# How do I calculate light years to years?

by Kronos5253
Tags: light
 P: 111 I have a question, sparked by this article, so I figured this would be the best place to put this. That is said to be 40 light years away... So if we're seeing a planet 2.7 times the size of earth with liquid water on it, and it's 40 light years away, how do you calculate how far into the past we are seeing this planet?
P: 1,717
 Quote by Kronos5253 I have a question, sparked by this article, so I figured this would be the best place to put this. That is said to be 40 light years away... So if we're seeing a planet 2.7 times the size of earth with liquid water on it, and it's 40 light years away, how do you calculate how far into the past we are seeing this planet?
If it is 40 light years away, then the light we see reveals the planet as it was 40 years ago.
P: 111
 Quote by sylas If it is 40 light years away, then the light we see reveals the planet as it was 40 years ago.
In terms of light years, yes... I basically mean 'Earth years'.. Meaning if we were to travel to that planet, how old would we be when we got there?

I tried finding a formula to figure that out, but I haven't really found anything.. So any help would be appreciated :)

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## How do I calculate light years to years?

Entirely depends on how fast you travel there. Need to know acceleration, top speed, coasting time and deceleration.

The more of your journey you spend at near c, and the closer you get to c, the shorter the duration. There is no limit in principle to how short the duration can be by maximizing these two factors.

In my treatise on Gliese 581 I used the distance of 22.6 ly and an acceleration/deceleration of 1g. The result was a 22.6ly journey with the crew experiencing a mere 6.1 years.
Mentor
P: 21,652
 Quote by Kronos5253 In terms of light years, yes... I basically mean 'Earth years'.. Meaning if we were to travel to that planet, how old would we be when we got there? I tried finding a formula to figure that out, but I haven't really found anything.. So any help would be appreciated :)
Here's the equation and a calculator that you can plug sample values into if you don't want to do the calculation yoursefl: http://www.1728.com/reltivty.htm

Note that this question is completely different than the one you asked in your first post.
P: 111
 Quote by russ_watters Here's the equation and a calculator that you can plug sample values into if you don't want to do the calculation yoursefl: http://www.1728.com/reltivty.htm Note that this question is completely different than the one you asked in your first post.
Yeah I realized that... And I realized that the answer to the first post is the answer I was looking for haha... I figured it would be a bigger number, but it makes sense to me.

Thanks for the answers though everyone :)

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