Greenwich time

This screensaver is a reasonably accurate simulation of the famous Shepherd clock at Greenwich in London. If you have a Mac, you can download it from this site. See the Software page on this site, or download it from this link.


In 1852 Charles Shepherd installed a new clock outside the gate of the Royal Observatory at Greenwich. This was an electrically operated clock, one of the earliest ever made, and it was controlled by a master clock mechanism inside the main building.

While Shepherd provided the engineering know-how, the original idea had come from the Astronomer Royal, George Airy. With the arrival of the railway network, England had recently found that a single time standard was needed to replace the various incompatible local times then in use across the country. Airy decided that this standard time would be provided by the Royal Observatory. His idea was to use what he called ‘galvanism’ or electric signalling to transmit time pulses from Greenwich to slave clocks throughout the country. The new submarine cable recently installed between Dover to Calais in 1851 raised the possibility of sending time signals almost instantly between England and France – this would allow longitude differences to be measured very accurately, for the first time.

In September 1851, Airy wrote to Shepherd asking for proposals and estimates. He included a request for the following clocks:

One automatic clock. One clock with large dial to be seen by the Public, near the Observatory entrance, and three smaller clocks, all to be moved sympathetically with the automatic clock.

He also wanted the Greenwich time ball to be electrically operated, so that it would drop down its flagpole at exactly 13:00.

By August 1852, Shepherd had built and installed the network of clocks and cables in the observatory, although the costs were considerably higher than the original estimates. Shortly after, for the first time, Greenwich mean time was transmitted along cables from Greenwich to London Bridge, and thence to clocks and receivers throughout England. The primary pulse originated from this unlikely-looking master clock in the observatory.


By 1866, time signals were sent from this clock to Harvard University in Cambridge, Massachusetts via the new transatlantic submarine cable.

The public clock at the gate originally indicated astronomical time, in which the counting of the 24 hours of a day started at noon every day rather than midnight.

This photograph was taken in the 1870s, and shows the clock approaching midday, at the astronomical time of 23:20.


A similar photograph, from 1870, shows the clock early in the morning, with the dial showing 18:52. The clock faces east, and the sun is high up to the north east (early morning). Obviously at midday astronomers can precisely observe the sun crossing the meridian so as to set or reset a clock. The 0 here means “start counting again”.


Following the International Meridian Conference, the clock was altered to show civil time with effect from 1 January 1885. Ever since, it’s shown Greenwich Mean Time, rather than British Summer Time (Daylight Saving Time).

The dial has seen numerous minor design changes over the years:

The original:

After a (not very successful) 1910 repainting:
after 1910 repainting

In 1940, a bomb destroyed the gates and damaged the dial. In 1947 a new replacement dial was installed:


The 1981 repainting thankfully reverted to an exact copy of the original design:


The clock is still ticking happily away today, although it’s now controlled by a quartz mechanism inside the main building. Here’s one of thousands of recent photographs:


The master clocks are still on display, as are the famous series of chronometers made by John Harrison. In the shop you can buy postcards and lapel pins of the clock (but no fridge magnets!) and there’s also a small 24 hour quartz clock on sale. It’s like a replica, although the complexities of the original Shepherd design have been replaced by a simpler interpretation.

When attempting my own version of this clock, the hardest parts were definitely the roman numerals. I couldn’t find any font which matched and had to draw them again from scratch. If you compare my copy and the original you’ll notice how much better the original is. Many subtle variations in size and placement were employed to produce an acceptable solution to an intractable problem: how to make numbers of such different widths do the same job. It’s not a pretty design, I have to say, but it has a good solid Victorian heaviness to it, which matches its history well.

For even more information about the clock, refer to this article on the Royal Observatory Greenwich website.


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