Light Years

[mrs_overall]

My 12 year old has just asked me this one. How did someone originally work out what a light year was?
Thanks in advance.

[Loosehead]

Well the would have taken the distance light travels in 1 second, 186232 miles, the multiplied it by the number of seconds in a year, ie 3153600 x 186232 = 5.873x10E12.

Do you mean who and why did someone perceive a unit of distance based on light?

[tompaine]

light trvels through space at 186000 miles/sec

there are 31,536,000 in a year so light travels

5865696000000 miles per year (5.86 trillion miles)

Its quite an easy calculation (with a caculator) and a light year is much easier to grasp than 5.86 trillion miles.

[fo3nix]

As the above say, a 'light year' is just the distance light travels in one year. So this new planet (Super Earth as it seems to be called sometimes), is about 20 light years away. This means that it would take 20 years for light to reach it from us.

So, the problem really is not working out what a light year, but actually what the speed of light is. This can be done in several ways, but one way (theoretical, with maths), was after some great work by a guy called Maxwell in the late 19th century. Essentially it relates the speed of light to some fundamental constants, making the speed of light itself a fundamental constant.

The equation at the bottom of the overview section in this article is what I'm talking about:
http://en.wikipedia.org/wiki/Speed_of_light#Ov erview

[mrs_overall]

Thanks for your answers. At the age of 12 she didn't really understand but I've managed to cobble an answer together for her based on your answers!

[THECORBYLOON]

If you imagine a car travelling at the speed of light just ask her how far it would travel in a minute, hour, day etc and a light year is how far that car would travel in a year.

[Buenchico]

Mrs_overall:
Providing answers to science questions posed by 12 year old girls isn't easy. I've spent over 30 years trying to answer this one ;-)
http://www.theanswerbank.co.uk/Science/Questio n140971.html

Chris

[jake-the-peg]

Light years have stuck in the imagination but are not the favoured units used by astronomers, kind of because as you imply they are rather arbitary, you have to measure the distance some way and then work out how many light years that equates to

If you hold your finger out at arms length and open and look fist with one eye and then the other it will seem to move against the background because of the seperation of your eyes.

We see a similar effect on the earth. The closer stars seen to move more against the night sky when photographed 6 months apart because the Earth goes around the Sun. We call this parallax
http://people.brandeis.edu/~wardle/ch4/ch4-ima ges/p6.4.parallax.gif

Something that seems to move by 1 second of arc that is 1/3600 of a degree is said to be one parsec away.

It turns out that a parsec is about 3.2 light years just under the distance to the nearest star.

[TechGirl]

I just read your pat question Buenchico. Are you any closer to your answer yet?

[TechGirl]

*past*

[Buenchico]

(With apologies to Mrs_overall for 'stealing the thread'):

TechGirl: I'm somewhat clearer in my own mind about the difference between 'white' and 'silver' but, despite all of the helpful suggestions I received from ABers, I still don't know exactly what I should have said to that young girl!

Chris

[mrs_overall]

Thanks to all of you who took the trouble to answer. xxx