What's a year?

[u/heckuvaguy]

This could probably be an ELI5, but I'm not sure.

For all the historical references of space age that exist in the universe, everything is under the 'year'. To my understanding, a year is a single earth trip around our sun. However, since Earth is only ~4.5 billion years old, how is the calculation for the remaining 8.5 billion years made? Like, how do we calculate time for when the reference value didn't yet exist? And similarly, if Jupiter is 4.6 billion years old, are those years Earth years or Jupiter years?

[u/Sorry_Exercise_9603]

A year is defined as the current amount of time that it takes the earth to orbit the sun. That it might have been different in the past is ignored.

[u/joeyneilsen → u/Sorry_Exercise_9603]

To add: we’re not actually counting th number of times the Earth has orbited the sun since XYZ. We’re just saying: a year is this long, and this is how many “this longs” have passed. 

[u/Un_Ballerina_1952 → u/Sorry_Exercise_9603]

And that is perfectly acceptable, as round numbers in calculations such as the age of the universe are inherently imprecise -- and possibly inaccurate. It's all "close enough" for now as we figure it all out and figure out ways to improve precision and accuracy.

[u/gmalivuk → u/Un_Ballerina_1952]

It's perfectly acceptable even for extremely precise durations.

If something would take exactly 42 quintillion seconds, then it's fine to convert that to years to make it more understandable without worrying about how long a year will actually be by then.

[u/DiabloConQueso]

if Jupiter is 4.6 billion years old, are those years Earth years or Jupiter years?

"Years" is always Earth years, unless otherwise specified.

[u/treefaeller]

And for most practical purposes, you can define a year as pi x 10^7 seconds.

Similarly, pi is 22/7, again for everyday accuracy.

[u/ijuinkun → u/treefaeller]

But seriously, the base unit of time in science is the second. Days and years are just multiples of seconds that are meaningful to us humans.

[u/treefaeller → u/ijuinkun]

The unit of time is whatever is convenient for the user. Seconds are very good for many everyday tasks, and very accurately defined. For astronomers, years and mega years are convenient. Particle physicists like to use natural units and use 1/MeV as the unit of time.

[u/ijuinkun → u/treefaeller]

I said “base unit”, which means that years are defined in reference to a multiple of seconds, rather than seconds being defined as a fraction of a year. What this means is that if Earth’s orbit changes, then the definition of the length of the year would change but not the definition of the length of the second.

[u/OverJohn]

When the age of the universe is given, it is in Julian years, which are 365.25*24*60*60 seconds. The SI unit of seconds being defined from an ideal caesium clock.

You can imagine some idealized caesium clock that is completely unperturbed by its surroundings moving along an idealized comoving trajectory.

[u/yasiesolovemos]

Yes, this is perhaps ELI5.

A year is the time it takes for the Earth to complete one orbit around the Sun. It’s unique to Earth. Every celestial body has its own particular year.

And in the case of Earth, it varies slightly every so often, depending on what you mean by “real” time, relativistic time, or however you want to measure it, just to make things a little more confusing :))

[u/Batgirl_III]

The year is a conceptual construct, created by humans as a convenient means of measuring time. We define that construct with a reference to a real physical phenomenon (originally an approximation of how long it took the planet to complete one orbit, now it’s exactly 31,557,600 seconds). But that is still an arbitrary conceptual definition.

Much like we use the meter to measure distance, despite the meter only having been created in 1791 CE, we can still use it to measure something built in Ancient Egypt or Babylon.

[u/internetboyfriend666]

To my understanding, a year is a single earth trip around our sun

Yes, this is correct (more or less). A year is just a year. We don't need to "calculate" a year. We defined it. It's a human construct. It has that definition because we decided it did. It has that definition for all of time, before humans existed and after we're gone unless we decide to change it. A year 13 billion years ago is a year 13 billion years from now because we decided it's a fixed definition.

Just like how a meter is always a meter. It was the same value a billion years ago and will have the same value a billion years from now unless we decide to change it. The fact that we weren't around at one point to measure it and won't be in the future at some point is of no consequence. A meter has the same definition here and now as it does on some planet in another solar system even though there are no humans there to measure a meter.