EDIT: I believe I have kicked a hornet’s nest. Did not expect this to blow up! But I am still looking for the “ur time”. the basis for it all. Like, maybe the big bang, or something.
In: 5399
The international coordination of time is controlled by the International Earth Rotation and Reference Systems Service, so they are like the time lords. They use atomic clocks to make sure time is as accurate as can be.
It depends. There are a few different “master” clocks in use across the world.
For example, the US Military uses the atomic clocks located at the US Naval Observatory and maintained by the Precise Time Department. They use dozens of cesium-beam standards and hydrogen masers, which, when averaged together and sampled every 100 seconds, provided a uniform time scale with a precision of about one nanosecond (10-9 s) per day, averaged over a year.
Those clocks don’t “refer” to anything. They are the standard and what ever they say the time is is the time.
Time used to be based on the sun, going from sunrise to sunset. But this means days have different lengths in different areas and in different seasons. At some point we all agreed what the worldwide time is and we keep track of that ever since.
There are 2 simple answers. Yes, there’s a master clock (sorta), but the real marker of time is the Sun.
First, yes, there is a master clock. There are hundreds of atomic clocks in many countries that keep time more accurately than humans are capable of noticing. Coordinated Universal Time or “UTC” is how all these atomic clocks work together. In a sense, it’s like a single master clock.
Second, the real master clock is caused by the Earth’s movement and the Sun’s position in the sky. So we can measure the “true” time relative to Sun’s position over a specific point on the Earth. Sometimes, human clocks (including UTC) have to be adjusted in very tiny amounts because they need to match the Sun more closely.
Time is relative, and not just in an E=MC2 kinda way. It used to be that time was based on noon. Whenever the sun was at its highest point was noon. Sun dials made it more predictable by having a clock but usually there was a big clock on the town square that everyone had to use(like Big Ben. If you could afford a watch you simply sync it with the town clock.
As transportation became faster with trains this made it clear that every town having a different time would complicate train schedules. Eventually time zones were created to make a uniform system.
Now we have computers that synchronize to a clock like the atomic clock in Colorado. There is also one in Greenwich England. They use a lot of very accurate astronomy but basically use the same principle of “noon is when the sun is highest” but on a very precise scale.
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Fun fact: Chances are, you’ve never in your life cared about your true local time. You think your local time is based on your time zone. Unless you’re really, really lucky though your timezone won’t give you the “correct” time based on your exact location – it’s an approximation for a given geographic area. Your “true” local time would be best measured by setting noon to when the sun is at its highest elevation each day.
Edit: Might as well share some of my other “Astro” related knowledge. On my first deployment with the Navy, I spent most of the 6-month trip filling the role of “Astro boy”. I had to do all the Astro calculations for each day. This was back before the internet or an app could do it for you. I’d calculate:
* Sunrise/Sunset (Sun halfway above the horizon)
* Civil Morning/Evening Twilight (Center of the sun’s disk 6 degrees below the horizon)
* Nautical Morning/Evening Twilight (Center of the sun’s disk 12 degrees below the horizon)
* Bearing Amplitude of the Sun/Moon (The compass bearing where the sun/moon rises/sets)
* And just for kicks and to prove how precise my calculations were, True Local Time. Which in a moving ship, also requires you to extrapolate your position at noon accurately.
Much like True Local Time, most people don’t think about bearing amplitude of the sun/moon. You think the sun rises in the east and sets in the west. That’s generally true, but not very accurate. If you’re well north of the equator, the sun will rise and set south of a perfect east/west heading, and vice versa if you’re south of the equator. Once upon a time, mariners would use bearing amplitude to check and calibrate their navigation equipment. By the time we started using laser ring gyros, Radio Aids to Navigation, Inertial navigation systems, and GPS, it was more just a way to torture junior officers.
Answer: We used to base time on astronomical events. A day used to be the amount of time that it takes for the sun to come back to the exact same position in the sky.
So we would take that length of time, divide it in 24 hours, divide each hour in 60 minutes and divide each minute into 60 seconds. In fact, the definition of a second used to be 1⁄86400th of a day.
24 × 60 × 60 = 86400
In the mean time, we kept creating ever more accurate clocks. The mechanical pendulum was replaced by a diapason, then by a quartz crystal, then by caesium 133.
So now, instead of measuring down from a day, we measure up from a caesium atom.
The catch is that the Earth’s rotation is not consistent. The 2011 earthquake in Japan [was strong enough to delay the day by 1.8 microsecond](https://www.space.com/11115-japan-earthquake-shortened-earth-days.html).
So on the following New Year’s Eve, we corrected the 1.8 microseconds during the twelve strokes of midnight.
So we not have clocks that are so stable that the Earth gets out of sync with them.
Yes, it’s called [Coordinated Universal Time](https://en.wikipedia.org/wiki/Coordinated_Universal_Time) (UTC). All time zones are defined as being a certain number of hours and minutes ahead or behind UTC.
The time itself is based on a weighted average of several atomic clocks located in laboratories all over the world.
Noon is set by when the sun is directly overhead if on the equator or due South in the northern hemisphere, due North in the Southern Hemisphere.
World time is set from Greenwich (in London England) going back to Colonial days.
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