When the Earth, Sun and Moon are on the same axis, the gravitational effects are at maximum and the tidal bulge is largest. As the Earth spins within the bulge, the sea level rises as the bulge approaches, reaches a maximum and then falls to a minimum about 6 hours later.
The difference between the height of the high water and the height of low water, the range, is a maximum and this is known as a Spring Tide.
Spring tides occurs twice a month.
- In the open ocean, the tidal effect is very small, with a daily rise and fall of sea level due to gravity of approximately only 0.3 metre or 1 foot.
- where the proximity of land channels the tidal bulge, the rise and fall is increased and values of up to 15 metres (nearly 50 feet) are seen in some parts of the world.