Tides are the rise and fall of sea levels. Roughly every 12 hours, the oceans on each side of the globe rise a little and then fall back. These tides are caused by the combined effects of the gravitational forces between the rotation of the Earth and the Moon and the Sun.
When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are very high and the low tides are very low. This is known as a spring high tide. Spring tides are especially strong tides. They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.
During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another. Neap tides occur during quarter moons.
Tide changes proceed via the following stages:
|1||Sea level rises over several hours.||flood tide.|
|2||The water rises to its highest level.||high tide|
|3||Sea level falls over several hours.||ebb tide|
|4||The water stops falling.||low tide|