Understanding Tides

Tides and tidal currents are caused by gravitational forces of the Sun and Moon acting upon the oceans of Earth as it spins. Because of its proximity to the Earth, the Moon exerts roughly twice the tide-raising force of the Sun. So the Moon plays a dominant role in controlling our tides and dictating their cycles each day.

Our tides repeat themselves once every 12 hours and 25 minutes – or twice a lunar day – which is the time it takes for Earth to rotate once relative to the Moon.

Tides produced by the Moon:

  • M2 (semidiurnal lunar) 1/2 lunar day = 12h 25min
  • O1 (diurnal lunar) 1 lunar day = 24h 50min

Tides produced by the Sun:

  • S2 (semidiurnal solar) 1/2 solar day = 12h 00min
  • K1 (diurnal solar) 1 solar day = 24h 00min

Perigean-spring vs. Apogean-neap tides

The greatest tidal range and fastest water speeds occur during “perigean spring tides.” During spring tides, the Sun and Moon’s gravitational pulls are aligned so they pull together (at the new and full Moon, when the Sun, Moon and Earth form a line). During perigean tides the Moon, in its elliptical orbit, is nearest Earth (“perigee”) and the lunar tide is greatest.

“Apogean neap tides” yield the lowest tidal range and slowest currents (the Moon is in first or third quarter, so the Sun and Moon are separated by 90° and their gravitational forces partially cancel each other, and “apogee” is when the Moon in its elliptical orbit is farthest from Earth resulting in the smallest lunar tide).

Fundy tides are unusual in that they respond as strongly to the perigee-apogee influence as they do to the spring-neap influence.  There is roughly a 7.4-day interval between spring and neap tides, a 13.8-day interval between perigean and apogean tides, and the extra-large, perigean-spring tides recur about every 206 days.

Fundy Tides

The primary cause of the immense tides of The Bay of Fundy is a resonance of the Bay of Fundy/Gulf of Maine system. The system is effectively bounded at its outer end by the edge of the continental shelf with its approximately 40:1 increase in depth. The system has a natural period of approximately 13 hours, a Q-value (efficiency) of about 5, and is driven near resonance, not directly by the Moon, but by the dominant semidiurnal tides of the Atlantic Ocean.

The gentle Atlantic tidal pulse pushes the waters of the Bay of Fundy/Gulf of Maine basin at nearly the optimum frequency to cause a large vertical range of the tide in the Bay of Fundy, particularly at its eastern end in Minas Basin.