# One light year distance

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• KingGambit
In summary, the conversation discussed the standardization for 1 light year distance, which is defined as the distance light travels in one second, multiplied by the number of seconds in one year. It was noted that there are some subtle differences in the definition of a year, but in this context, it is defined as 365.25 days. The speed of light is also defined as 299 792 458 ms-1, making 1 light year exactly 9,460,730,472,580,800 meters. The conversation also touched on the standardization of prefixes such as kilo and kibi, which are defined as 1,000 and 1,024 respectively. There was also mention of the different definitions

#### KingGambit

TL;DR Summary
Light year metric
Dear PF Forum,

It's been a while since I logged in here. And I really do appreciate all the answers that I've been getting here.
Now, I wonder. Is there any standardization for 1 light year distance?
Is it 10 trillion kilometers, or
299,792,458 * 60 * 60 * 24 * 365.256 = ..... 9,460,885,884,991,030 KM
Okay, okay I know what a kilobyte is. Be it 1000 or 1024 bytes, I know when I buy a RAM module that says it's 8GB, or that I buy a harddisk that says it's 1 terabyte. But it's just that the light year definition.
Is there any standardization for light year?

Thank you very much for your answer.

KingGambit said:
Is there any standardization for 1 light year distance?
Yes, it's the distance light travels in one second, times the number of seconds in one year. The value is not rounded off to anything.

KingGambit
Worth noting that there are some subtleties around the definition of a year. For these purposes, it's apparently defined as 365.25 days.

Klystron, russ_watters and KingGambit
PeterDonis said:
Yes, it's the distance light travels in one second, times the number of seconds in one year. The value is not rounded off to anything.
Thanks Peter

Ibix said:
Worth noting that there are some subtleties around the definition of a year. For these purposes, it's apparently defined as 365.25 days.
Thanks Ibix

Ibix
What everyone has said so far is accurate. I'd just further clarify that in this context, a "day" is defined as precisely 86,400 (=60*60*24) SI seconds, according to the International Astronomical Union:

https://www.iau.org/public/themes/measuring/

So a Julian Year is 365.25 of those days. (Not a Sidereal Day nor a Mean Solar Day). An SI second is the time required to go through a certain number of cycles for a photon of the very specific frequency emitted when a cesium atom undergoes a specific hyperfine transition between energy levels. So although the definition uses the words "day" and "year," it's no longer tied to cyclic processes in nature that are not as regular as we first thought/hoped, such as the spin & orbit periods of the Earth. It's just dependent on atomic processes, and those are the most regular periodic things we know about in nature.

Last edited:
Klystron, phinds, vela and 3 others
What has been said already is correct, but just to tie it all together:
• A day is defined as 86,400 seconds (by the IAU).
• A year is defined as 365.25 days (by the IAU), meaning that there are 31,557,600 seconds in a year.
• The speed of light is defined as 299 792 458 ms-1 (by BIPIM as one of the seven SI defining constants).
• Combining these definitions, a light year is therfore exactly 9,460,730,472,580,800 m.
Note also that the prefix "kilo" is defined as 1,000 in the SI and so a kilobyte (kB) is exactly 1,000 bytes; ISO 80000 defines the prefix "kibi" as 1,024 and so a kibibyte (KiB) is exactly 1,024 bytes.

Klystron, berkeman, vanhees71 and 2 others
pbuk said:
What has been said already is correct, but just to tie it all together:
• A day is defined as 86,400 seconds (by the IAU).
• A year is defined as 365.25 days (by the IAU), meaning that there are 31,557,600 seconds in a year.
• The speed of light is defined as 299 792 458 ms-1 (by BIPIM as one of the seven SI defining constants).
• Combining these definitions, a light year is therfore exactly 9,460,730,472,580,800 m.

Thank you very much for your reply @pbuk

pbuk said:
Note also that the prefix "kilo" is defined as 1,000 in the SI and so a kilobyte (kB) is exactly 1,000 bytes; ISO 80000 defines the prefix "kibi" as 1,024 and so a kibibyte (KiB) is exactly 1,024 bytes.

Yeah, tell that to the computer seller.
For Harddisk and file size. 1 Kb is sometimes 1000 bytes or 1024 bytes. And sometimes 1 Mb is 1000 * 1024 bytes, sometimes it's 1024 * 1024 bytes.
For RAM module and flash disk. 1 Kb is exactly 1024 bytes. 1 Mb is exactly 1048576 bytes.
And 1 Gb is... heck, can't remember the number

But I do remember 4GB is 4294967295,
It's the largest value for a DWORD variable.

Anyway, thanks for your reply and summary @pbuk

KingGambit said:
Yeah, tell that to the computer seller.
For Harddisk and file size. 1 Kb is sometimes 1000 bytes or 1024 bytes. And sometimes 1 Mb is 1000 * 1024 bytes, sometimes it's 1024 * 1024 bytes.
For RAM module and flash disk. 1 Kb is exactly 1024 bytes. 1 Mb is exactly 1048576 bytes.
And 1 Gb is... heck, can't remember the number

But I do remember 4GB is 4294967295,
It's the largest value for a DWORD variable.

