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The far side of the Moon is the hemisphere directed approximately away from Earth. “Far” describes the direction to Earth; it does not describe illumination by the Sun. The far side of the Moon experiences both sunlight and darkness during the lunar phase cycle. ^[ref].

Facing a companion and facing the source of light are different relations. The Moon keeps its near side of the Moon approximately toward Earth, while the direction to the distant Sun is nearly fixed during the idealized orbit shown below. A fixed surface patch therefore turns toward, across and away from sunlight. The far side of the Moon has day and night too; its name does not mean permanent darkness. ^[ref].

If the Sun direction has fixed angle $\alpha$ and the marked surface patch has outward normal vector at angle $\theta$, its direct illumination is proportional to $\max (0,\cos (\theta -\alpha ))$. With synchronous rotation, $\theta =f+\pi$, so the patch goes through day and night even though it always faces Earth. In reality the Earth-Moon system advances around the Sun: the lunar spin and orbital cycle relative to distant stars is about $27.3$ Earth days, while the lunar phase cycle has a synodic period of about $29.5$ Earth days. These numbers describe the actual system, not the fixed-sunlight cartoon. ^[ref].

For a synchronously rotating exoplanet whose orbital companion is itself the illuminating star, the direction to the star turns with the orbit. On an ideal circular orbit, with perpendicular spin axis and no additional oscillations, this direction is fixed relative to the surface, giving persistent day and night hemispheres. Day-night heat redistribution then helps determine the nightside temperature; permanent lack of direct starlight does not imply zero temperature. Orbital eccentricity and departures from this ideal spin geometry modify that simple picture.

The hemispheric contrast involving thicker, generally higher feldspathic crust on the far side of the Moon, compared with generally thinner crust, more extensive lunar maria and the concentration of KREEP-rich material on the near side of the Moon. Crustal mass, topography, chemistry and later volcanic deposits are different observables. A proposed mechanism must explain their relationship rather than identify the contrast solely with visible dark maria.

The old, commonly bright and heavily cratered lunar terrains whose crust is rich in feldspathic material, including anorthosite. They occur on both lunar hemispheres, although the far side of the Moon has more extensive highlands and generally thicker crust. Their ancient crustal formation must be distinguished from later impact modification.

The Moon is Earth's natural satellite. Its synchronous rotation keeps approximately the same near side of the Moon facing Earth, while both the near side of the Moon and the far side of the Moon experience sunlight and darkness. See illumination of a synchronously rotating moon for the geometry. ^[ref].

The lunar crustal dichotomy combines several observables: the far side of the Moon has generally thicker, higher, feldspathic lunar highlands, whereas most visible lunar maria and the Procellarum KREEP Terrane occupy the near side of the Moon. These need not all have been produced by a single event. The old anorthosite crust formed mainly before the younger basaltic mare infill. Gravity-based crustal models combined with sample and seismic constraints demonstrate that crustal thickness and porosity must be considered alongside surface color.^[ref]

An early thermal and chemical asymmetry is one physically motivated option. In a lunar magma ocean, crystallizing, relatively buoyant plagioclase can accumulate to form feldspathic crust. On a young Moon that rapidly attained synchronous rotation, radiation from a still-hot nearby Earth would preferentially warm the nearside. The primordial Earthshine hypothesis proposes that this imposed a thermal gradient in the atmosphere and accreting material: crust-forming refractory constituents condensed preferentially toward the cooler farside, favoring thicker farside crust. This is more specific than simply saying that the farside cooled faster; it requires a persistent hemispheric chemical contrast.^[ref]

Subsequent concentration of incompatible elements in KREEP-rich late liquids can help preserve a thermal asymmetry: potassium, uranium and thorium provide radiogenic heating, supporting later melting in the nearside source region. Lower surface elevation and a thinner crust also facilitate some magma transport and pooling, although crustal thickness alone cannot specify eruption rates. The origin of the crustal contrast and the later production and emplacement of mare basalt are separate parts of the explanation.

The thermal option is consistent with a thick feldspathic farside crust, a hemispheric chemical contrast and the existence of an early hot Earth. It is not uniquely demonstrated by those observations. Vigorous magma-ocean mixing could erase the proposed gradient; the required irradiation must persist during the relevant condensation and crust-building interval, not merely occur at some early time. A failure to find the predicted bulk compositional contrast in demonstrably ancient deep crust, or a chronology placing the asymmetry after complete crystallization, would challenge this particular mechanism. The existence of farside mare deposits and large thin-crust basins also warns against treating thickness as an on/off volcanic switch.

Accretion of a smaller companion is a contrasting option. A low-speed collision of a smaller co-orbiting moon with the lunar farside could add a broad layer of relatively cold silicate material rather than excavate an ordinary deep crater. Numerical impact models demonstrate that such a depositional event can build thick highlands and displace underlying residual liquid toward the opposite hemisphere.^[ref] Broad crustal excess and asymmetric evolved material are compatible with that mechanism, but are not unique evidence for it. The necessary companion, collision timing and impact geometry must be dynamically feasible. A predicted buried depositional interface, age discontinuity or lateral fabric should survive somewhere; their systematic absence in well-resolved, unmodified ancient crust would count against the model. Closely related bodies could have similar isotopes, so failure to find a large isotopic difference alone would not disprove accretion.

Both scenarios allow later basin impacts, partial melting of the lunar mantle and mare emplacement to modify the original contrast. A useful explanation must reproduce crustal mass, chemistry, chronology and volcanic distribution together, and state which observations would distinguish its initial cause.