# Light years?

#### wegs

##### Matter and Pixie Dust
Valued Senior Member
So, one light year is the distance light travels in one year and this is roughly equal to six trillion miles. And a galaxy is 100,000 light years.

I don’t fully understand this; so light is traveling six trillion miles in one year? Thought I understood this, but was watching a snippet of news this morning where they were discussing billions of light years, and maybe I don’t understand.

Why do astronomers use light years to measure how far away other galaxies may be?

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I don’t fully understand this; so light is traveling six trillion miles in one year?
Correct.

A light year is just a conveniently unit to use for huge distances. In other words writing Alpha Centauri is 4.4 light years away is easier than writing it is 26,400,000,000,000 miles. Just like it is easier to say that some city is 20 miles away instead of saying the city is 105,600 feet away.

For planets in our solar system we usually use miles or kilometers for distance, however using light minutes as a measure of distance makes it easier to easily see the relative distances of the planets. For instance Mars is 12.7 light minutes from the sun and Jupiter is 43 light minutes from the sun.

I don’t fully understand this; so light is traveling six trillion miles in one year? Thought I understood this, but was watching a snippet of news this morning where they were discussing billions of light years, and maybe I don’t understand.
Yes. So A galaxy one billion light years from us has light reaching us having travelled six billion trillion miles - 10^9 x 10^12 = 10^21 - or six sextillion miles.

For planets in our solar system we usually use miles or kilometers for distance, however using light minutes as a measure of distance makes it easier to easily see the relative distances of the planets. For instance Mars is 12.7 light minutes from the sun and Jupiter is 43 light minutes from the sun.
And in a very practical sense for both galaxies and planets - using light as a measurement unit tells us the signal delay.

It takes 43 minutes for a signal from Jupiter (such as a probe's data) to reach us, and 2.5 million years for a signal from Andromeda (such as a supernova) to reach us.

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[...] Why do astronomers use light years to measure how far away other galaxies may be?

In their papers and professional discourse, astronomers may often tend to use parsecs (which includes kiloparsecs, megaparsecs, gigaparsecs) for applicable extrasolar distances. (The Alpha Centauri system barely surpasses one parsec.)

Media outlets prefer light years for public consumption.

Yes. So A galaxy one billion light years from us has light reaching us having travelled six billion trillion miles - 10^9 x 10^12 = 10^21 - or six sextillion miles.

Do you really think that the galaxy (or whatever it is now... you said now ?) who is not actually one billion light years away from us, that have emmited light a billion years ago, has travelled one billion light years ?
The only thing we should say, in my opinion, is : If light travel during 1 billion year (duration) then it has travelled one billion light year (distance).

Do you really think that the galaxy (or whatever it is now... you said now ?) who is not actually one billion light years away from us, that have emmited light a billion years ago, has travelled one billion light years ?
I cannot decipher what you are asking here. Is a galaxy that we've established to be one billion light years away, not one billion light years away?
There is some amount of Cosmological Expansion, so not today, no. But a billion years ago, yes.

The only thing we should say, in my opinion, is : If light travel during 1 billion year (duration) then it has travelled one billion light year (distance).
OK. If that floats your boat.

Is a galaxy that we've established to be one billion light years away, not one billion light years away?

Was, a galaxy that we've established it was one billion light years away, not one billion light years away ?
No.
Was, a galaxy that we've established it is one billion light years away, not one billion light years away ?
No.
Is, a galaxy that we've established it was one billion light years away, not one billion light years away ?
No.
Is, a galaxy that we've established it is one billion light years away, not one billion light years away ?
No.

There is some amount of Cosmological Expansion, so not today, no. But a billion years ago, yes.

A billion years ago it was one bilion light years away ?

OK. If that floats your boat.

The only thing i can say is :
Light has been emited from a galaxy one bilion years ago (because it is what we suppose) and this light has traveled more than one bilion light years (the reality because expansion is reality).
Ligth has traveled one bilion years and this light has traveled one bilion light years (by definition).

Correct.

A light year is just a conveniently unit to use for huge distances. In other words writing Alpha Centauri is 4.4 light years away is easier than writing it is 26,400,000,000,000 miles. Just like it is easier to say that some city is 20 miles away instead of saying the city is 105,600 feet away.

For planets in our solar system we usually use miles or kilometers for distance, however using light minutes as a measure of distance makes it easier to easily see the relative distances of the planets. For instance Mars is 12.7 light minutes from the sun and Jupiter is 43 light minutes from the sun.
Thank you for this explanation - suppose the “source” of the light moves or changes its position, would the speed of light stay constant? I want to say yes, it does stay constant.

But, why?

No matter what light “travels through” it won’t slow down?

Yes. So A galaxy one billion light years from us has light reaching us having travelled six billion trillion miles - 10^9 x 10^12 = 10^21 - or six sextillion miles.

