Greenwich Mean Midnight

the instant of lower transit across the prime, or Greenwich, meridian by the fictitious point on the celestial sphere known as the mean sun. Universal, or Greenwich mean, time is reckoned from Greenwich mean mid-night through 24 hours to the following midnight. Universal time is the mean solar time at the meridian that passed through the meridian circle of the Royal Greenwich Observatory until the observatory was moved from London to Herstmonceux, 70 km southeast of London.