of the bound rotation, which occurs after some millions of years in relative proximity, at distant celestial bodies but only after billions of years, or never.

Effect on the web

For the entire system, the law of conservation of angular momentum is considered. The total angular momentum is composed of the intrinsic angular momentum of the two bodies (rotation) and their orbital angular momenta in the orbit around the common center of gravity. In the tidal friction, the proportions of rotation are

  • Smaller when the period of rotation is smaller than the orbital period, which results in an increase of the web portion, and thus an increase in the web due to the diameter of the effect of conservation of angular momentum
  • Greater when the rotation period is greater than the orbital period, so the web diameter shrinks


Basically both partners practice their tidal force on each other, so that both celestial bodies can be rotationally bound. However, often a partner of a system is much more massive than the other, the tidal force is also unevenly distributed, with the result that the bound rotation occurs first in the less massive partner.

However, if the difference between the two masses low, so it may be a two-sided rotary bonding, the so-called co-rotation, in which both partners show each other the same page. An example of this is the system Pluto - Charon.

Spin-orbit resonance

In some cases, it can lead to a spin-orbit resonance. Such a resonance occurs when the ratio of the rotation period to the orbital period by two low natural numbers can be expressed, for example, 1:2 or 3:2 and a gravitational bond is present.

The spin-orbit resonance occurs when the web relatively eccentric and tidal power is relatively weak. Cycles through the rotation of the companion during the tidal braking the area of ​​a resonance with its orbital period, so the rotation period can be captured in what leads to a further deceleration of the rotation does not occur.

An example of this is the Mercury. When Mercury is in front of a 3:2 resonance, that is, in time, rotating in the Mercury three times, he performs two Sonnenumläufe.