# When does a body turn

## Why is the earth rotating?

When we set an object in motion in our everyday life, it returns to a state of calm on its own. A carousel rotates as long as the motor drives it. If you switch off the engine, it slowly comes to a standstill. Doesn't that mean that the movement of the heavenly bodies also has to be constantly driven by a force so that they do not come to a standstill?

It is friction that leads us to this wrong conclusion: Frictional forces influence all movements in our everyday environment and slow them down by converting kinetic energy into thermal energy. In fact, until the 17th century, naturalists believed that rest was the natural state of all bodies and that a permanent force was necessary to maintain movement.

The earth

It was not until the English physicist and astronomer Isaac Newton realized that bodies maintain their state of motion when no forces act on them. Forces change the state of motion of a body: The force of the motor accelerates the carousel, the frictional force slows it down. Friction does not play a major role in cosmic movements (we will come back to exceptions later), so the movement of the planets does not slow down even over billions of years.

A body completely free of force would, however - since its state of movement does not change - always move in a straight line. As Isaac Newton also recognized, a central force - the gravitational pull of the sun - is necessary to keep the planets on their orbit. In the simple case of an exact circular path, gravity always acts perpendicular to the direction of movement and therefore only changes the direction, but not the speed of the movement: The planet therefore moves with constant movement in a circle around the sun. In reality, it is a bit more complicated, since most celestial bodies move on elliptical orbits in which both direction and speed change along the course of the orbit. But here, too, the following applies: As long as no other force acts alongside gravity, the orbital movement remains unchanged overall.

But with this we still have no explanation for the rotation of the earth. Since the sun and most of the planets and moons rotate in the same direction, it can be assumed that the rotation of the celestial bodies has something to do with the history of the formation of the solar system. Our solar system was formed about 4.6 billion years ago from a large cloud of gas and dust that slowly contracted. As with a figure skater who slowly draws her arms during a pirouette, the self-rotation of this gas cloud has also increased during the contraction. Physicists call this phenomenon "conservation of angular momentum".

The planetary orbits in our solar system

However, the angular momentum of the cloud was far too great for a star - the centrifugal force that occurs during the rapid rotation would have prevented the formation of our sun from the outset. Therefore, a flat rotating gas and dust disk formed around the emerging sun, which has absorbed a large part of the angular momentum. The planets and moons then emerged from this disk. In fact, our sun contains 99.9 percent of the mass of the solar system, but only 0.5 percent of the angular momentum! The lion's share of the angular momentum lies in the orbital motion, a smaller part of it in the natural rotation of the planets and moons.

In the early days of the solar system, however, collisions between celestial bodies also influenced the rotation of the planets. Venus, for example, owes its retrograde rotation - i.e. counter to the prevailing direction - presumably to the impact of an asteroid. And the formation of the moon through the collision of the primordial earth with a body roughly the size of Mars has probably also changed the rotation of the earth considerably.

According to today's knowledge, this event accelerated the rotation of the earth so that the day on our planet was initially only around 14 hours. And with that we come back to friction. The tides generated by the moon on earth act like a huge brake and have thus slowed the rotation of the earth to the current value. Currently, tidal friction is increasing the length of the day by about 23 microseconds per year. At the same time, the moon moves 3.8 centimeters away from the earth every year - this is again a consequence of the conservation of angular momentum.

The tidal friction has also led to the fact that the moon now has a so-called bound rotation, i.e. the earth always faces the same side. Tidal friction also plays an important role in other planet-moon systems, planets in narrow orbits and close binary stars.