Tides

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Tides are often explained as the Moon simply pulling the ocean toward itself, which explains the high tide on the side of the Earth facing the Moon but leaves an obvious puzzle: there is also a high tide, at the same time, on the exact opposite side of the planet. The full explanation needs one more piece. The Earth and Moon do not orbit a fixed Earth; they orbit their shared center of mass, a point inside the Earth but off its exact center, and every point on Earth is swinging around that shared center along with the planet as a whole. On the side facing the Moon, the Moon's gravitational pull is slightly stronger than the average pull felt at the Earth's center, producing a bulge of water toward the Moon; on the far side, the Moon's pull is slightly weaker than average, and the orbital motion produces a matching bulge away from the Moon. The two bulges, one toward the Moon and one away from it, are what produce two high tides roughly every lunar day rather than one.

The Sun contributes a smaller version of the same effect, roughly forty six percent as strong as the Moon's despite the Sun's far greater mass, because tidal force falls off with the cube of distance rather than the square, and the Sun is vastly farther away. When the Sun and Moon align, at new moon and full moon, their tidal effects add together, producing the largest tides of the month, called spring tides. When the Sun and Moon sit at right angles to each other, at the Moon's first and last quarter, their effects partly cancel, producing the smallest tides of the month, called neap tides.

A lunar day, the time it takes the Moon to return to the same position overhead, runs about fifty minutes longer than a twenty four hour solar day, because the Moon is also moving in its own orbit around the Earth in the same direction the Earth is rotating. That fifty minute lag is why high tide arrives roughly fifty minutes later each day rather than at the same clock time. Isaac Newton's Principia, in 1687, gave the first correct account of tides as a consequence of gravity acting between the Earth, Moon and Sun, but Newton's own equilibrium theory assumed an idealized ocean with no landmasses or currents in the way; it took Pierre-Simon Laplace's later dynamic theory of tides, accounting for the real ocean's basin shapes and the momentum of moving water, to explain why real coastlines see tidal timing and height that diverge substantially from Newton's simpler prediction.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Most coastlines in the world see a tidal range, the vertical difference between high and low tide, of no more than a few feet. The Bay of Fundy, between the Canadian provinces of Nova Scotia and New Brunswick, regularly exceeds fifty feet, the largest tidal range recorded anywhere on Earth. The gravitational pull driving that tide is the same pull acting on every other coastline in the world; the Bay of Fundy's extremity comes entirely from the shape of the bay itself.

The bay is long, narrows and shallows toward its inner end, funneling an enormous volume of water into a progressively tighter space as the tide advances, the same effect that speeds a river through a narrow gorge. More importantly, the bay's own natural resonant period, the time it takes a wave to slosh from the bay's mouth to its inner end and back, happens to be close to the roughly twelve and a half hour period of the tide itself. When a system's natural resonance closely matches the period of the force driving it, the response amplifies dramatically, the same physical principle that lets a properly timed push send a playground swing far higher than an equally strong push at the wrong moment. The Bay of Fundy is, in effect, a giant water container tuned almost exactly to the tide's own rhythm, and that near-perfect tuning, not unusually strong gravity, is what produces its extraordinary range.

The consequences are visible and economically significant. At the head of the bay, the incoming tide can reverse the flow of rivers, producing a tidal bore that runs upstream against the current, a phenomenon visitors travel specifically to watch on the Shubenacadie River. The same enormous volume of moving water has long made the bay a target for tidal power generation, from the Annapolis Royal tidal station built in the twentieth century to more recent in-stream turbine projects, though the extreme currents and ice conditions that make the bay attractive for power generation have also made sustained tidal power extraction there a genuinely difficult engineering problem, one still being actively worked out.