The movement of the Earth’s seas and oceans can be seen twice daily when the tide comes in and goes out. The cause of this movement is a complex relationship between forces.
- Seas and oceans on the side of the Earth that are nearest the Moon are pulled upwards by the Moon’s gravitational effect. This is also repeated on the opposite side of the Earth, causing two bulges and troughs in the oceans.
- As the Earth spins, the bulges in the oceans created by these gravitational forces stay more or less in the same place while the Earth spins underneath them. The result is that most shorelines have two high tides and two low tides as the landmasses pass the troughs and waves.
At full and new moons, the tidal force of the Sun (usually a lesser effect) is added to that of the Moon causing high spring tides, while at quarter moons the forces are opposed causing low neap tides.
