The one reason why we have 24 hours in a day is that people decided that this is the most convenient way to estimate time. There is nothing in nature that determines the duration of days, minutes, and seconds which are the main time units.


But it is different if we speak about a time period corresponding to what we call a day. The continuance of the day is the time interval during which the Earth makes a full turn around its axis. This period of time is always the same.

You may be surprised, but in fact a day does not equal exactly 24 hours. Scientists have calculated the time of the whole Earth rotation and it is quite simple. They have detected at which moment the certain star is above the meridian, this moment is considered the beginning of the day. The end of day is marked at the second when the same star again appears above the same meridian. The time interval measured in this way is called astronomical and is equal to 23 hours 56 minutes and 4.09 seconds.

It is interesting that time was not always measured in this way. For example in ancient Greece the length of a day was determined by the time between dawn and dusk. Night hours were simply not counted.

The ancient Romans began to count time the same way as we do now: from midnight to midnight.

Before the clock was invented, both day and night were counted as 12 even intervals. Such a system was inconvenient, as the length of the day and night is not constant. In summer each day is larger than the night, while in winter it is the other way around.

So the ancient Romans doled the whole day into 24 identical parts (hours). Today time is measured in this way everywhere.