Nightside

Nightside is a term used in science and fiction with several distinct meanings. In astronomy, it denotes the hemisphere of a rotating body that faces away from the Sun, opposite the dayside. The terminator marks the boundary between the illuminated and dark halves. The nightside is typically cooler and darker, though it may emit light from stars, aurorae, or human-made sources. Magnetic and atmospheric processes can transport energy to the nightside, influencing phenomena such as auroras and plasma circulation.

In fiction, Nightside refers to a prominent urban fantasy setting created by British author Simon R. Green. The Nightside is a hidden, supernatural city accessed through portals, often described as a dark, magical counterpart to London. It is inhabited by a range of beings including vampires, demons, gods, and other unusual entities. The setting serves as the backdrop for Green's Nightside series, which follows John Taylor, a private investigator who takes on cases in the Nightside while navigating its dangers and politics.

The term also appears in broader cultural usage to evoke the idea of a “night-side” or dark aspect of a place or phenomenon. However, the two primary uses—an astronomical concept and the fictional Nightside—are the most widely referenced.