# Light year

1. Aug 17, 2004

### lakshmi

what is the value of 1 light year

2. Aug 17, 2004

### pmb_phy

It is the distance light travels in one year. It, in itself, is a measure of distance so perhaps you're interested in converting this to meters? Miles?

Pete

3. Aug 17, 2004

### GOD__AM

A measure of distance? How can light speed be an accurate measure of distance when it travels with respect to an observer?

4. Aug 17, 2004

### Nereid

Staff Emeritus
'distance' as in 'within your own frame of reference' (or one not in relative motion to yours); to obtain a value for the distance as observed by someone in a different frame, just apply Relativity!

5. Aug 17, 2004

### decibel

The valua at which light travels in a year is approx. 9,500,000,000,000 kilometers.

6. Aug 17, 2004

### pmb_phy

Because the speed of light is independant of the observer. Therefore since all observers measure the same speed of light, 1 light year is a well defined quantity (although different observers measure different distances between events, but that's a whole different topic).

Pete

7. Aug 17, 2004

Here is the full Calculation:
$$v=\frac{d}{t}$$

$$d=vt$$
$$d=cy$$

$$d=(3.0*10^8\frac{m}{s})(3.15*10^7\frac{s}{year})(\frac{1km}{1000m})$$

$$d= 9 460 800 000 000km$$

8. Aug 17, 2004

### Prometheus

I think that you are taking liberties with the meaning of the word value.

9. Aug 17, 2004

### pervect

Staff Emeritus
Take a look at the defintion of the SI meter sometime

10. Aug 17, 2004

### robphy

Here's a nice order of magnitude calculation. (No calculators needed.)

$1\ yr \approx \pi\times 10^7\ s$
$c \approx 3\times 10^8\ m/s$

$1\ light\ yr=(c)(1\ yr) \approx (\pi\times 10^7\ s)(3\times 10^8\ m/s)=3\pi \times 10^{15}\ m$

You can work out the analogous approximation in feet using
$c \approx 1\ ft/ns$