The oldest sundial found in the Valley of the Kings, Egypt, dates back to the 13th century. The rest of the time, the angle of the shadow gradually changes. At noon, the shadow disappears or lays strictly from north to south. The working principle of a sundial is based on the direction of the shadow cast by a vertical object - gnomon. Therefore, milliseconds, microseconds and other smaller fractions of time are now decimal. But, by the time a second became too coarse to measure even shorter periods, the sexagesimal system had long gone out of fashion and was replaced by the decimal one. People have used this system for more than one millennium. Sexagesimal number system is one of the oldest on earth, and it was also very popular with ancient astronomers, who were the first to measure the exact time intervals. Why there are 60 of them is known quite reliably. The hour had to be divided into lesser units to orientate in time even more accurately. The number 12 is much better for division than number 10 because 10 has only two divisors: 2 and 5. It also can be divided without remainder by 4, as well as by 6. Dividing by 3, we get morning, afternoon, and evening - each interval of 4 hours. For example, dividing by 2, we get the first and second half of the day, 6 hours each. However, the number 12 is notable in that it is divisible without remainder in many different ways. The shadow cast by a long vertical object changed its length and direction, reflecting the passing of the day.Īncients divided the scale of the sundial into 12 equal parts, hours. This periodical phenomenon was used to determine the time during the day using a sundial. Every morning the Sun rises in the east and sets in the west in the evening.

It once seemed that there is nothing more stable for time measuring than the regular change of day and night.

Nuclear time scale