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A proposed explanation of the lunar farside highlands in which irradiation from a nearby hot early Earth imposes a thermal gradient on a synchronously rotating, accreting Moon. Preferential condensation and deposition of refractory crust-forming material toward the cooler farside seeds a lasting chemical and crustal-thickness asymmetry. It requires early synchronous rotation, appropriate transport times and incomplete homogenization by subsequent mixing. It is a testable formation hypothesis, distinct from ordinary present-day illumination of the lunar night side.^[ref]

A nearside lunar geochemical region enriched in KREEP-associated material and heat-producing elements. Its distribution offers a constraint on the Moon's chemical and thermal evolution. Enrichment can help sustain melting but does not, by itself, identify the process that first made the hemispheric crustal contrast.

A sidereal month is the Moon's orbital period relative to distant stars, about $27.32166$ days. It is shorter than the synodic month because the sunward direction changes as Earth travels around the Sun. The lunar spin period associated with synchronous rotation matches the sidereal month in the mean. ^[ref].

A solar eclipse occurs when the Moon obscures part or all of the Sun as seen from a specified location. Totality allows observations of nearby background stars that ordinary daylight hides.

A spring tide has an enhanced high-to-low tidal range because the Moon and Sun are approximately aligned as seen from the Earth, so their principal tidal contributions reinforce. Such alignments occur twice in a lunar phase cycle.

Synchronous rotation means one prograde turn about the body's own axis per orbital period, measured against fixed directions in an inertial frame. This $1:1$ spin-orbit resonance approximately describes the Moon's synchronous rotation toward Earth. ^[ref].

The striking geometric idea is that one turn in space can keep one face toward the companion: a fixed orientation in the frame turning with the orbit is a changing orientation in an inertial frame. This distinction helps us predict illumination: a synchronously rotating exoplanet orbiting its illuminating star can have persistent day and night hemispheres, whereas the Moon, synchronized to Earth, receives changing illumination from a different body, the Sun.

A synodic month is the synodic period of the Moon relative to the Sun, about $29.53059$ days on average. A lunation is commonly measured from one new Moon to the next. Actual lunations vary, so the constant-speed relation derived in lunar phase is useful for understanding the mean cycle rather than predicting exact dates. ^[ref].

A trajectory whose dynamics require gravitational forces from more than one attracting body. Around a synchronously rotating moon, a maintained trajectory involving the planet–moon system is distinct from a simple moon-centred stationary Kepler orbit; the moon's Hill sphere limits the latter approximation.

Tidal locking settles a body's spin into a sustained relation with its orbital motion through tidal forces and tidal dissipation. The simplest case is synchronous rotation, a $1:1$ spin-orbit resonance. On an ideal circular orbit, with spin axis perpendicular to the orbital plane, the body keeps the same face toward its companion, as the Moon approximately does toward Earth. ^[ref].

This article retains the broader sense of tidal locking as capture into a spin-orbit resonance; the term is also commonly used specifically for synchronous rotation. Tidally evolved rotation need not be $1:1$: Mercury has a $3:2$ spin-orbit resonance with the Sun, turning three times in two orbital periods. The distinction matters when inferring illumination or climate from a spin period. ^[ref].

A tide is a periodic change in ocean level driven mainly by the differential gravitational forces of the Moon and Sun, together with the Earth's rotation. Their tidal forces act across the extended planet; a uniform acceleration shared by the whole Earth does not by itself generate a tide. Ocean-basin resonance and coastal geometry modify the local response.