1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Unit Conversion: Astronomical Units, Parsecs and Light Years

  1. May 17, 2012 #1
    Hi everyone,
    It's been years since I solved a physics problem for school. Now that I'm back at it I could use some help on this problem:


    "The astonomical unit (AU) is defined as the mean centre to centre distance from Earth to the Sun, namely 1.496 x 10^11 m. The parsec is the radius of a circle for which a central angle of 1s intercepts an arc of length 1AU. The light year is the distance that light travels in 1 year. (a) How many parsecs are there in one AU?

    The attempt at a solution

    I can do simple conversions of units, but I'm having trouble figuring out the connection between AUs and parsecs. I know that Arc length = r * angle (in rad) which leaves me with 1 AU = 1 parsec * 1s. But then again s is seconds, not radians...
    Another way I attempted the problem was to assume that the arc of a circle whose radian is 1 parsec is so large that I can assume that the arc is a straight line, is that a fair assumption?

    I'd really appreciate any corrections or just a hint in the right direction, not the full answer, and then hopefully I can work out the other parts of the problem myself. Thank you in advance!
  2. jcsd
  3. May 17, 2012 #2


    User Avatar

    Staff: Mentor

    Hi iwantpi, Welcome to Physics Forums.

    The '1s' above is 1 arc second, an angle. Remember that degrees are divided up into 60 arcminutes, which are in turn divided into 60 arcseconds.
  4. May 20, 2012 #3
    1 parsec = 206264.984626 au
    I got it from aconverter.net, which I usually used. hope this help.
  5. Jun 15, 2012 #4
    Thanks for all the help you two and sorry bout the late reply. I figured it out. It wasn't so hard once you guys told me about the arc second thing. Thanks again.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook