<p>You're forgetting that one year is not 365 days, but 365.2422 or something close to that.</p>

<p>That change will give you a number much much closer to the google provided number.</p>

CC BY-SA 3.0
2016-08-29T06:12:33.783Z
Ah got it now.. the calculator rounded the darn answer for some reason.
CipherBot
CC BY-SA 3.0
2016-08-29T06:22:10.800Z
There's always websites on the internet who simplifies a lot of things, this is one such example. Not surprised actually.
CipherBot
CC BY-SA 3.0
2016-08-29T21:42:31.283Z
See [Toby's answer](http://astronomy.stackexchange.com/a/18116/53).
Keith Thompson
CC BY-SA 3.0
2016-08-30T03:11:41.533Z
His days per year is wrong, and his answer didn't answer the op either. However it is a nice explanation/calculation of a light year.
LaserYeti
CC BY-SA 4.0
2021-09-01T22:32:54.533Z
The average length of a year on the Gregorian calendar (over a 400-year leap-year cycle) is exactly 365.2425 days.  Astronomically, it's between 365.2416 and 365.2428 days depending on which moment of Earth's orbit you're measuring it from.
dan04
CC BY-SA 4.0
2022-12-13T16:27:56.863Z
My days per year is _not_ wrong for this context - as I said, astronomy uses a *Julian year*, not a *Gregorian year* to define this length.  And my answer does answer the question, giving the exact length (I notice yours doesn't, so that criticism seems a bit rich!).
Toby Speight
CC BY-SA 3.0
2016-08-29T02:37:59.423Z
2016-08-29T02:37:59.423Z
LaserYeti
<p>By convention, astronomy uses the <strong>Julian Year</strong> for the computation of a <a href="//en.wikipedia.org/wiki/Light-year" rel="nofollow noreferrer">light year</a>:</p>
<blockquote>
<p>Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified.</p>
</blockquote>
<p><a href="https://en.wikipedia.org/wiki/Light-year#Definitions" rel="nofollow noreferrer">Wikipedia</a> gives the length as</p>
<blockquote>
<p>31 557 600 s ✕ 299 792 458 m/s = <strong>9 460 730 472 580 800 m</strong> (exactly)</p>
</blockquote>
<p>The reason that it's exact (and not subject to experimental error) is that the metre itself is defined in terms of the speed of light, so the quantities are fixed by definition.</p>

CC BY-SA 3.0
2016-08-29T13:39:56.193Z
One way to avoid this ambiguity would be to lookup the length of a lightsecond (where second is a well-defined SI unit) and then convert that into whatever notion of years you care about. But for astronomy purposes you probably just want to use the usual convention.
R.. GitHub STOP HELPING ICE
CC BY-SA 4.0
2022-12-13T16:31:05.967Z
One light-second is **299 792 458** metres of course, and that's exactly the conversion that's being done in the quoted text.
Toby Speight
CC BY-SA 4.0
2016-08-29T08:38:39.753Z
2022-12-13T16:24:44.867Z
2022-12-13T16:24:44.867Z
Toby Speight
<p>What is the exact measurement of a light year? I searched google for the answer in meters and came up with $9.461\cdot 10^{15}$ meters. When I calculated the answer considering $299\,792\,458 \;\text{m/s}$ as the speed of light, I came up with:</p>

<p>$$
299\,792\,458 \times 365 \times 24 \times 3\,600 = 9\,454\,254\,955\,508\,926 \;\text{m}
$$ Why is there such a gap? Did I miss something to add in the equation or is it just wrong?</p>

CC BY-SA 3.0
2016-08-29T21:44:38.587Z
9,460,514,622,032,012 meters is not quite correct; it assumes a year of about 365.2417 days.
Keith Thompson
<light><distances><speed>
Exact measurement of a light year
CipherBot
# Exact measurement of a light year

(Asked by: username_0 on 2016-08-29)

What is the exact measurement of a light year? I searched google for the answer in meters and came up with $9.461\cdot 10^{15}$ meters. When I calculated the answer considering $299\,792\,458 \;\text{m/s}$ as the speed of light, I came up with:

$$
299\,792\,458 \times 365 \times 24 \times 3\,600 = 9\,454\,254\,955\,508\,926 \;\text{m}
$$ Why is there such a gap? Did I miss something to add in the equation or is it just wrong?

**Tags:** light, distances, speed
> **Comments:**
> **username_1**: 9,460,514,622,032,012 meters is not quite correct; it assumes a year of about 365.2417 days.  

---

## 2 answers

### Answer by username_4 on 2016-08-29. Score: 29 | Accepted answer

By convention, astronomy uses the **Julian Year** for the computation of a [light year](//en.wikipedia.org/wiki/Light-year):

> Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified.

[Wikipedia](https://en.wikipedia.org/wiki/Light-year#Definitions) gives the length as

> 31 557 600 s ✕ 299 792 458 m/s = **9 460 730 472 580 800 m** (exactly)

The reason that it's exact (and not subject to experimental error) is that the metre itself is defined in terms of the speed of light, so the quantities are fixed by definition.

> **Comments:**
> **username_5**: One way to avoid this ambiguity would be to lookup the length of a lightsecond (where second is a well-defined SI unit) and then convert that into whatever notion of years you care about. But for astronomy purposes you probably just want to use the usual convention.  
> **username_4**: One light-second is **299 792 458** metres of course, and that's exactly the conversion that's being done in the quoted text.  

---

### Answer by username_2 on 2016-08-29. Score: 6

You're forgetting that one year is not 365 days, but 365.2422 or something close to that.

That change will give you a number much much closer to the google provided number.

> **Comments:**
> **username_0**: Ah got it now.. the calculator rounded the darn answer for some reason.  
> **username_0**: There's always websites on the internet who simplifies a lot of things, this is one such example. Not surprised actually.  
> **username_1**: See [Toby's answer](http://astronomy.stackexchange.com/a/18116/53).  
> **username_2**: His days per year is wrong, and his answer didn't answer the op either. However it is a nice explanation/calculation of a light year.  
> **username_3**: The average length of a year on the Gregorian calendar (over a 400-year leap-year cycle) is exactly 365.2425 days.  Astronomically, it's between 365.2416 and 365.2428 days depending on which moment of Earth's orbit you're measuring it from.  
> **username_4**: My days per year is _not_ wrong for this context - as I said, astronomy uses a *Julian year*, not a *Gregorian year* to define this length.  And my answer does answer the question, giving the exact length (I notice yours doesn't, so that criticism seems a bit rich!).  

---

