i just shoved some figures into a calc and came up with:

light year = 9413822700000 km

anyone got a more precise figure

1 ly = 1556926 seconds * 299792458 meters/second = 9460528412464108 meters

- Warren

1 light year = 31,557,600 sec. * 299792458 meters/sec. = 9,460,730,472,580,800 meters. (365.25 days/yr)

Let's have a little American hubris.

1 light year = 186,282.6 mi/sec * 3600sec/hour * 24hour/day * 365day/year = 5,874,608,000,000 miles, a more accurate estimate of the classic six trillion figure.

299,792,458m/s is defined as exact for the constant c. So I won't do anything more with metric; chroot and 2inquisitive got me here.

depends what time you measure it...
http://www.newscientist.com/news/news.jsp?id=ns99996092

ok, the general consensus is around:

9,460,000,000,000,000

The general consensus is irrelevant.
There are 31,556,926.0 +/- 0.1 seconds in a year. (The length of a year varies, but not much - maybe half a second a century.)

There are exactly 299,792,458 metres in one light second.

Multiplying the two, we get one light year = 9,460,528,410,000,000 +/- 30,000,000 metres.

The year is not an SI unit, so you won't get any more precise than that (unless the light-year is defined on some partciular year... is it?)

But wait!

WordIQ (http://www.wordiq.com/definition/Light_year), and indirectly supported by NIST (http://physics.nist.gov/Pubs/SP811/appenB9.html), says that a light-year is specifically based on the Julian year of 365.25 days, which means that 2inquisitive's figure is the true one with zero error.

One light-year is exactly 9,460,730,472,580,800 meters.

The "exact" measurement of a "light year" cann't be defined. *Looks at Pete*
Their to busy working on Pi.

You can't get an exact measurement. I would like it if someone knew the "exact" calculation to work out the distance of one light year.

Or, does anyone know the "exact" speed of light? To no significant figures either.

If anyone knows any of this information, it would be helpful for my calculations for a 'light drive'.

Regards,
J.

The speed of light (and thus the light-year) are not measured, they are defined. The speed of light is defined to be 299,792,458 m/s. Because it is a definition, it is exact.

- Warren

The speed of light (and thus the light-year) are not measured, they are defined. The speed of light is defined to be 299,792,458 m/s. Because it is a definition, it is exact.

- Warren

Chroot, surely you can't be right about this! I would have thought that every scientific definition has it's quantity derived through processes of observation and measurements of data. Something has to be measured, be it direct or indirect, in order to define it. Please correct me if I'm wrong or tell me how you can define the speed of light (and thus the light-year) without measuring something???

Chroot, surely you can't be right about this! I would have thought that every scientific definition has it's quantity derived through processes of observation and measurements of data. Something has to be measured, be it direct or indirect, in order to define it. Please correct me if I'm wrong or tell me how you can define the speed of light (and thus the light-year) without measuring something???
warren is correct, you are incorrect. The second is defined to be 9,192,631,770 periods of oscillation of the frequncy between two hyperfine levels of Cs-133.

the meter is defined to be the distance travelled by light in a vacuum in 1/299,792,458 seconds, where the second is defined as above.

According to these two definitons, the speed of light is exactly 299,792,458 m/s, by the very definition of the meter. If you measure something different, then your meter stick is not actually a meter. Thus, in fact, if you do an experiment measuring the speed of light, you are actually measuring the meter.

Thus the stuff that spidergoat mentions above, that the speed of light changes in time, is incorrect. The speed of light, by definition, cannot change. What might change is the fine structure constant.

The general consensus is irrelevant.
There are 31,556,926.0 +/- 0.1 seconds in a year. (The length of a year varies, but not much - maybe half a second a century.)


It's a little more, I'm afraid. We've tacked on 32 extra (leap) seconds since going to an atomic time standard in 1967.

Yes, that's consistent with the length of a year increasing by maybe half a second per century.

