How far can a human see at night?

I am not talking about seeing in total darkness, which I understand is not possible. I am looking more about a question how far can a human see during starry night or with half moon/full moon. I tried googling for this, but found no useful results.

EDIT I think I need to clarify: I am thinking more in terms of immediate surroundings. I understand we can a source of light from afar, but what about a tree on a flat land - from how far would we see it in star light?

• A better, more scientific way to ask this question would be to ask about visual acuity as a function of luminance... there is no meaningful way to answer your title question without specifying other key parameters. Oct 7 '19 at 20:07
• You would need to further specify it, for example, at a certain lumination (half-moon), from how far you could recognize a tree as an oak (from the shape of its leaves). Anyway, half-moon can be quite bright...
– Jan
Oct 8 '19 at 7:14
• Hmm.. I would like to build some kind of intuition about this, not exactly have a formula. Would I be able to discern a tree shape from star light only in an empty field? From what distance? How about full moon? Are there any resources I could take a look at or examples? Examples would likely work best for me. Oct 8 '19 at 17:25

2 Answers

At night you can actually see further than during the day. At night you can see stars that are many light years away, but during the day the furthest thing you can see is probably the sun, which is 8 light minutes away. There's probably no definite answer because the brighter and bigger the object (and the darker the surrounding and the clearer the air), the greater the distance from which you can see it.

According to Sky and Telescope, the most remote stars (as part of Andromeda Galaxy) you can see with a naked eye are 2.5 millions light years away.

Talking about the objects on earth, at half-moon, you could see to "the end," that is to the horizon.

• Thanks a lot for the answer, I clarified my question. Oct 7 '19 at 19:52

With clear, dark skies (no urban light pollution), about 2.5 million light years. That's the distance to the Andromeda galaxy, which is visible (in the northern hemisphere) with ordinary visual acuity: https://en.wikipedia.org/wiki/Andromeda_Galaxy#Amateur_observing

Vision does not depend on distance, but on the brightness of the things being viewed, and their size. That is, we see stars as points because their diameter is too small to resolve (angular resolution) but they still put out a lot of photons. We see Andromeda as a fuzzy patch of light, because it is composed of lots of stars spread over a large distance.

For more ordinary things, it depends on the amount of light, and the size. Flying in a small plane on a clear winter night with half moon or more, it's possible to see snowcapped mountains 50 miles (80 km) or more distant.

• Thanks a lot for the answer, I clarified my question. Oct 7 '19 at 19:52
• The clarification doesn't really help. What do you mean by seeing the tree? Do you need to see the light reflected from it, or is it sufficient to see the shape of the tree blocking starlight? You could probably calculate an approximate answer to the first case from intensity of starlight * reflective index of the tree * the inverse square of the distance, and comparing it to visual acuity. Oct 8 '19 at 3:21
• OK, to clarify further: In an empty field, with star light, from what distance could I discern a shape of a tree? How about a person? And how about with a half moon or full moon? Oct 8 '19 at 17:26