A time ball or timeball is a time-signalling device. It consists of a large, painted wooden or metal ball that is dropped at a predetermined time, principally to enable navigators aboard ships offshore to verify the setting of their marine chronometers. Accurate timekeeping is essential to the determination of longitude at sea.
Royal Observatory, Greenwich, London. Installed in 1833, a time ball sits atop the Octagon Room
The Boston Time Ball (1881)
Sydney Observatory with time ball
The time ball at Port Lyttelton, New Zealand, started signalling Greenwich Mean Time to ships in the harbour beginning in 1876. The Lyttelton Timeball Station was destroyed by an earthquake in 2011 but was rebuilt and reopened in 2018.
A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT), and the time at the current location found from observations of celestial bodies. When first developed in the 18th century, it was a major technical achievement, as accurate knowledge of the time over a long sea voyage was vital for effective navigation, lacking electronic or communications aids. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent experimentation and testing that revolutionized naval navigation.
A marine chronometer by Charles Frodsham of London, shown turned upside down to reveal the movement. Chronometer circa 1844-1860.
The marine "Chronometer" of Jeremy Thacker used gimbals and a vacuum in a bell jar.
Henry Sully (1680-1729) presented a first marine chronometer in 1716
John Harrison's H1 marine chronometer of 1735