The Hong Kong Observatory is an official timekeeper in Hong Kong. The provision of an accurate time service, particularly to mariners, was one of the original duties of the Observatory since its establishment in 1883. In the early days, astronomical observations were taken at the Observatory using a 6-inch Lee Equatorial and a 3-inch Transit Circle, of which, observations taken by the Transit Circle was mainly used for the determination of local time. The time signals were indicated to as many people as possible by dropping a 6-feet diameter time ball from a mast in front of the Marine Police Station at Tsim Sha Tsui. Each day about 12:50 p.m., the barrack sergeant of the Marine Police in conjunction with the Observatory, raised the ball to the top of the mast and dropped it at exactly 1p.m. The time ball was first dropped on 1 January 1885. The time ball tower was later moved to Blackhead Point in January 1908.
In 1904, Greenwich Mean Time was adopted as the basis for Hong Kong Time (8 hours in advance of Greenwich Mean Time). With the introduction and wider use of time signals on radio broadcast, the time ball was dismantled on 30 June 1933.
After the war, pendulum clocks were installed and regulated by radio time signals from other centres. Timing accuracy gradually improved from seconds to within one-fifth of a second per day.
In 1966, a crystal-controlled timing system was installed to replace the pendulum clocks. Direct broadcasting of the 6-pip time signal from the Observatory on 95 MHz also commenced in the same year until 16 September 1989.
On 1 January 1972, Hong Kong adopted UTC as official time standard. A timing system based on a Caesium beam atomic clock was first acquired in 1980 and a replacement clock was installed in 1994. The frequency standard controlling the clock is a primary standard and is traceable to the primary standard of the Communications Research Laboratory in Japan.
In 2004, the Observatory installed a high accuracy time transfer system and employed the Global Positioning System common-view method to provide time information of HKO's atomic clock to the Bureau international des poids et mesures (BIPM) for determination of UTC. HKO also adjusts its atomic clock based on time information provided by BIPM to maintain an accuracy of better than 1 millionth second.
Today, the Hong Kong Observatory's time service is based on the Caesium beam atomic clock with an accuracy of fractions of a microsecond a day. Such a level of accuracy is important to scientists, industry and to other professionals whose work requires it. The public can check the time using the Observatory's Web Clock and Dial-a-weather System, as well as listening to the 6-pip time signals broadcast by RTHK. They can also synchronize computers using the Network Time Service of the Hong Kong Observatory.