Forces of nature

The contrast between the moonlight and the darkness of the sea.

 Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by theMoon and the Sun and the rotation of the Earth.

Most places in the ocean usually experience two high tides and two low tides each day (semidiurnal tide), but some locations experience only one high and one low tide each day (diurnal tide). The times and amplitude of the tides at the coast are influenced by the alignment of the Sun and Moon, by the pattern of tides in the deep ocean (see figure 4) and by the shape of the coastline and near-shore bathymetry.

Most coastal areas experience two high and two low tides per day. The gravitational effect of the Moon on the surface of the Earth is the same when it is directly overhead as when it is directly underfoot. The Moon orbits the Earth in the same direction the Earth rotates on its axis, so it takes slightly more than a day—about 24 hours and 50 minutes—for the Moon to return to the same location in the sky. During this time, it has passed overhead once and underfoot once, so in many places the period of strongest tidal forcing is 12 hours and 25 minutes. The high tides do not necessarily occur when the Moon is overhead or underfoot, but the period of the forcing still determines the time between high tides.

The Sun also exerts on the Earth a gravitational attraction which results in a (less powerful) secondary tidal effect. When the Earth, Moon and Sun are approximately aligned, these two tidal effects reinforce one another, resulting in higher highs and lower lows. This alignment occurs approximately twice a month (at the full moon and new moon). These recurring extreme tides are termed spring tides. Tides with the smallest range are termed neap tides (occurring around the first and last quarter moons).