The only reason a day has 24 hours in it is that man has decided he would like to work out a time that way. Nothing occurs in nature or in the world that has anything to do with hours or minutes or seconds. These divisions of time were made up by man for his convenience. But something does happen that has to do with what we call a “day ’. And that something is the rotation of the earth on its axis from west to east. Every time it goes around once, a specific amount of time has passed. We call that time a “day”.
Scientists can measure that time exactly and they use the stars to do it. Observatories have what are called “sidereal” clocks. A sidereai day begins the instant tbat a given star crosses a meridian and lasts until the instant that it rocrosies the same meridian.
Since man has broken up the day into hours, minutes, and seconds, we can say how long a sidereal day is. It is 23 hours 56 minutes and 4-09 seconds long, which means that there are 366 aidereal days in a year. We base our time (solar time) on the Earth’s rotation relative to the Sun, which gives us a slightly longer day and 365) days in a yr, hence we have a leap year every fourth year where there is oneextra day, tomake up.
To early people, a day meant simply the space between sunrise and sunset. The hours at night were not counted. The Greeks counted their day from sunset to sunset. For the Romans, it was from midnight to midnight.
Before clocks were invented, day and night were divided into 12 hours each. This division was not practical as the length of the two periods differs with the seasons. Today, countries have a day which, by law, extends over the 24-hour period from midnight to midnight, following the Roman method.