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How far is the Horizon?

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A typical human eye with “perfect” eyesight can resolve about 2-1’ (arcminutes, or 1/10800 to 1/21600 of a full circle) using typical “white” light. Taking all this into account, a human being can tell that two light placed 1 meter apart are two, not 1, up to between about 10 to 20 km.

Brilliant. That was my next question - what is the angular resolution of the human eye for visible light. And you answered it well.

Since all light from non emitting sources is reflected, that calculation should hold for everything.

That is - you could distinguish two individual objects 1 meter apart up to about 10 to 20km.

TFS

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I didn't totally think about everything that has to be taken into account.

I might be way off, but, is the angular resolution more or less the 'event horizon' of the eye, as long as we use a metre distance between the two objects ? Because if you'er to measure two lights a kilometre apart, the angular resolution would be further ? (I hope I'm not repeating the exact same questions for already answered answers. :o )

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I might be way off, but, is the angular resolution more or less the 'event horizon' of the eye, as long as we use a metre distance between the two objects ?
I wouldn’t myself use the term “event horizon”, for fear of causing someone to think I was talking about something having to do with black holes, but I think, tmaromine, you’re correctly appreciating that angular resolution provides a good way of determining how far away the human eye can perceive the shape of well-illuminated objects of specific sizes.

You can apply it to objects at astronomical distances. At roughly 400,000,000 m distance, 2 arc-minutes equates to about 40,000 m. So, the smallest “feature size” a human being standing on earth under perfect viewing conditions could see with their naked eye on the surface of the moon would be 40 km. A smiley face and a random squiggle scratched in the regolith would look the same, unless the lunar artist spaced the eyes, mouth, etc at least 40 km apart. Features on earth like the Nazca Lines, drawings of dogs, spiders, monkeys, and other designs, the largest of which is about 300 meters across, would be visible to the human eye at an altitude of no more than about 300 km, just a bit higher than the lowest altitude in which a satellite can orbit without its orbit rapidly decaying due to upper atmospheric drag.

We discussed similar issues involving resolving objects on the moon using telescopes in several posts spanning several years in the ”Flags on the Moon”.

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I understand what you mean by EV's relation with black holes, but the term event horison did exist before black holes. :) (You probably know that already ; I guess it's just that in the scientific community it's like how we relate a "programme" to computers instead of TV shows now. :P )

I see how distance apart between two things and how far the observer is from those two things, and how light and cetera is all a factor now... I don't precisely understand the arcminutes and stuff, because we never made it to circles in geometry last year...... «Flags of the Moon» thread looks interesting ; I'll read into it some.

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• 2 years later...

At lake level I can see longpoint, Canada 24mi. away....at 185' above I can see more of longpoint, Canada but not the Canadian mainland 30+mi away.

100.4 m

(329.5 ft) 15.27 m

(50 ft 1 in) Wyoming 1909-1924 sunk[26] This American ship had a tendency to flex in heavy seas, causing the long planks to twist and buckle.[27] This allowed sea water into the hold, which had to be pumped out.[28]

I have serious doubts as to a wooden vessel being built any larger.
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At sea level looking out towards where the ocean meets the horizon. The horizon is approximately 15 miles away.

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At sea level looking out towards where the ocean meets the horizon. The horizon is approximately 15 miles away.

Thank you and welcome to Hypography! I'm amazed at all the confusion that has drawn out this simple question so long.

Of course, the distance to the horizon is also a function of the vertical distance of the observer from the plane of the horizon, as is explained at How to Calculate the Distance to the Horizon - wikiHow. Still, the question and the protracted debate seem fairly strange to me when your answer is pretty much on target. There are lots of other Gordian knots around here, so keep your sword honed.

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