Grant Ruthven, of Momona School, asks :-

What makes the world spin?

Pam Kilmartin, an astronomer at the Mt John University Observatory, responded.

The world spins because it was made from stuff that was spinning. The planets formed from a flat cloud of dust and gas that circled the sun. The stuff closer to the sun spun faster than the stuff further out, like a whirlpool. As the dust and gas gathered together to make planets, some of the rotation energy went into making the planets spin.

On earth spinning things are slowed down by friction. The spin energy is used up in rubbing against the air, the ground or the bearings. So the spinning stops. In space there is no friction so planets spin forever, unless the spin energy is given to something else.

The earth is slowly losing its spin energy to the moon. The moon's gravity pull makes tides in the sea. The tides drag on the earth, slowing down its spin. This speeds up the moon and makes its orbit bigger. Both changes are very slow. Over one century the drag on the earth's spin makes the day longer by just a thousandth of a second. The moon's movement away from the earth is just centimetres per year.

Where planets are close to the sun, or moon's close to a planet, the tidal force can be very strong. It can bend the whole planet or moon out of shape and slow the spin quickly. Our moon has had its rotation locked by the earth's pull so it always keeps the same face turned towards us. Similarly, the planets Mercury and Venus have lost most of their spin because of tides raised by the sun.