The standard second is based on the length of a day in about 1820.
If each year is now a second longer than that year, then we need a leap second every year or so.

warren is correct, you are incorrect. The second is defined to be 9,192,631,770 periods of oscillation of the frequncy between two hyperfine levels of Cs-133.

the meter is defined to be the distance travelled by light in a vacuum in 1/299,792,458 seconds, where the second is defined as above.

According to these two definitons, the speed of light is exactly 299,792,458 m/s, by the very definition of the meter. If you measure something different, then your meter stick is not actually a meter. Thus, in fact, if you do an experiment measuring the speed of light, you are actually measuring the meter.

Thus the stuff that spidergoat mentions above, that the speed of light changes in time, is incorrect. The speed of light, by definition, cannot change. What might change is the fine structure constant.

Understood, but you still had to *MEASURE* the number of oscillations to define what a second is!!!

For all you know, that measurement may change depending on where you are relative to gravitational fields. There is no science without measurement. There are no definitions without measurement. Can you not see my point?

Yes, that's consistent with the length of a year increasing by maybe half a second per century.

The standard second is based on the length of a day in about 1820.
If each year is now a second longer than that year, then we need a leap second every year or so.

Of course - I misread your statement to mean that we add a half second or so per century.

Understood, but you still had to *MEASURE* the number of oscillations to define what a second is!!!

For all you know, that measurement may change depending on where you are relative to gravitational fields. There is no science without measurement. There are no definitions without measurement. Can you not see my point?
you are correct that you cannot do science without measurement. But I think you are failing to appreciate something: you can't make a measurement without a standard unit of measurement. This unit is something that has to be defined, it cannot be measured.

In older times, we defined the meter to be some stupid bar sitting in a vault in France. this meter could not be measured. with that definition, it was possible to measure the speed of light. In modern times, we define the meter in terms of the speed of light itself. The speed of light can no longer be measured, but the meter can be.

it is arbitrary what you choose to be a defined quantity and what you choose to be a measured quantity. the current definitions are useful because they don't require you to go visit a stupid vault in france to calibrate your instruments.

If I define a new weight unit, the ME, to be my weight, then I will always weigh exactly one ME. Even after dinner.

And yes, cesium atoms vibrate more slowly at the bottom of a gravity well than at the top, so seconds are longer in some places than in others.

so seconds are longer in some places than in others.
and of course, this holds for any clock, for any unit of time, no matter how you define it. Time dilation is not a flaw in your clock or definition of unit of time, but rather an artifact of the geometry of spacetime.

I agree with dav57 and Jolonar.

The speed of light is not a defined quantity. The meter is a defined quantity, based on the speed of light. The precision of any distance measurement we make ultimately depends on how well we can measure the speed of light.

So, we we say "the speed of light is exactly 299,792,458 m/s", we are not actually saying anything useful about light* - we are only saying something about how long a metre is.

The same goes for a light year. While it is true that one light year is exactly 9,460,730,472,580,800 meters, this isn't really giving us useful info* about the distance of a light year.

It is really better to say that one metre is exactly 1/9,460,730,472,580,800 light-years.


* It is useful in an informal way, because most of us are aware of the length of a metre less precisely from secondary standards.

But I think you are failing to appreciate something: you can't make a measurement without a standard unit of measurement.

I disagree. A distance is an actual physical quantity that exists independently of any standard units.

I can unambiguously communicate a distance without a unit of measurement, simply by holding out my hands and saying "this long".

This is how I see it…

Firstly, there is absolutely no point defining any fundamental unit in this universe unless you are intending to compare it to something else. Therefore, every arbitrary unit you care to fix will be directly or indirectly compared to something else, otherwise don’t bother defining anything in the first place!

Secondly, when comparing something with your fundamental definitions, I’m afraid there is no valid comparison without measurement.

Finally, your measurements are not confined to one place, where as the defined values usually are. Nobody can be sure how the defined values may alter as a result of your location in the unioverse, which makes everything complete b*ll*cks.

Thanks for listening

I can unambiguously communicate a distance without a unit of measurement, simply by holding out my hands and saying "this long".
Then your "this long" is a new standard unit of measure. Now we have twice "thislong", thrice "thislong", a jillion times "thislong", whatever you need.