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

Is it really possible to see down to mag 8?

  1. Apr 25, 2014 #1
    I saw this thread about Neptune's naked eye visiblity: https://www.physicsforums.com/showthread.php?t=362668 . I found this thread when I was searching for an answer to the very same question that is asked there.

    In every source I've seen that actually details the Bortle Dark Sky scale it has been said that one can see down to Mag 8 under perfect conditions (I think I've seen some sources that have said 8.5, which makes this even more puzzling.) Neptune's magnitude is sometimes as bright as 7.78. Yet, every single source I've seen has said that Neptune is not naked-eye visible even under perfect conditions.

    To be clear here, I do understand the basic issues relating to apparent magnitude and such. I am aware that Neptune is way farther from earth than the naked eye planets, and that that is the reason why it is so dim. But... it seems like according to the Bortle Dark Sky Scale it would be just barely within naked eye range under absolutely perfect conditions. Is this true? I've had trouble finding anything that actually addresses the issue, most of the sources that say you can go down to mag 8 do not address Neptune in particular, while most sources that talk about the naked eye planets just say "Neptune is too dim."

    So... is it actually possible to see Neptune with the naked eye?
  2. jcsd
  3. Apr 26, 2014 #2


    Staff: Mentor

  4. Apr 26, 2014 #3


    User Avatar
    Science Advisor
    Gold Member

    I've always read that the limiting magnitude for the naked eye is 6-6.5, not 8. For example here: http://arxiv.org/abs/astro-ph/0011310 (fig 6.) or here: http://www.astrometry.org/magnitude.php [Broken]

    There's always some variation between individuals, so perhaps that can account for rare cases of people being able to see down to mag 8, but for the majority of observers it should be too dim.
    Last edited by a moderator: May 6, 2017
  5. Apr 26, 2014 #4
    I do know that its really, really hard to find a perfect dark sky site. Most places still get some light pollution, even if only just a tad. But, it seems like according to the Bortle Dark Sky Scale it is possible to see 8.0 under absolutely perfect conditions with good eyesight. I'm not sure what the angular diameter thing is though. That might be the answer to the question. Does angular diameter have anything to do with apparent magnitude?
  6. Apr 26, 2014 #5


    Staff: Mentor

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook