3D view of how people measure the time

3D view of how people measure the time

Understanding and measuring the passage of time already played an important role in ancient farming and in the lives of agricultural peoples. The first calendars appeared at the same time as the emergence of writing and the development of mathematics and astronomy. They contained the fundamental units for measuring time: day, month and year.

The year is a fundamental unit for measuring time. There is a distinction between a sidereal year and a calendar year. A sidereal year is the time it takes for the Earth to complete one revolution of its orbit, as measured against the fixed stars. A calendar year is made up of all the days contained in the sidereal year. While a normal year has 365 days, a leap year consists of 366 days.

Another fundamental unit for measuring time is the month. A month is the period of a complete revolution of the Moon around the Earth. This period is difficult to describe, and the period of a cycle is not constant. Still, in calendars, for example in our Gregorian calendar, a month is made up of entire days and a calendar year is divided into 12 months. The third fundamental unit for measuring time is the day. A day, an astronomical unit of time, is the span of time it takes for the Earth to make one entire rotation. A calendar month can be either 28, 30 or 31 days. February usually has 28 days and 29 in leap year.

One of the most ancient devices for measuring time is the sundial. According to written sources, it was already used by the ancient Greeks. There are several types of sundials. On the horizontal sundial, the stylus is parallel to the axis of the Earth. Since the gnomon is not perpendicular to the surface of the sundial, its shadow will not move at a uniform rate during the day, making it more difficult to mark the hour lines. As the Sun moves across the sky, the shadow-edge of the gnomon aligns with the different hour-lines, indicating the correct time. Even in the Middle Ages, sundials were still considered the most precise time measuring device.

When and where the first hourglass appeared is uncertain. It is certain, however, that it was widely used in the 16th century. The hourglass consists of two vertical glass bulbs of equal size mounted on a stand. Initially the lower glass bulb was filled with sand, while the top one was empty. When used, the hourglass was inverted, so the sand would flow into the lower glass bulb. Factors affecting the period of time that the hourglass measured included the amount of sand and the width of the neck.

The water clock was already used in ancient times. According to written sources, it was first used in Egypt and later it spread to Mesopotamia and Ancient Greece. The water clock consisted of a waterproof clay container with markings that showed the passage of time. There was a hole at the bottom of the container through which water was drained. Water was poured into the container up to a given height. Then, as the water left the container, an increasing number of markings were visible to the observer. The span of time measured could be adjusted by making an appropriate selection of the markings and changing the diameter of the hole.

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