Spring tides have nothing to do with spring, and neap means powerless
Twice a month, when Sun, Moon and Earth line up, their pulls combine and tides swing to their extremes. Those are spring tides, named for water that leaps up, not for the season. A week later, with Sun and Moon at right angles, the ranges shrink to neaps, an Anglo-Saxon word meaning without the power.
Tides arise because gravity weakens with distance. The Moon pulls a little harder on the side of Earth facing it and a little more gently on the far side, stretching the planet along the line between them. Solid rock barely yields, but seawater is free to shift, especially sideways. Because the Moon orbits in the same direction Earth spins, it returns to the same spot in the sky roughly every 24 hours and 50 minutes, so the dominant lunar tide, called M2, repeats about every 12 hours and 25 minutes.
In most places the sea rises and falls about twice daily, flooding over the intertidal zone and then ebbing to expose it, with slack water as the current turns. Some coasts get one cycle a day, and many see two unequal highs. The Moon's distance matters too: when it is nearest, at perigee, ranges grow, and six or eight times a year perigee coincides with a new or full moon, producing the year's biggest ranges, by as much as a foot in some locations.
Real tides are sculpted by basin shapes, coastlines, sea-floor depth, friction and resonance, so predicting them takes about 62 separate constituents. Mapmakers draw cotidal lines joining places that reach high water at the same moment. In an enclosed basin these lines converge on an amphidromic point with almost no tide at all, and high water sweeps around it every 12 hours, generally counterclockwise in the Northern Hemisphere and clockwise in the Southern, thanks to the Coriolis effect.
The oceans are not alone in feeling this pull. Earth's crust rises and falls by centimetres in what is called Earth tide, and the atmosphere shows tidal swings in pressure and wind, partly driven by solar heating. Wind, heat expansion and air pressure add non-tidal changes on top, including storm surges in shallow coastal seas. Tide gauges at fixed stations track water level against a datum, usually mean sea level, while ignoring waves shorter than a few minutes.
Source: Tide
