Daysidan

Daysidan is a Swedish term meaning the day side, used to describe the hemisphere of an astronomical object that is illuminated by the Sun. It is commonly employed in Swedish-language astronomy and space science to distinguish the sunlit side from the nightsidan (the nightside). The concept applies to planets, moons, Earth, spacecraft, and other celestial bodies, as well as to observations and models dealing with illumination, temperature, and energy balance.

Etymology and usage: The word is a compound of dag (day) and sidan (the side). In standard Swedish, daysidan is treated as a singular noun, with plural daysidor and possessive form daysidans. The term is widely used in educational materials, research papers, and media discussions when describing where sunlight falls on a body and how that affects conditions such as temperature and albedo.

Applications: On the Moon, daysidan refers to the Sunlit hemisphere during a given phase, where temperatures can rise sharply compared with the nightsidan. For tidally locked exoplanets, the daysidan may be permanently lit, leading to extreme day–night contrasts on the terminator. In Earth and planetary science, daysidan denotes the portion facing the Sun, corresponding to daylight and the region of solar energy input that drives weather, climate, and imaging observations. The concept also appears in space weather when describing the sunlit side of planetary magnetospheres.

See also: Nightsidan, day side, sunlit hemisphere, terminator.