Diptych Dial
 © Museum of the History of Science, Oxford
Four hundred years ago,sundials were the most important things to be based on mathematical theories. They were the only way that people could tell the time accurately because reliable watches hadn't been invented. Sundials are able to tell the time from the position of the sun in the sky. The sun casts the shadow of a spike called a 'gnomon' (pronounced 'no-mon') onto a series of lines that have been calibrated to show the hours in the day.

The 'gnomon' of a sundial absolutely always has to point directly to the Pole Star. If you see a sundial in a garden or on a wall you can always tell which direction north is because the gnomon will be pointing that way. The hour lines on a sundial can be drawn onto any sort of surface, whether flat or curved, that is in any direction at all. Because of this some sundials have even been made in the shape of spoons, cups, and even shoes. However, it requires a lot of very complicated maths to draw the hour lines for a sundial correctly. This is partly because although the sun is always in the same direction at the same time every day, it is at a different height every day. For example, in both summer and winter the sun is always exactly south at midday, but in summer it is much higher in the sky than it would be in winter.
 © Museum of the History of Science, Oxford

This sundial is even more complicated because not only is it pocket sized, but it can be used to tell the time using a number of different time systems. Believe it or not, people didn't only tell the time using the system of 'equal' hours that we use today, in which one hour is always the same length of time throughout the whole year. They also used a system of 'unequal' hours, in which the actual length of an hour changed every day according to how many hours of darkness there were and how many hours of light there were in the day. In winter, when the days are short and the nights long, the system had twelve daylight hours, each individual hour being quite short, and twelve night hours, each individual hour being quite long. Only on the equinoxes were the day hours and the night hours equal. This may sound crazy but it was the most common system of telling time in the Renaissance.

This sundial is called a 'diptych dial' because it is made of two separate plates hinged together like 'diptych' paintings in a large church. It is made of solid ivory and is therefore very precious, coming from a time when elephants were not as threatened with extinction as they are today. Like lots of old mathematical instruments it has been made into a work of art. All its surfaces, inside and out, are covered with very beautiful decoration. The decoration has been engraved and punched into the ivory and filled with different coloured paints. There is even a little picture of a cherub resting its head heavily on its hand. This is to remind us that time flies and that we should make the most of life while we have it. This sundial was made in Nuremberg which was the leading place for sundial making in the 16th century. It was made by Paul Reinmann who was a very famous maker of sundials. It is not known who the sundial was made for, but it is likely that it would have been a rich person.