The eight planets rotate on their own axes at different speeds. Generally, the greater the mass, the faster the speed. Jupiter, the largest planet, has also the fastest speed of rotation of 9 hours, 50 minutes. Mars, the smallest of the outer planets, rotates in 24 hours 37 minutes. The Earth is about 10 times as massive as Mars, yet it rotates in about the same time: 24 hours. Why does the Earth rotate so slowly?
If the Earth were to have no moon, its rotating speed would be 15.5 hours. However, the Earth’s rotation has been slowed down by the tidal effects of its huge satellite, the Moon. While the Earth is affected by the Moon’s gravity, the Moon is affected by the Earth’s gravity as well. The Earth’s gravity is 81 times stronger than the Moon’s gravity. The Moon’s rotation has been slowed to an extremely slow speed(27.321582 days to turn one round) with respect to the Earth. That is why one of its side always faces the Earth. In other words, the Moon is in synchronous rotation with the Earth.