Sundials: not just a pretty face

Talk by David Cook at Priory Street, 1 March 2019.

There is more to sundials than things plonked in a garden on screwed to a church wall and you would be surprised how accurate they can be.

David Cook
David Cook, presenter of the talk

David presented an interesting talk for the Society at the Priory Street Centre on Friday 1 March. He started out by saying that sundials are ‘rubbish’ and then went on tho show that a lot of them aren’t, demonstrating a range of different sundials from his own collection.

David showed us that in principle a sundial will show the time (as long as the sun is shining!), but a number of factors combine to make them inaccurate. These are:

  • Because the earth’s orbit around the sun is an ellipse it travels at different speeds at different times of the year.
  • The rate of the Sun’s passage through the constellations changes over the course of the year because of the tilt of the earth. It is fastest at the solstices, and slowest at the equinoxes.

David explained that the equation of time is a measure of the variation in the length of each day through the course of the year, because of these effects. In certain months of the year, days can be up to 20 seconds longer or shorter than 24 hours. He pointed out that, for any particular day of the year at the same time the sun would be in a different position and this draws out the shape know as an Analemma.

David demonstrated a simple graph that shows the amount of time to be added or deleted from that shown on the sundial. More sophisticated sundials have an adjustable apparatus so that this calculation is unnecessary. In addition, the longitude of the sundial in relation to Greenwich must be accounted for. Finally the gnomon, which casts the shadow, must be parallel to the earth’s axis, so depending on the latitude the angle of it will vary.

David completed his talk by showing images of sundials from around the world, and then invited the audience to view some of the examples he had bought with him. See below for some examples.

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