How far can the naked human eye see if unobstructed? (straight ahead on perfectly flat land). Is there a stopping point or is it infinite when you look in the sky? Is there any situation that the human eye can see infinite?

What are the factors that determines how far we can see? The photon flux density and...?

Physics and biological point of views will both be interesting to discuss.

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    $\begingroup$ In practice, 2.5 million light years (M31 or Andromeda). $\endgroup$
    – jinawee
    Sep 18, 2013 at 20:18
  • $\begingroup$ I should add there is a time-distance limitation on that. A photon from a distant source has to overcome the expansion of the universe in order to reach us and we can see it. $\endgroup$ Sep 27, 2013 at 1:22
  • $\begingroup$ @jinawee: In practice, it's more like the observable size of the universe, even though those blurry microdots are just averagings of the finer grained pieces. $\endgroup$
    – phresnel
    Apr 18, 2016 at 14:38
  • $\begingroup$ The most far object visible with naked eye is the Andromeda galaxy (some millions of light years). $\endgroup$
    – peterh
    Jan 6, 2018 at 20:10

2 Answers 2

  1. due to the curvature of the earth, you can only see flat land for about 5km give or take a few hundred meters.

  2. from a sky object you are limited by the photon flux emitted or reflected by that object (50-150 photons in a 1ms burst, and all landing within a 10 arcminute spot on your eye)

  3. the wavelength of those photons affect your sensitivity (green photons are best)

  4. the duration your eyes have spent in darkness (eye sensitivity increases 100 fold by 5th minute in darkness, and 1000x by 15th minute)

apart from 1., all the above information can be found in this famous paper: http://arapaho.nsuok.edu/~salmonto/vs2_lectures/Lecture6.pdf

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    $\begingroup$ every visible star to the naked eye happens to have a high enough intensity to exceed the visual threshold from their respective distances. $\endgroup$
    – gregsan
    Sep 19, 2013 at 9:13

We can see objects much more distant than the Andromeda Galaxy with the naked eye if they have unusually high intrinsic brightness (supernovae, gamma-ray bursts, etc.)

According to the following link, the current record is estimated to be 7.5 billion light years away: https://www.space.com/5153-stellar-explosion-distant-object-visible-naked-eye.html

  • $\begingroup$ While this factoid might be useful, this doesn't seem to really answer the question asked (which is about the limiting factors of seeing far-away things). $\endgroup$
    – Kyle Kanos
    Jan 7, 2018 at 1:28

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