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
We started out defining the second as a division of the day. In 1967, the definition was shifted to cesium atomic clocks, which is much more precise. From that time, it’s been obvious just how variable the rotation of the earth is. If you damn a river flowing toward the equator, you hold more mass closer to the poles (and therefore closer to the axis of rotation). Like when a spinning skater pulling in their arms, the earth speeds up. There are a bunch of other factors that effect earths rotation and it’s not fully understood, but we’ve had leap seconds about three dozen times since we went to the atomic clock standard.
Leap seconds tend to play havoc with the internet though, because a bunch of servers suddenly find themselves a second off from others during the transition. It was recently decided to stop the practice and just let solar time drift off of UTC. It’s not like you’ll notice though. The drift is about a minute per century.
It gets so much more confusing and precise 😀 calibrating clocks across the world with NTP.. high precision local clock calibration down to nanoseconds in a small network with PTP, atomic clocks, universal coordinatedtime (UTC) vs international atomic time (TAI).. leap seconds, the leap smear ..
Time is way complicated!
Originally? They used a sundial. A second was 1/60 of a minute. Which was 1/60 of a hour. And an hour was 1/12th of the time from sunrise to sunset on the equinox.
The babylonians loved 12 and 60 (and 360) because it made trigonometry not need a lot of fractions. Plus they can be easily divided into whole numbers.
I think the responses are missing the heart of your question.
You’re NOT asking how do we make sure that the second is standardized.
What i think you’re really asking is, how did we choose the length of the second such that the day is evenly divided by 86400 seconds. The answer is, we didn’t. We didn’t choose a second first. Like another commenter said, we chose hours first. We didn’t initially have seconds, we had hours, then minutes, then seconds.
The second was initially defined to be a specific fraction of a day. When we got better and better mechanical ways of measuring seconds, we just built that in. More recently, it became more important to have the second have a constant definition, rather than tied to earth’s rotation rate. But it’s still really close.
Imagine you have a tiny clock inside a cesium atom. This clock doesn’t have hands that move around like the clock on your wall. Instead, it “ticks” in a special way. Inside the cesium atom, there’s a tiny part called an electron, which can jump back and forth between two spots. Each time it jumps, it’s like a tick of our tiny atomic clock.
Scientists have found out that if you watch a cesium atom, the electron will jump back and forth exactly 9,192,631,770 times in one second. Because this number is always the same, it helps everyone in the world agree on what a second is.
To get the second from the cesium atom, scientists use a special tool called an atomic clock. This clock watches the cesium atom very closely. It counts how many times the electron jump.
So, thanks to the cesium atom’s steady ticking, we know exactly how long a second is, and we can keep time very precisely for all sorts of important things, like computers, GPS, and even when we launch rockets into space!
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.
A leap year isn’t a whole year. It’s a day. Every four years.
Why do we have that day?
Because the rotation of the Earth isn’t 24 hours. It’s actually off from that by a few seconds. Add up a few seconds by 365, and you get a little less than .25 days. Do that 4 times, and you have almost a day left over, which is why we do leap days/years. Since it’s not quite .25 days, we also have to skip every 25th leap year to accomodate.
So you see, leap years are actually leap days that are made up of leap seconds.
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