Anyway, thanks for your reply and summary @pbuk
I'm sorry.
4GB can't be odd number: 4294967295, I think it's 4294967296.
It's just the largest value that I remember. I'm a computer programmer myself.

KingGambit said:
Yeah, tell that to the computer seller.
For Harddisk and file size. 1 Kb is sometimes 1000 bytes or 1024 bytes. And sometimes 1 Mb is 1000 * 1024 bytes, sometimes it's 1024 * 1024 bytes.
For RAM module and flash disk. 1 Kb is exactly 1024 bytes. 1 Mb is exactly 1048576 bytes.
And 1 Gb is... heck, can't remember the number

But I do remember 4GB is 4294967295,
It's the largest value for a DWORD variable.

Anyway, thanks for your reply and summary @pbuk
I guess this is off topic, but yeah by the old definitions based on powers of 2, computer storage sellers were indeed being dishonest, because every MB they sold you was 1 million bytes and not 10242 bytes. But once k and M prefixes in computing were redefined as powers of 10 to be consistent with the rest of SI, the sellers' inaccurate statements became accurate. The only problem is that the OS didn't change its notation, so it still reports your capacity in MiB using an "MB" label. Thus, what you have still appears to be less than what you thought you purchased. A GiB would just be 10243 = 230 bytes, whatever that number is. I certainly don't claim to know it, because I only have my powers of 2 memorized up to 216

Btw, you probably know this, but it should consistently be uppercase 'B' for bytes. Lowercase 'b' refers to bits. E.g. a bandwidth of 1 Gbps means 1 gigabit per second, not 1 gigabyte per second.

pbuk said:
there are 31,557,600 seconds in a year.
...which is ##\pi\times 10^7## seconds/year to better than 0.5% accuracy, if you can't be bothered to carry too many digits around in your head. So a light year is about 0.4% larger than ##3\pi\times 10^{15}\mathrm{m}##.

KingGambit said:
I'm sorry.
4GB can't be odd number: 4294967295, I think it's 4294967296.
It's just the largest value that I remember. I'm a computer programmer myself.
That difference of 1 is the difference between the largest integer that can be stored in a 32-bit container, ##2^{32}-1##, and the number of bytes that can be addressed by such an integer, ##2^{32}## numbered from zero to ##2^{32}-1##. Software engineers have relied on context to distinguish the two meanings of “4G” since forever, or at least since we stopped doing the same thing with “64K” in the mid-seventies.

Welcome to PF, where every topic eventually morphs into metrology.

While the messages above are correct, they kind of miss the point. Very few things - quite possibly nothing - is actually measured in light year, much less light years to 16 decimal places.

The distance ladder starts with parsecs. To get light years, one takes parsecs and divides by 3.26.

It may well be that these light years differ from the formal "correct" definition in the 16th decimal place or maybe even earlier. Nobody cares,

Motore, jbriggs444, russ_watters and 2 others
pbuk said:
What has been said already is correct, but just to tie it all together:
• A day is defined as 86,400 seconds (by the IAU).
• A year is defined as 365.25 days (by the IAU), meaning that there are 31,557,600 seconds in a year.
• The speed of light is defined as 299 792 458 ms-1 (by BIPIM as one of the seven SI defining constants).
• Combining these definitions, a light year is therfore exactly 9,460,730,472,580,800 m.
Note also that the prefix "kilo" is defined as 1,000 in the SI and so a kilobyte (kB) is exactly 1,000 bytes; ISO 80000 defines the prefix "kibi" as 1,024 and so a kibibyte (KiB) is exactly 1,024 bytes.
I will never mention kibblebytes.

We can not measure distances to stars and so on with that accuracy, so it does not matter what definition of year you use.

russ_watters
Oh please guys, the correct answer to "is there any standardization for Avogadro's number" is not "it doesn't matter because we can't count that high anyway", it is "yes, it is was (re)defined by BIPIM as 6.022 140 76×1023 in 2019" .

The question in the OP was:
KingGambit said:
Is there any standardization for 1 light year distance?
and the correct answer to that was given in #7.

LastScattered1090, vela and andrew s 1905
Thank you pbuk for caring enough to answer King Gambit's question.

KingGambit
pbuk said:
Oh please guys, the correct answer to "is there any standardization for Avogadro's number" is not "it doesn't matter because we can't count that high anyway", it is "yes, it is was (re)defined by BIPIM as 6.022 140 76×1023 in 2019" .
Oh.

Thank you

Actually, the first correct answer was given in post #6
which also provided a reference

KingGambit and Motore
Whoever gave the correct answer, thank you for caring enough to do so.

malawi_glenn said:
Actually, the first correct answer was given in post #6
Actually, all of the answers in posts #2, #3, #6, and #7 were correct; the later ones added more relevant details. At any rate, the OP's question has now been well answered. And that seems like a good note on which to close this thread. Thanks to all who participated.

KingGambit