Eclipses, Tides & the Earth–Moon Barycenter: Structures & Vocabulary

The Earth–Moon system is governed by shared orbital motion and differential gravity. Solar and lunar eclipses occur when the Sun, Earth, and Moon align closely enough for one body's shadow to fall on another, tides arise because the Moon's gravitational pull differs across Earth's diameter, and both Earth and Moon orbit their common center of mass, the barycenter.

An eclipse requires phase plus node alignment: at new moon the Moon can cast an umbral or penumbral shadow on Earth, while at full moon Earth can shadow the Moon. Tides result from the gradient in lunar gravity, not from a uniform pull on all parts of Earth. In orbital mechanics, Earth and Moon both accelerate around the barycenter, so neither body is perfectly stationary while the other simply circles it.

Eclipse paths can be predicted centuries in advance using orbital ephemerides. Tide gauges show periodic responses to lunar and solar geometry, while satellite laser ranging measures the Moon's orbit and gradual recession. JPL ephemeris systems explicitly distinguish Earth's center from the Earth–Moon barycenter when calculating positions.

The Moon's orbit is tilted about five degrees to the ecliptic, so eclipse seasons occur near orbital nodes. Tidal bulges are produced because the Moon's gravity differs across Earth's diameter, with the Sun adding a secondary tidal contribution. The Earth-Moon barycenter lies inside Earth but offset from its center.