Days on the young Earth lasted about six hours, and they keep lengthening
When Earth formed, a full day probably lasted around six hours. The Moon's tidal pull has been braking the planet ever since, and the average solar day now runs about two thousandths of a second longer than 86,400 seconds, growing roughly 2 milliseconds more every century. Fossil corals preserve part of that slowdown.
The palaeontologist John W. Wells worked out how to read ancient day lengths from growth rings in fossil corals, whose biology responds to the tides. Stack enough daily bands against yearly ones and you can count how many days an ancient year held. Modern timekeepers feel the same slowdown. Because atomic seconds no longer match Earth's spin, leap seconds are occasionally added to Coordinated Universal Time; between 1972 and 2022 there were 27 of them, announced in advance by the service that monitors the planet's rotation.
Even the ordinary day is slippery. Measured by the Sun's return to its highest point, a day can run up to 7.9 seconds long or short of 24 hours, because Earth's orbit is an ellipse and its axis is tilted; clocks use an imaginary mean Sun to smooth that out. Measured against the distant stars, one rotation takes only 23 hours 56 minutes and about 4 seconds. The gap exists because Earth also moves along its orbit each day, so it must turn nearly 361 degrees to face the Sun again.
Where a day begins is a matter of culture. The Jewish day starts at sunset or nightfall, echoing Genesis, where evening comes before morning, and medieval Europe counted hours after sunset too; Christmas Eve and Halloween are leftovers of holidays that began the night before. Ancient Egypt reckoned from sunrise, while the Romans and Chinese started at midnight. Astronomers once began the day at noon so a whole night's observations shared one date.
Standard time zones arrived with railway timetables in the mid-nineteenth century, and most major countries had adopted them by 1929. The word day itself goes back to Old English dæġ, cousin of German Tag.
Source: Day
