Nightsidan

Nightsidan is a term used in astronomy and in Swedish-language science communication to describe the portion of a planetary body that is shielded from sunlight, i.e., the night side. It contrasts with the daysidan, the sunlit side. The term is commonly used when describing planets, moons, or other bodies with a defined dayside and nightside.

In practice, nightsidan is defined by the direction opposite to the Sun. The boundary between dayside and nightside is the terminator, a moving line that shifts with the body's rotation and with seasonal tilt. On tidally locked bodies, the nightsidan can be permanently dark or experience long periods of darkness, resulting in large temperature contrasts and distinctive atmospheric or surface processes. In planets with an atmosphere, heat transport by winds can reduce these contrasts.

In observational astronomy and exoplanet science, the nightsidan is important for studies of thermal emission, phase curves, and climate models. While the dayside may be hot and brightly illuminated, the nightsidan emits primarily infrared radiation and can host different chemical and physical conditions.

In Swedish usage beyond science, nightsidan can also be used metaphorically to refer to the darker or hidden side of events, places, or societies.