Friday, February 5, 1999 Published at 17:34 GMT
Six pip salute
The Dent clocks: Precision instruments
The "pips" have been used by millions of people in the UK and around the world to set their clocks and watches. Heard before the main news bulletins on radio, they have also preceded some of the great announcements of the 20th Century, from the invasion of Poland in 1939 to the death of Diana, Princess of Wales, in 1997.
The timepieces have been in storage for several years, but, according to Jonathan Betts, the observatory's Curator of Horology, are still in perfect working order.
"All that's been required is some light cleaning and re-lubrication," he told BBC News Online. "They'll be ticking."
Time was marked by two weight-driven astronomical regulators - one was a back-up in case of breakage.
The clocks - No 2016 and No 2012 - were actually built in 1874 by E Dent and Company of London for use in finding objects in the night sky. Although still regarded as precision instruments in 1924, technological improvements meant that more accurate timekeepers were soon being used to regulate the Dent clocks.
"It was a wonderfully conceived system," said Jonathan Betts. "An audible signal, as astronomers have known for many years, is far more useful than any visual signal - you listen while you check your watch.
"Also, they were the first - everybody internationally has followed the BBC."
Today, the Corporation keeps its own time with two atomic clocks sited in the basement of Broadcasting House. These are kept in step by signals received from the Global Positioning Satellite (GPS) system and by information received from the Rugby radio transmitter facility which is operated by BT Aeronautical and Maritime under contract to the National Physical Laboratory (NPL).
This is far more regular than Greenwich Mean Time (GMT) which drifts as the speed of the Earth's rotation changes. Differences between the two mean that "leap seconds" occasionally need to be added to the time signal, by means of an extra pip. This keeps the two types of time roughly in step.
This oddity is one of the reasons why the last pip is slightly longer than all the rest.
"As a consequence of going over to atomic time, ever so occasionally we need a seven-pip time signal like there was at the end of December," said John Chambers. "And if you have a seven-pip time signal, people are going to do a double take unless you lengthen the last pip."