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Light year

  1. Aug 17, 2004 #1
    what is the value of 1 light year
  2. jcsd
  3. Aug 17, 2004 #2
    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?

  4. Aug 17, 2004 #3
    A measure of distance? How can light speed be an accurate measure of distance when it travels with respect to an observer?
  5. Aug 17, 2004 #4


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    '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!
  6. Aug 17, 2004 #5
    The valua at which light travels in a year is approx. 9,500,000,000,000 kilometers.
  7. Aug 17, 2004 #6
    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).

  8. Aug 17, 2004 #7
    Here is the full Calculation:
    [tex] v=\frac{d}{t} [/tex]

    [tex] d=vt [/tex]
    [tex] d=cy [/tex]

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

    [tex] d= 9 460 800 000 000km [/tex]
  9. Aug 17, 2004 #8
    I think that you are taking liberties with the meaning of the word value.
  10. Aug 17, 2004 #9


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    Take a look at the defintion of the SI meter sometime
  11. Aug 17, 2004 #10


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

    [itex]1\ yr \approx \pi\times 10^7\ s[/itex]
    [itex]c \approx 3\times 10^8\ m/s[/itex]

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

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