Tides are not just an ocean event. The solid crust under your feet rises and falls by up to about half a meter each day, stretching as much as 55 centimeters at the equator, because the Moon and Sun squeeze the whole planet. The Sun’s gravity on Earth is 179 times stronger than the Moon’s, but tidal force falls off with the cube of distance, so the Moon supplies about 69 percent of the effect and the Sun about 31 percent. The real driver is the horizontal, or tractive, component of that pull, which sweeps water sideways into two bulges, one facing the Moon and one opposite. Because the Moon orbits as Earth spins, a tidal cycle runs about 12 hours and 25 minutes. Alignment at new and full moon, called syzygy, brings spring tides, while quarter moons at quadrature bring weak neap tides.

Understanding took centuries. Pytheas linked tides to the Moon around 325 BC, Seleucus of Seleucia blamed the Moon around 150 BC, and the Venerable Bede in 730 AD tracked tides running about four fifths of an hour later each day. Kepler credited lunar gravity, Galileo mocked the idea and blamed Earth’s motion, and Newton’s Principia settled it in 1687. Real coastlines scramble the math into amphidromic points where the water barely rises, 16 to 17 meter tides in the Bay of Fundy, opposite tides on either side of Cook Strait, and a double high tide at Southampton. Lord Kelvin built gear driven machines to predict local tides. All that friction dissipates about 3.75 terawatts, has lengthened the day by about two hours over 600 million years, and pushes the Moon away by roughly 3.8 centimeters a year.

• Sailors use the rule of twelfths to estimate how fast water rises each hour after low tide.
• The atmosphere has its own tides in air pressure and wind.
• Galactic tides from the Milky Way can knock comets out of the Oort cloud.
• GPS networks and observatories must correct for the ground’s daily rise and fall.
• Laser reflectors left by Apollo astronauts measure the Moon’s drift.