Sinus Contagiosus: The Moon's "Bay of Contagion" Explained

Sinus Contagiosus (Latin for "Bay of Contagion") is a small lunar mare (basaltic plain) on the Moon's near side, forming a northwestern extension of Oceanus Procellarum (Ocean of Storms). It lies at approximately 7.5° N, 39.5° W, north of the crater Kepler and east of Herodotus and the Aristarchus Plateau.

Geological Context

• Formation: Flood basalt eruption ~3.5–3.8 Ga, filling low-lying basin.
• Morphology: Irregular, bay-shaped, ~70 km diameter. Smooth, dark floor (low albedo) typical of maria.
• Notable features: Wrinkle ridges (tectonic compression), ghost craters (buried by lava), sinuous rilles (lava channels).
• Adjacent: Rimae Prinz (rilles), Mons Herodotus, Vallis Schröteri (largest lunar rille).

Name Origin

Named by Giovanni Battista Riccioli in his 1651 lunar map Almagestum Novum. Riccioli used thematic nomenclature: "seas" for maria (Tranquillitatis, Serenitatis), "bays" (sinus) for smaller extensions. Contagiosus ("contagious") likely reflects the superstitious association of the Moon with disease/epidemics in pre-modern thought—not a geological descriptor.

Exploration & Observation

• Visible in small telescopes (best at First Quarter / Last Quarter terminator).
• Imaged by Lunar Orbiter, Apollo, LRO, Chandrayaan, amateur astronomers.
• No landing missions; proximity to Aristarchus (high scientific interest) makes it a potential future target.

Clarification

Despite the name, Sinus Contagiosus has no connection to infectious disease. It is a geological feature on the Moon. The similarly named Sinus Iridum (Bay of Rainbows) and Sinus Medii (Central Bay) follow the same naming convention.

A fascinating piece of lunar geography with a name that echoes historical fears—now a landmark for selenographers and Moon watchers.