Lnight

Lnight is a term used in speculative astronomy and science fiction to describe the length of the nocturnal phase on a planetary body. It denotes the period when the host star's direct irradiance is absent or below a defined threshold, resulting in practical darkness for surface observers or biospheres.

The Lnight value is defined as the duration, in local time, between sunset and sunrise when instantaneous surface irradiance I(t) remains at or below I_thresh. A commonly used threshold is 0.001 lux, corresponding roughly to starlight-level darkness on Earth; other thresholds may be used for different scientific or narrative purposes. Lnight can be expressed in hours per local solar day or as a fraction of the planetary rotation period.

In planetary science and fiction, Lnight is used alongside Lday to describe diurnal cycles on worlds with varying rotation rates, axial tilt, or orbital configurations. It is especially relevant for tidally locked planets, slow rotators, and planets with thick atmospheres whose scattering alters perceived darkness.

The term has appeared in academic-style thought experiments and world-building literature to discuss the implications for habitability, circadian biology, and human activities such as exploration and colonization on distant worlds. As a concept, it remains primarily descriptive and is not tied to a single canonical source; different authors may define the threshold and measurement convention to suit their model.

See also: Day length, Polar night, Circadian rhythm, Exoplanet habitability.