The Ground Beneath Us Has Tides Too

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Ocean tides are the familiar result of gravity from the Moon and, to a lesser extent, the Sun. But the same tidal forces also deform the solid Earth itself. The ground is not perfectly rigid: as the Moon and Sun change their positions relative to Earth, the crust and the rest of the planet respond with tiny, slow changes in shape and height. These movements are called Earth tides or solid Earth tides. At some locations, the vertical movement can approach roughly 40 centimeters during a tidal cycle. The main lunar component repeats on a roughly 12.4 hour period, so the surface rises and falls on a regular twice-daily rhythm. The Sun also contributes, strengthening or weakening the overall effect depending on its alignment with the Moon. Around new and full moons, their tidal effects reinforce each other, producing larger tidal ranges. That sounds enormous until you consider the scale involved. A huge region of the planet is moving together, so a person standing on the ground does not normally feel the motion as the sensation of a hill suddenly lifting beneath them. Over ordinary distances, nearby points can move by nearly the same amount, making the change difficult to notice with everyday measurements. The difference becomes important when scientists need extremely precise positions over very large distances. Earth tides therefore matter in precision science and engineering. Geodesists, gravity surveyors, radio astronomers and other researchers account for the effect when working with measurements that can detect changes far smaller than everyday human perception. High precision positioning systems and instruments can measure or correct for these regular movements. USGS also notes that Earth tides are useful when studying groundwater behavior and calibrating sensitive instruments used to monitor changes in the Earth. The phenomenon is a useful reminder that solid ground is not completely motionless. On human time scales it looks rigid, but on planetary scales it flexes continuously under the gravitational influence of bodies in space.