3 ms·

If an object is measured to be a billion light years away, that means that light from that object takes a billion years to get here.

• yeukhon 11y ago
  Right... but how do you know it takes a billion year to get here?
  • cheez 11y ago
    Measuring distances is an always active part of astronomy, as I understand it, but it starts with parallax. See this page for more info: https://en.wikipedia.org/wiki/Cosmic_distance_ladder https://en.wikipedia.org/wiki/Cosmic_distance_ladder
    • yeukhon 11y ago
      Thank you!