Formal Gardens Sundial
Background
The sundial in the formal gardens of the Nonsuch Park mansion house is thought to be well over 100 years old. However, the concept of measuring time by the relatively steady passage of the Sun across the sky has been used for well over 3,000 years, although it is likely that humans have been using the concept of using the Sun's shadow to measure time well before this. Mechanical clocks were being developed from medieval times, however these were notoriously unreliable and had to be reset frequently, usually by reference to a sundial. By the 19th century, time balls (such as the one on top of the observatory in Greenwich installed in 1833) were used to set clocks, especially for marine chronometers, which were essential in ships navigation on the open sea. The time was determined again by reference to the Sun or the passage of stars across the meridian line.
In 1924 the BBC commenced the broadcast of the time pips, to indicate the precise time of the hour of the day. The time for these was set by mechanical clocks at Greenwich observatory. We now also have timepieces that keep time by reference to a radio signal, and adjusting the timepiece automatically. Parallel to these improvements in determining the exact time, the timepieces themselves have improved in accuracy, meaning they had to be corrected less often. Hence by about the late 19th century the humble sundial lost its significance as a timepiece, but still retains popularity as a garden ornament or even art installation.
How do Sundials Work
There are many different designs of sundials, but the one we are likely to be most familiar with is the bronze disc, set horizontally on a pedestal. It was also common to see these positioned vertically, such as on church towers. The sundial would have a raised spike or wedge (the gnomon) that casts the shadow of the Sun onto the face of the sundial. The time is then read off against by reference to the shadow as it passes across graduations etched on its circumference, indicating the time.
Limitations of Sundials
- The most obvious limitation of sundials is that they need the sun to be shining in order to work! Hence they are only of ornamental value at night time or on overcast days.
- Adjustments need to be made to allow for the geometry of the Earth's orbit about the Sun. Our orbit is not perfectly circular, and the plane of the orbit is also inclined, meaning that the exact time can vary slightly either side of the sundial's markings depending on the time of year.
- The time per the sundial also needs to be adjusted for summer time.
- Time based upon a sundial is also specific longitude. Hence, for example, when the sundial indicates it is midday in London, a sundial in Bristol would be showing 11:50am, due to Bristol being further west. Prior to the advent of the railways in the 19th century people would have worked on this 'local' or 'Sun' time, so local time in Bristol was 10 minutes later than local time in London. The need to have unambiguous railway timetables nationwide led to the concept of GMT as a nationwide standard time which was in widespread use by the 1850s and officially adopted in 1880.
