The gravitational pull of the Moon is the main reason for the tides on Earth. The Moon attracts the water in the seas, pulling up a bulge of water towards it. The Earth itself is also attracted and so another bulge of water is ‘left behind’ on the opposite side of the Earth from the Moon. Places on the Earth beneath the two bulges have high tides at that moment while places at right angles to these have low tides at that momen.
N.B. The diagram is not to scale.
The Earth rotates beneath these two bulges and as it does so every place on Earth has two high tides and two low tides a day.
There is not exactly 12 hours between the high tides at a particular place on the Earth because the Moon is not only pulling on the water but also moving round the Earth as it does so. As it moves it 'pulls’ the bulge of water round with it.