The red “time ball” on the roof of the Royal Observatory, Greenwich, has been signalling Greenwich Mean Time since 1833. In those 175 years the timescale kept by the astronomers at Greenwich has become part of our daily life (David Rooney writes).

Each year 1.3million visitors make the pilgrimage to our courtyard, snapping photos of each other standing across the Prime Meridian — the line that defines GMT. And since 1884 the time at Greenwich has been at the heart of the world's time zone system.

In 1880 Britain's legal timescale was set at GMT — but the time now given out by the BBC's pips, the BT speaking clock and the official British time signal from Cumbria is no longer GMT. Since the 1970s it has been a timescale called UTC.

The two timescales are never more than a fraction of a second apart, by definition, and are tied using the neat trick of leap seconds.

Greenwich time means a lot to us. When the BBC was set up, in 1922, the time signals emerging from the transmitter on the roof of Selfridges provided GMT. The Greenwich pips followed in 1924.

In 1935 the Post Office held a talent competition to find the “girl with the golden voice” for their speaking clock service. Callers to “TIM” heard Greenwich time to a fraction of a second — precisely.

GMT was also the stock-in-trade of Maria and Ruth Belville who, from the 1850s, travelled London selling the time to watchmakers and others using a corrected pocket watch named Arnold. Ruth, Maria's daughter, was in business until 1940. Londoners kept time by the Belvilles' beat for a century.

Trying to find a timescale that suits everyone is nigh-on impossible. In 1894 a French anarchist named Martial Bourdin blew himself up outside the Royal Observatory in a botched attempt to destroy the clock outside our gates. The heartbeat of Britain ticked on.

Specialists who want a leap-second-free timescale can use one of two that exist for the purpose. Astronomers and navigators can get the old variable GMT to great accuracy for their observing needs.

But we ordinary folk want the best of both worlds. We want our civil timescale to be even, because technologies such as mobile phone networks and computers need that. But we want our time to match closely the time by the Sun. So we tie UTC to GMT with leap seconds. Cutting the link between our clocks and the stars would be a bold step.

- David Rooney is curator of timekeeping, Royal Observatory, Greenwich, and author of Ruth Belville: The Greenwich Time Lady (NMM, 2008)

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