The Sundial of Trevithick 

Since it’s a lovely sunny day in Cardiff – and already very warm – I thought I’d step outside the office of the Cardiff University Data Innovation Research Institute which is situated in the Trevithick Building and take a picture of our new sundial:

This flat sundial was installed by a company called Border Sundials and is designed very carefully to be as accurate as possible for the particular wall on which it is place. It’s also corrected for longitude.

However, I took the photograph at about 10.30am, and you’ll notice that it’s showing about 9.30. That’s because it hasn’t been corrected for British Summer Time so it’s offset by an hour. Moreover, a sundial always shows the local solar time rather than mean time which is shown on clocks. These differ because of (a) the inclination of the Earth’s orbit around the Sun relative to the equator and (b) the eccentricity of the Earth’s orbit around the Sun, which means that it does not move at a constant speed. The difference between mean time and solar time can be reconciled using the equation of time. The maximum correction is about 15 minutes, which is large enough to be seen on a sundial of this type. Often a graph of the equation of time is placed next to a sundial so one can do the correct oneself, but for some reason there isn’t one here.

The sundial adds quite a lot of interest to what otherwise is a featureless brick wall and we often notice people looking at it outside our office.

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