I learnt this at Greenwich actually. The Time Ball (invented by Captain Robert Wauchope) is a signaling device in which a ball is dropped at a given time in order for ships' masters to determine the error and rate of their chronometers whilst in harbour. The Time Ball Tower in the Waterfront was built in 1894 and is situated next the Harbour Engineer's former residence (Dock House). It remained in use for 40 years after which new technology led to it lying idle for 63 years before being restored and officially recommissioned in November 1997.