dayside

Dayside refers to the hemisphere of a planet, moon, or other celestial body that is illuminated by the Sun at a given moment. It is opposite the nightside, the half in darkness. The boundary between them is the terminator, and the point directly beneath the Sun is the subsolar point. As the body rotates, the dayside moves across the surface, producing regular diurnal day–night cycles with the Sun’s apparent path depending on latitude and season.

On tidally locked worlds, the same hemisphere may face the star permanently, creating a lasting dayside and nightside and often producing extreme thermal contrasts and distinctive atmospheric dynamics. By contrast, bodies with regular rotation exhibit a daily cycle of solar heating and cooling.

The dayside receives solar radiation that is absorbed, reflected, and re-emitted as heat. The resulting surface and atmospheric temperatures depend on albedo, atmospheric composition, and the greenhouse effect. Planets with thin or no atmospheres, like Mercury, show large day–night temperature swings, whereas those with dense atmospheres, such as Venus, display high daytime temperatures with smaller relative fluctuations.

Scientific and practical relevance includes studying energy balance, climate patterns, and weather on the dayside, as well as planning observations and missions. In exoplanet research, dayside temperatures and phase curves help constrain albedo, circulation, and atmospheric structure. For solar power and communications, the orientation and illumination of the dayside influence operations and data transmission.