In a television interview I saw a few weeks ago, French performance artist Philippe Petit noted how he could still vividly recall that during his 1974 high-wire walk between the twin towers of the World Trade Centre he was able to gaze out across the New York skyline and observe the curvature of the earth – a phenomenon I also once heard described before by my father, who was once a commercial pilot.
However, can’t for the life of me work out why it is that anybody with an elevated position would be able to perceive the horizon as anything different to a person standing near to sea level. Surely, assuming that the Earth is one big sphere, the horizon would always appear as a straight line that circles the observer, regardless of their altitude?
Could it be that there’s some sort of atmospheric conditions that causes some sort of variation in the apparent position of the horizon?
Or, is it that the planet’s actual shape as an oblong spheroid becomes apparent to any jerk who thinks that its a bright idea to stand on a piece of string suspended 400 metres in the air?
I doubt you could see the Earth’s curvature from atop a building, as per the link in the previous post. I’ve been in a zillion airplanes and you can’t really tell from thousands of feet up.
One might be fooled into thinking they are seeing the Earth’s curvature by some optical illusion, though. From an elevated height you would be able to perceive the horizon being below you in 360 degrees, or close to it, giving the planet below you the distinct appearance of being a disk that it doesn’t have when you’re standing at the corner of 38th and Lexington. Your brain might think it’s seeing the earth’s curvature, as opposed to just the circle of the horizon. That would explain why, when I look out my airplane window and only see a slice of the pie - say, 40-50 degrees of the horizon - I don’t see curvature.
You’d have to run an experiment to see if this was true though.
Obviously, at some point you’re going to start seeing curvature. So the question remains: at what altitude is it apparent? Definitely higher than even Burj Dubai.
Define “seeing curvature”. If you’re standing on the Moon and looking up at Earth, you see a circle of Earth embedded in the sky. If you’re standing in the middle of Kansas and look at Earth, you also see a circle of Earth embedded in the sky. The only difference between the two cases is the size of the circle.
Hypothetically, could you say in that huge field in Kansas, if you are standing in it and look around in all directions you see nothing but field…we’ll say it’s a large enough field to be able to do that.
Now, stay with me here, say you spray paint a solid black square, you remaining in the center.
Continue to make the square large and larger until the corners start to disappear (if they would even disappear)…
Would this, optically, accentuate the curvature of the earth?
Unless he’s referring simply to the extra distance at which the horizon appears when your observation height increases, I agree that this doesn’t make a lot of sense.
A phenomenon observable at moderate altitude (say, 7000’ above the ground) happens when you have extremely good visibility and plentiful high cumulus clouds. The limit of visibility is then due to the clouds curving down to the horizon - you are looking into the bottoms of the distant clouds.
It’s not that large. Picture the Earth as a circle, and your head as a point somewhere outside that circle. At some nontrivial angle on either side, lines emanating from this point will stop intersecting the circle; the Earth will necessarily occupy less than 180 degrees of your field of view.
Granted, it may be very, very close to 180, so close that you can’t tell the difference, but that’s just what it means to say that the only difference between Kansas and the moon is one of degree.
Okay…I had to draw a diagram because I just want to make sure I’m following you and have too much time on my hands…Like this? With the yellow being my field of vision? And that less than half of the circumference comes in contact with the earth?
Wouldn’t it increase as I get closer to the earth?
Yes, it would increase as you get closer to the Earth, but it will never become 180*, which means that field in Kansas will never be the only thing you see “in front of” you (though it may be the only part of the Earth you see in front of you). There will always be at least some “forward” rays coming out from your eye which miss the Earth entirely. Thus, the visible Earth will always form a bounded circle, and there will be some points in your view outside that circle. Just like from the moon…
*: Well, it would become 180 once your eye was actually stuck in the ground of the Earth. Then all you could see would be, well, the particular point of the earth you were stuck in.
Probably confirmation bias. I’ve experienced that too from a plane . . . but not from the top of the WTC, which I visited a few times. That’s just not high enough.
I can see the curvature of the Earth quite well. Moreover, if I tilt my head just right, the curvature changes from concave to convex. Methinks it might be something to do with my glasses.
Even from the moon, you are not really appreciating the curvature of the Earth. It might well be a flat disk. It takes the moving line of shadow from the Sun to appreciate its curvature (sphericity?).
This is a great example of what I think helps us appreciate the curvature of the Earth. Can’t you just appreciate this from the ground, btw?
Interesting. At what point do you stop seeing it as a square? I think that quickly you start losing your appreciation of the corners and start seeing it round.
The locus defined by the lines tangent to the surface of the earth radiating from the eye of an observer (horizon) is always a cone with a cone angle limited between 180º when the observer is on the surface of the earth and 0º when the observer is infinitely far away. With the observer close to earth he cannot see the entire cone simultaneously because human vision can only cover a smaller angle.
From here on the question has to be answered by making certain assumptions. I would propose that there are two different points when one can see “roundness”.
One is when the observer is high up enough to put some feeling of significant distance to the surface of the Earth, even if the entire horizon cannot be observed simultanously at that point. I would say this height is very subjective and dependent on the observer. I would say 400 m certainly qualifies.
Another point is when the entire horizon can be observed simultanously. This happens much higher up and can be calculated more easily.
Still, the question is more psycological than geometrical.
