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Time and Calendars · entry 33 of 52
A second was once defined as a fraction of a day, but atoms now define it.
The longer version
The second began as a division of the solar day: split the day into 24 hours, each into 60 minutes, each into 60 seconds. That made the second a slice of Earth's rotation rather than an independent unit. Tides and shifting mass inside the planet slow the spin gradually.
Rotation is not steady enough for modern needs, so the definition was moved to a fixed count of oscillations of a particular atomic transition. The chosen number preserved the old length as closely as measurement allowed, so nothing about daily life changed, while the unit itself became reproducible anywhere. Now the day is measured against the second, not the reverse.
Where this entry sits
These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.
More From Time and Calendars
Eight more entriesThere are 60 seconds in a minute and 60 minutes in an hour.
Read 35Babylonian mathematicians favored the number sixty, which is why minutes and hours use it.
Read 36Ancient Egyptians divided daylight into twelve parts, which helped shape the twelve-hour clock.
Read 37Mechanical clocks first appeared in medieval Europe, often mounted in towers for public use.
Read 38Clock faces usually move clockwise because that is the direction a sundial's shadow travels in the north.
Read 39Early mechanical clocks had no faces and simply struck bells to announce each hour.
Read 40Wristwatches became common only in the twentieth century, replacing pocket watches for most people.
Read 41Digital clocks display numbers directly instead of using hands that rotate around a dial.
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