# Time unit

A time unit is a physical unit of time measurement . In the international system of units , the basic unit of time is the second (s), defined as 9,192,631,770 times the period of a microwave that is in resonance with a selected transition between two energy levels in the cesium atom .

In addition to the second, there is the minute (1 min = 60 s), the hour (1 h = 60 min), the day (1 d = 24 h), the week (7 d), the month (4 weeks + 0– 3 d or 28–31 d), the year (1 a = 12 months or 52 weeks + 1–2 d or 365–366 d), the decade (10 years), the century (10 decades), the millennium (10 Centuries). A million years is abbreviated to Ma = "mega-years" (megannum), a billion years with Ga (gigaannum) based on the SI units .

## history

### 24 hours a day

The division of the day into twelve parts probably comes from a version of the sundial of the ancient Egyptians. Since the timing was long done with the help of sunlight, the night could not be divided so easily. Later, Egyptian astronomers discovered 36 stars that divided the sky into equal parts. The night could be divided with 18 of these stars. Three stars each were used for dawn and dusk, as it was difficult to see the stars at that time. The time of total eclipse was determined by the remaining twelve stars. Between 1550 and 1070 BC This system was simplified by using 24 stars, twelve being for the night. The Klepsydra was also used for timing.

Later, between 147 BC , Hipparchus BC and 127 BC Suggested dividing the day into 24 hours of equal length. However, such a classification only became common in the 14th century with the advent of mechanical clocks.

### 60 minutes and 60 seconds

The Babylonians made astronomical calculations in the 60s system, also known as the sexagesimal system. Hipparchus took over this later. Eratosthenes used the 60s system to divide a circle into 60 parts. With this he established an early system for latitudes . Hipparchus improved this system. Claudius Ptolemy divided the 360 ​​degrees in his work Almagest into two stages, each with 60 units. He called the first unit partes minutae prima - the minutes - the second partes minutae secundae - our seconds.

## Individual evidence

1. Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day? Scientific American , March 5, 2007, p. 1 , accessed June 1, 2012 .
2. Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day? Scientific American , March 5, 2007, p. 2 , accessed June 1, 2012 .