And in a very practical sense for both galaxies and planets - using light as a measurement unit tells us the signal delay.

It takes 43 minutes for a signal from Jupiter (such as a probe's data) to reach us, and 2.5 million years for a signal from Andromeda (such as a supernova) to reach us.
Ahh! That makes sense - how long it takes for a signal to reach us. Thanks!

Same question above applies - would the time the signal (of light) takes to reach us, ever not be constant?

Was, a galaxy that we've established it was one billion light years away, not one billion light years away ?
No.
Was, a galaxy that we've established it is one billion light years away, not one billion light years away ?
No.
Is, a galaxy that we've established it was one billion light years away, not one billion light years away ?
No.
Is, a galaxy that we've established it is one billion light years away, not one billion light years .
I don't see how this complementary lecture is helping the OP get their question answered. It seems like more of an ego striking opportunity for you.

Thank you for this explanation - suppose the “source” of the light moves or changes its position, would the speed of light stay constant? I want to say yes, it does stay constant.

But, why?

No matter what light “travels through” it won’t slow down?
There's nothing in a vacuum that will slow light down, but it does slow in some materials such as water and glass.

2.5 million years for a signal from Andromeda (

Why I don't call home to often, so long to get any sort of reply

I don't see how this complementary lecture is helping the OP get their question answered. It seems like more of an ego striking opportunity for you.

Thank you for this explanation - suppose the “source” of the light moves or changes its position, would the speed of light stay constant? I want to say yes, it does stay constant.

But, why?

Look, he is thankfull, not understanding how wrong you are.
You now just need to help him again, explaining "why" the speed of light is constant.
I am sure you also have some "good" answer for that.

There's nothing in a vacuum that will slow light down, but it does slow in some materials such as water and glass.

No, thats not the question... are you tricking him ?
Why is the speed of light constant (in vacuum of course...) even if the object that emmit light accelerate.

weg said:
suppose the “source” of the light moves or changes its position

Look, he is thankfull, not understanding how wrong you are.
You now just need to help him again, explaining "why" the speed of light is constant.
I am sure you also have some "good" answer for that.
OK, So you're just here to troll then, not to constructively contribute to the OPs question.

Look, he is thankfull, not understanding how wrong you are.
You now just need to help him again, explaining "why" the speed of light is constant.
I am sure you also have some "good" answer for that.

(First, OP is a “she.”) Second, why do you believe Dave is wrong?

OK, So you're just here to troll then, not to constructively contribute to the OPs question.

You mean pointing wrong answers is trolling ?
I understand better why you see a lot of trolls surrounding you.

No, thats not the question... are you tricking him ?
Why is the speed of light constant (in vacuum of course...) even if the object that emmit light accelerate.
Or slows down. Or moves (somehow) further away. But suppose the observer changes position?

Dave answered my question in part - that it wouldn’t slow down in a vacuum, that it remains constant but this assumes it would change if the way it was traveling (no longer in a vacuum) changed, yes?

On the same track - so, when I see a bright star twinkling in the night sky, that light is only just reaching me in that moment but the light from the star left it long before I’m actually seeing it?
Same with the sun (although that’s only minutes to see the light emission) and so on…

(First, OP is a “she.”)

OP is he for me because it is not a title, but only a noun. But if you are female i can speak of "you" instead of the OP.

wegs said:
Second, why do you believe Dave is wrong?

I do not believe, and it should be obvious for all who know how things work.
If you read carefully, you can understand that it is nonsense to say the thing like he did (but many just repeat thinks like he did without understanding anything, so it is not new to me).
If you dont understand, then it mean the answer he gave you does not allow you to understand.

dicart said:
The only thing we should say, in my opinion, is : If light travel during 1 billlion year (duration) then it has travelled one billion light year (distance).

wegs said:
But once it reaches us, wouldn’t it be

No, because here you assume that the galaxy itself is actualy one billlion light year away.
"You dont need this hypothesis" like gallilee would say.
The only thing you can say is :

One day light went from a distant galaxy.
This light started to travel and in our reference frame "it has traveled one bilion years" (a duration, dont know how you know this, but let suppose it).
Here you can assume, because of the definition of the speed of light, that this same light has traveled one bilion light years (a distance).

You can not say this :
Dave said:
So A galaxy one billion light years from us has light reaching us having travelled six billion trillion miles - 10^9 x 10^12 = 10^21 - or six sextillion miles.

To be much clearer let say the same thing with the same units :
"So A galaxy one billion light years from us has light reaching us having travelled one billion light years."

1. There is no galaxy at one billion light year.
2. If you consider that it was some galaxy (at least) one billion light years away from us when it emmited the light, the light could not have traveled one billion light years, because of the expansion : It has traveled more.