Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Calculating how long ago light was emitted from a star?

  1. May 29, 2012 #1
    hey folks,

    i was wondering if anyone could give me help on a wee problem i have.

    using a distance of a star of 2.6pc from earth, how would one calculate the time it took for light to be seen on earth today...

    my books have rounded that 1pc = 3.3 ly
    so i would take 2.6 pc to be the same as = 8.6 ly

    so i believe that it would have take 8.6 lightyears for the light to reach earth today,

    how would this be converted into 'years' from light years or am i missing something?

    any help would be excellent!

  2. jcsd
  3. May 29, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    A light-year is a measure of distance. It is the distance that light travels in one year. Thus it take 8.6 years for light from a star 8.6 light-years away to reach us.
  4. May 29, 2012 #3
    thanks for replying...

    so 1 lightyear = 1year

    ∴ 8.6 lightyears = 8.6 years

    i thought i was missing some further calculations there and maybe i was just over complicating the question?

  5. May 29, 2012 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    NO! You cannot equate a time and a distance! In physics, the two sides of an equation must have the same dimensions. Time = distance is just as nonsensical as saying temperature = force or similar.

    The correct statement is that it takes light a TIME of 8.6 years to travel a DISTANCE of 8.6 light years.
  6. May 29, 2012 #5
    could you give me advice as to how i would go about calculating the time in years for light seen in earth today emitted by a star whose distance is 2.6 parasecs away?

    i'm a bit lost...

  7. May 29, 2012 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What cepheid was referring was the mixing of units of distance and unit of time.

    For example if two towns are 150 miles apart and you drive from one to the other at 50 mph, it will take you 30 hrs to make the trip. But you would not say that

    150 miles = 3 hrs.

    Because miles are units of distance and hrs are units of time, thus they cannot be directly equated to each other.

    Instead, you would say that

    150 mile/50 mph =3 hrs.

    With your problem, light-years is the distance and years is the time. The speed is measured in light-years per year. Light travels at a speed of 1 light-year per year.

    So you would say that

    8.6 light-years/ 1 Light-year per year = 8.6 years.
  8. May 29, 2012 #7
    thank you for your help, much appreciated!!
  9. May 29, 2012 #8
    Fo nearby stars all that has been posted is fine...the ageof the light is about the time of travel time for the light.

    But for very great distances we must also take into account that as the light travels to get here, the distance it travels increases during transit time. Light gets further delayed. For example light at the most distant points in the universe is about 45 blyr away now, but since it was emitted 13.7 byr ago the universe has expanded by factor of 1090, so at the time of emission the surface of last scattering was only 45blr/1090 = z or about 41mly away!!!
  10. May 31, 2012 #9
    ^Are you referring to the inflation theory due to horizon problem ?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook