Calendar, chronometer, clock… Where does this division of time come from? Why is the year divided into 365 days, 12 months, and the day into 24 hours? Why does an hour count as 60 minutes? Explanations of these quasi-universal units of time.
It is the alternation of day and night that is the first sign of the times for all mankind: the duration of one day is that of a “sunrise” to a “sunrise” the next day, even if it’s not quite the sun , which rises at the beginning of the day, but the earth which revolves around itself. The earth also revolves around the sun. These two movements have different durations: it takes about 365 days and a quarter to complete the journey around the sun.
From observing nature
Another natural complication explains the variety of calendars. The ancient and agricultural world, closer to nature, observed the phases of the moon (marked by new moon, first quarter, full moon, last quarter): these last seven days, hence the division into days of the week common to a large part of humanity ; The duration between two phases of the moon is approximately 29.5 days or one lunar month.
The choice of months of 30 or 31 days in a year of 12 months totaling 365 days is historical, dating back to the Roman Empire, but many civilizations work with lunar months of 29 or 30 days and years of 12 or 13 months old.
Movement of the earth around the sun – day/night – seasons. © PCCL – Physics – Chemistry – College – High School
Calendar: calculations and units of measurement
Why by the dozen and by the half dozen?
Dividing the day into 24 hours of 60 minutes is more universal than calendars because it is a purely human process that is not subject to the complex constraints of nature.
The numbers 24 and 60 are only familiar to us for counting time, as we live in a more “decimal” world, with tens, hundreds and thousands. However, we buy eggs or oysters by the dozen or “half dozen”.
The decimal system is natural only for counting on fingers, but for dividing it is often useful to be able to divide into 2 or 3, even into 4 or 6. Dividing a cake into 5 or 10 is less convenient than into 12, and even into 24. And dividing into 60 (or into 20) is hardly more complicated than dividing by 5… Also, in watches with hands, the dial is divided into 12 hours.
Counting 60 minutes in an hour or 60 seconds in a minute requires precision engineering instruments such as a stopwatch. On the other hand, dividing the day into hours was a matter of course long before clocks were invented. It is enough to observe the movement of the sun across the sky when it is sunny, and then dividing the day is as easy as cutting a cake into 6, 12 or 24.
human time, earth time
The sundial, without a shadow of a doubt
Without explaining in detail how a sundial works, one can simply follow the shadow of a (vertically placed) stick. Noon is the time when the sun is highest in the sky during the day: here the shadow is shortest and the sun points in a fixed direction, the south, regardless of the season. Only on the equinoxes (around March 20 and September 22, when there are 12 hours day and 12 hours night) does the sun rise, precisely indicating the direction to the east and the stratum to the west.
In summer, the sun rises higher in the sky, rising in the east (northeast) and setting in the west (northwest). However, half a day, or 12 hours, elapsed between the times the sun moved from geographic east to west. And if we draw a circle around the staff whose shadow we are observing and divide it into 24, the hour will be marked by the direction of the sun. So when the sun is in the east, it is always six o’clock in the morning, six o’clock in the evening when it is in the west.