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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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In practice, 2.5 million light years (M31 or Andromeda). – jinawee Sep 18 '13 at 20:18
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. – Ali Abbasinasab Sep 27 '13 at 1:22
@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. – phresnel Apr 18 at 14:38
  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:

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So what you are saying is that there is a threshold intensity for vision? However, one can have a distant point source with arbitrary intensity. How does this put a limit on how far a person can see? – dj_mummy Sep 19 '13 at 4:07
every visible star to the naked eye happens to have a high enough intensity to exceed the visual threshold from their respective distances. – gregsan Sep 19 '13 at 9:13

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