F. Pearce of Dunedin asks :-

Why does the Earth revolve and the Moon doesn't? Do all planets revolve?

Frank Andrews, an astronomical educator with the Carter Observatory in Wellington, responded.

All large objects in space like moons, planets, stars and galaxies etc form as a result of turbulence starting the gravitational collapse of very diffuse clouds of gas and dust. As the gas starts to fall towards the center of collapse small random currents in the gas and dust moving in different directions result in rotation as the body (planet, star etc) forms. This process is called "accretion". As all objects in the universe form by accretion, all objects have spin.

The Moon does rotate! It rotates in exactly the same time as it takes to complete one orbit around the Earth (27.322 earth days). That is why the Moon always shows the same face towards the Earth.

Why does the Moon, and quite a lot of other moons, rotate in exactly the same time they take to orbit their planet? Earth tides are caused by the moon's gravity pulling on the Earth which distorts its shape slightly. As well as "ocean tides" the moon's gravity also causes "atmospheric tides" and "earth tides". In New Zealand the earth's surface actually moves up and down by about 30 centimeters twice a day due to earth tides. We don't notice this movement because it is small and very smooth.

The Earth also caused tides on the Moon. Because the Moon's mass is only 0.0123 of the earth's mass, the "moon tides" raised by the Earth on the Moon were considerably higher than the earth tides. It takes a lot of energy to cause the tides. Scientists have been able to show that this energy has come from slowing the spin of the Earth and Moon. Because the Moon is much less massive than the Earth it has slowed a lot more quickly than the Earth. Today it has reached a stable state in which there are no longer moon tides because it spins at the same rate as it orbits the Earth. This allows the tidal bulge to remain fixed in one place facing the Earth.

The interesting thing is that geologists have been able to demonstrate that the Earth used to rotate more rapidly in the past than it does today. The earth's rotation rate was about 16 hours in the distant past. The current rotation rate of 23 hours 56 minutes is still being gradually slowed by the moon's gravity and this will continue till the Earth also rotates at the same rate as the Moon takes to orbit our planet. This won't happen for billions of years and by that time the Sun will have become a Red Giant and destroyed the inner planets including the Earth.