meridian: [14] Etymologically, meridian denotes the ‘middle of the day’. It comes via Old French from Latin merīdiānus, a derivative of merīdiēs ‘mid-day’. This was an alteration of an earlier medidiēs, a compound noun formed from medius ‘middle’ (source of English medium) and diēs ‘day’. The application of the word to a circle passing round the Earth or the celestial sphere, which is an ancient one, comes from the notion of the sun crossing it at noon. => medium[meridian etymology, meridian origin, 英语词源]