|A Ring Dial - or two-ring universal equinoctial ring dial to give it its full name - was a cutting edge scientific instrument in its time, which is about 500 years ago! Essentially it is a type of sundial that allows you to tell the time wherever you are, so long as the sun is shining.Originally developed in the early 17th century by the English mathematician William Oughtred, it was a development of an earlier instrument, the astronomer's ring, which was used for measuring the altitude of stars and also terrestrial objects.You need to know your latitude, so this is the first adjustment that you need to make. Secondly you need to set the date using the sliding section in the middle. Then you rotate the inner ring until it won't turn any more, and your ring dial will now look like the picture above. Suspend the ring dial by the cord, such that the sun shines through the pinhole in the middle, and hits the centre of the inner ring. From here you can read off the time. When you have finished, you can fold the ring dial flat again. The ring dial does not allow for the 1 hour change in summer time, so you need to factor that in to your calculations. It can work in any latitude from 20 degrees to 75 degrees, in either hemisphere.What is clever is that, unlike a normal sundial that needs to be aligned and requires you to know the exact direction of north, the ring dial does not need this information.Needs no batteries!This modern replica is made of solid brass, and comes in a small cardboard presentation box, together with instructions.The dial is 6cm across.
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