meridies

Meridies is a Latin term that primarily denotes noon or the middle of the day. In historical astronomy and navigation, the word was also used to refer to the meridian, the imaginary great circle on the celestial sphere that lies in the local vertical plane and passes through the observer’s zenith as well as the north and south celestial poles.

In practical astronomy, the meridian serves as a reference line for local solar time. The Sun’s apparent transit across the local meridian marks solar noon, when the Sun reaches its highest point in the sky for that location. Devices such as sundials and astrolabes were designed around this concept, using the meridian to determine time from the Sun’s position.

The term meridies appears in Latin texts from classical to early modern periods and is largely historical today. In English and many modern languages, meridian and noon have supplanted the usage of meridies, though the word survives in scholarly, literary, or Latin-language contexts. The concept remains central to discussions of timekeeping, celestial navigation, and the geometry of the celestial sphere, where the meridian defines the local reference for measuring longitude and the apparent movement of celestial bodies across the sky.