Indians knew the time accurately based on solar position. The first explorers carried watches and clocks, but not of sufficient accuracy. English horologist John Harrison proved in 1764 that a clock could be used to locate a ship's position at sea with extraordinary accuracy. A new Longitude Act, known as the Act 5 George III, followed in 1765. Chronometers, which measure time accurately in spite of motion or varying conditions, became popular instruments among many merchant mariners during the 19th century. Prior to the invention of timepieces, rough navigation by stars could be done, but time zones were not established. Time zones, as we know them today, were slowly adopted and were uniform by 1920.