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
So, this is Hard to ELI5, but basically we have MANY Systems to keep time. The first question you should ask is, what do you want to measure? Absolute time or relative time? We have “one second” very specifically defined as a Number of Vibrations of a specific elements Atom. That would mean we can measure this and therefore keep time very acurately by counting the Vibrations and exactly know how much time has passed since we started to measure. However, this does not Tell us yet what Day or month it is. For this, you need a reference. This is where is gets quite complex to understand. We use the Rotation of earth around the sun would be the simple answer. But how do we know where we where (e. G. at the 1st of January 2000,also called the J2000 reference date)? We can precisely locate pulsing quasars as reference and therefore Figure out orientation and location of earth relative to the sun.
This brings us to all Kinds of issues with the Atom-clock timekeeping. While it is super precise, it does not perfectly align with our arbitrary 12 month system and geogeaphic processes we are interested in (summer, Winter,..). So one issue is, that we have to skip a second now and then to keep it aligned with our average-person time. For scientific purposes, this gets even more complicated, especially for travel in space. This means we have up to 0.5 seconds differences of the “true” time we want to reference. A spacecraft can travel several miles/kilometeres in that timeframe and be somewhere completely different than expected (e. G. Where the antenna is pointing).
As you can see, the question itself is already quite complicated of what you want to know exactly.
the length of a second is defined by about 450 atomic clocks placed all around the world and there are 86400 seconds in a day. the atomic clock solved the problem if precise timekeeping, but it doesn’t solve the problem that the earth simply does what it wants with regards to it’s unpredictable wobbling so sometimes it rotates fast and sometimes it is slow. we solved this by getting humanity together to resync our times every so often. a leap second gets added to one of the days in the year and only on some years, decided by the IERS, which is an organisation with many national members (20+ countries). this is called UTC or universal coordinated time.
anybody can pick a timezone they want which is UTC with an offset, plus or minus any number of hours or minutes and most countries pick a whole number. the UK gets dibs on UTC+0 because as the first global maritime power, they pretty much invented the idea way back when it was called GMT, after the town Greenwich where the Royal Observatory is.
within reason, a country usually picks an offset whereby 12pm for them is where the sun is highest. this means a geographically large country can have multiple time zones (e.g. USA), or they can also force everyone to use one time zone (e.g. China, India). the reason is a weigh off between common sense (people want to work during light and sleep during dark) and commercial/trade (you want to be awake at the same time as your colleagues and trade partners). so almost every country is going to have 12pm slightly off the actual moment when the sun is the highest. if it’s more than an hour off, it’s probably because it’s decided to follow a larger country nearby who it likes to trade with.
time zones are based on Greenwich Mean Time, which was adopted by the British to enable their ships to navigate the world effectively, using shipboard clocks to calculate longitude (the distance from Greenwich)
As the British effectively conquered most of the world, this standard of time became accepted as a way of keeping everyone on the same page. It was succeeded by UTC, Coordinated Universal Time where the base time zone is still England (Greenwich) and the other time zones count ahead or behind that time.
the “Master clocks” are atomic clocks that are set to this UTC. Greenwich is still +-0, while other time zones count from there. I’m in Vancouver, so UTC-8
since time and space are the same thing, human measurement of time is entirely a construct; but we need some way of coordinating our activity, and labeling units of time is an effective way of doing that
Give the NIST clock a call to hear the time:
303-499-7111
I used to call this a lot more before the internet
There are fundamental physical truths that if you’ve never thought about, time can seem more baffling.
There is a point in the day that the sun is highest. This happens every day and it is always the same amount of time between the high points one day to the next – it makes no difference if it’s summer, winter etc. or if you are in the North Pole or on the equator. always the same. At that point in the day the number of daylight hours that remain is exactly the same as the number that have passed. I.e. it is the exact middle of the day. It follows that at that point plus or minus half the time between high points is exactly half way through the dark period of a day (night). Everything else follows on from these facts.
I think the less satisfying answer is that GMT (Greenwich Mean Time), made popular during the rise of rail travel, was landed on – it is noon GMT when the sun is directly above the Royal Observatory in Greenwich. From here, the eventual move to measuring cesium decay now “keeps us true” (accounting for leap years/seconds and the ever-changing rotation of the earth) to the time someone decided was accurate when GMT was introduced. And we just roll with it.
Then we’ve generally accepted both the current year (2023) and a Gregorian calendar with twelve months – allowing us to know the time of the day of the year, wherever we are. So, we (no one in particular but the various people in positions of influence along the way) “just decided”.
Time is accurate right now because we say it is!
Least that’s how I understand it…
Based on your edit, it’s very important to note that on a cosmic scale, it becomes even more difficult to define a specific time.
Special relativity is an area of physics that’s all about reference frames. Not to get too into all the details of it, the bottom line is observers of a specific phenomenon that are in two different reference frames (for example, somebody standing on the surface of a planet and somebody else in a nearby spaceship, accelerating away from it) will end up observing two different things happening with respect to time measurements.
This basically means that your idea of time is tied together to your idea of space and where you are.
So if you’re hoping that there’s some kind of way to, say, “measure time relative to some event” like the big bang, unfortunately everywhere in space there would be disagreement about it.
Here’s the problem with your idea of an “ur time”… Time progresses differently depending on gravity effects. The most accurate clocks we can make will actually progress at different speeds depending on how far above sea level they are. [Here’s an article about it](https://www.likejapan.com/en/life/optical-clocks-skytree/)
Time is relative so there can be no “ur time.”
“omg how did the first humans KNOW it was a tuesday??!”
dude, we made it up, all of it…. we “know” because we made it up
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