What is the minimum altitude one would require (relative to the area at which you are located, rather than to sea level) in order to be able to visually perceive the curvature of the Earth?
You can see the curvature of the earth at sea level. Go down to the ocean and watch a ship sail over the horizon.
While the effect of a ship coming over the horizon is as a result of the curvature of the earth, to the best of my recall (I haven’t been to the ocean for quite some time), you can’t look over the water and say to yourself that the earth appears to be a sphere. Rather, the ship looks like it is coming up from over an edge.
Can you actually perceive at sea level (or any other “flat” surface of the earth), along a roughly horizontal axis, that the earth is curved, or does it appear flat? If not, how high might you have to be to say, “well, it actually does look like a ball”?
Th question has no definite answer as it does not make sense. As friedo says you can see from the height of your eyes which is about 6 feet. As you rise the circle you are seeing gets smaller but it is always a circle from the beginning.
Well, the problem here is that there is no one specific altitude where all of a sudden you perceive the curvature; it’s a gradual thing. You can generally perceive the curvature fairly easily at around 20,000 feet or so. Perhaps lower if you’re paying attention.
:eek: I meant to say 60,000, not 20,000. I believe the first photos clearly showing the earth’s curvature were taken around 60,000 feet.
I need more sleep. 
I wish. (5’ 5 1/2")
That’s the answer to the question I should have asked. Thanks.
If you sight across a straight edge, such as a metre rule (and using a marine horizon) you can see it from sea level (ish)
Not really. The observer sees the earth as a circle beneath him. His eye is the tip of a cone which is tangent to the sphere of the earth. As he ascends the angle becomes gradually smaller but there is no definite point where there is any significant transition unless you want to define one. If you say “how high do I need to be to see the earth as a circle with a radius of X degrees?” then that can be answered. Otherwise the question has no answer.
You can go to this site Earth View and get a (simulated) view of the earth from above any point at any altitude
Yeah, Mangetout has it.
You can do it easily if there’s a wall or fence rail that you can sight against.
If you were standing on a flat surface that extended out indefinitely (infinitely), the horizon–if you could see one–would appear at eye level. If you were standing on a flat surface that stopped a fixed distance out (you were on a disk), then the horizon would still appear to be curved, and as you rose up the curve would become more noticeable.
We’ve all been up in airplanes 35,000 feet high–I didn’t notice much difference in how the shape of the Earth looked, looking out one of the windows.
Using Mangetout’s method will show that the horizon is curved.
That might only mean a disc-shaped earth, as RM Mentock suggests.
Combine it with one of the other methods, like observation of ships disappearing over the horizon, or the ability to see further from greater heights, and you’ve shown curvature in two orthogonal directions.
The simplest interpretation of that is that the earth is spherical.
It doesn’t rule out other geometries, though.
Edgar Allen Poe wrote a short story about a guy who flew a hot air balloon to the moon. Ridiculous thing, really… But one thing that stands out is the way Poe thought the world would look from very high altitudes.
He made a guess that at stratospheric heights, the world would have a concave shape, like a bowl underneath you. Poe even explained the optical illusion in a way that seemed very plausible (something to do with the horizon being very slow to drop from eye level).
Last time I was on a flight, it was a clear day and the ground was flat enough that I thought I’d have a look. Now, maybe it was only Poe’s posthumous suggestion, but it seems he was right… it wasn’t a very pronounced dip, but I do think there definitely was one.
While climbing Mt. Fuji, I could see the curvature well before I got to the top.