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.
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