It seems like everything with our calendar is based around 24hr days and the number of 24hr days to revolve around the sun. But a 24hr day can be broken down to 1,440 minutes and in turn 86,400 seconds. How did we (humans) calibrate the second so that exactly 86,400 would be 1 rotation of the earth to the point where we never need something like a “leap second” like we have with leap years?
In: Planetary Science
There is a whole history of how the second became more well defined:
– Originally it was defined as a 1/86400 of a day. There were various mechanical devices built to try to oscillate at a rate where the seconds could be counted. Most advanced countries had a master clock somewhere that provided their reference.
– In the mid 20th century, it was discovered that crystal oscillators were much more accurate than any mechanical device, so the second was defined by oscillations of such crystal oscillators. There were a few places that had crystal oscillators that people widely agreed as being the reference setters, with the most widely accepted one being NIST.
– More recently, desires to get an even more accurate definition led to defining the second even more exactly. This time it was defined as exactly 9,192,631,770 oscillations of a Cesium atom, which was to most stable practical reference known.
These measurements are now even more accurate that the Earth’s rotation. The Earth’s rotation varies, mostly as a result of water being at different levels due to climate trends, causing small shifts in rotational speed. This causes the need for “leap seconds” every now and then to keep the seconds on the calendar the same as the exact position of Earth’s rotation.
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