Tides
Tides are the slow, periodic vertical rise and fall of the sea surface. They are usually described as being either diurnal or semi-diurnal.
Diurnal tides have one high water and one low water in each lunar day (about 24.8 hours), while semi-diurnal tides have two high and two low waters in the same time period. While these tidal changes are easier to observe where land and water meet, they exist everywhere – even in the middle of the ocean.
Tidal ranges along the shoreline vary by location. For example, the tidal range in Canada’s Lake Superior is measured in centimetres, while Canada’s Bay of Fundy, west of Nova Scotia has the largest tidal range in the world at 17m.
High and low tides are the result of the attractive forces (gravitational pull) of the Moon and Sun on a rotating Earth. The closeness of the Moon to Earth (384,404 km), and the distance to the Sun (149,669,292 km) accounts for the Moon having a tide-rising force nearly 2.5 times greater than the Sun. The position of these celestial bodies results in significant variation in pulling forces causing above or below normal tidal ranges.
The range between a high and a low tide is greatest when the Sun, Moon and Earth are in alignment. These are spring tides. When the Sun and Moon are at right angles to the Earth their gravitational forces significantly reduce each other. This causes the neap tide; a period of decreased tidal range. The term neap is an acronym for near even as possible.