This Demonstration shows the two types of 24-hour analog dials used on some clocks and watches. The 24-hour dial is used on technical and specialist timepieces such as watches for pilots and clocks used as time controllers. This version includes another thin blue hand, which is a UT (Universal Time), "Zulu", or GMT hand.
We are all familiar with the 12-hour analog dial used on clocks and watches, but the less familiar 24-hour version of the analog dial is still in use today. In this version, all 24 hours of the day are shown on a single dial. The seconds and minutes are shown as usual, but the hour hand on a 24-hour dial completes just one revolution every 24 hours; on the more common 12-hour dial, the hour hand completes two revolutions every day. There is no possibility of confusion between AM and PM times on a 24-hour analog dial!
The earliest clocks, built in the Middle Ages, would most likely have had 24-hour dials rather than 12-hour dials. You can still see some of these early designs in old European clocks, particularly the astronomical clocks such as the ones in Exeter, Wells, Ottery, and Wimborne, in England, and many other examples in Germany, the Czech Republic, and Italy. The 24-hour dial survives to this day on technical and specialist devices.
The two main variants of the design show different starting points for the start of the day. With 12 at the top of the dial, midnight is at the base of the dial, and the hour hand approximately mirrors the apparent motion of the sun through the day, rising during the morning, and then dropping again during the afternoon. With "0" at the top of the dial, noon is at the base of the dial, and in this configuration the dial's appearance is closer to that of the sundial.
This version includes another hand, a UT, Zulu, or GMT hand. This is frequently found on pilots' 24-hour watches, as a reference point for the system of time zones.