Nachtseite

Nachtseite is a term used in German astronomy to denote the hemisphere of a planetary body that is not illuminated by its primary star, usually the Sun. It contrasts with the Sonnenseite, the day side that faces the star. The boundary between these hemispheres is the terminator, where illumination transitions to darkness and day-night effects are most pronounced.

In bodies with atmospheres, heat is transported from the day side to the night side by winds, oceans, or other processes, moderating extremes on the Nachtseite. In airless bodies, such as the Moon, the Nachtseite can be extremely cold and experiences rapid temperature changes when night follows day. On tidally locked planets, the Nachtseite may remain permanently dark, creating strong contrasts with the perpetually lit day side.

Studying the Nachtseite helps scientists understand thermal structure, heat transport, and climate dynamics of planets and moons. Observations often rely on infrared measurements and phase curves to infer temperatures and energy distribution between the day and night hemispheres. The Nachtseite can be difficult to observe directly because of glare from the day side, but its thermal emission provides essential information about composition and atmosphere.

In German-language writing, Nachtseite is the standard term for this concept. It is widely used in textbooks, articles, and popular science. While the term is primarily scientific, context may also describe phenomena related to the "dark side" in a more general or metaphorical sense.