Spring tides and neap tides
The height of the tide varies from day to day but the fluctuations are predictable. They are mainly caused by:
- the degree to which the sun and moon's influences are acting in the same direction; and
- the varying distance between the moon and the earth.
Twice each month, at the time of the new moon and the full moon, the gravitational influences of the moon and sun reinforce one another and cause the tides to rise to greater heights and fall lower than average tides. These are called spring tides from the Old English word "springan" which means to well up.
At the time of the quarter moon, when the sun, earth, and moon form a right angle, the difference between high and low tide is less than average. These are neap tides, from the Old English "nep", as in nipped in the bud.
Every 27 and a half days, when the moon reaches a point in its orbit closest to the earth (called perigee) the tidal range is increased. When perigean tides coincide with spring tides, extreme tides can be expected. In the Bay of Fundy, these conditions may create tides as large as 16 metres (53 feet). Conversely, when the moon is at its apogee, its farthest point from the earth, even spring tides are diminished.