That 230 miles is the curvature of the Earth. The Planet curves away from you in all directions equally.

The problem with the arc of the Earth (commonly confused with curve), is that we humans are very, very small. Compaired with the planet in fact, we are beyond microscopic.

A water bear on a beach ball would say the ball is flat, though we know it’s a ball, because it is small enough not to be able to see the arc

Earth is 24,000 miles (40,000 km) around and a human’s eyes are two inches (4.5cm) apart. So, 48 billion centimeters.

48 Billion divide 4.5 makes 10.6 billion times the scale we can see at as an individual. That’s how large the Earth is to us, 10.6 billion times.

Astronauts have to be about 120 miles (200 km) above the planet before the see a hint of the arc. It’s just that big.

To see the whole planet it’s about 18,000 miles. This Apollo 17 picture was taken from that far out.

Here's an experiment that should be a required assignment before anyone reposts that idiotic image that the horizon always rises to eye level.

Next time you’re in a plane get a window seat and take out your smartphone and open the level app. Sight along the long edge at the horizon and take a screenshot of the level app. Do it ten times and average the results.

At 28,000′ the horizon will have dropped 2.5 degrees. At 38,000′ the horizon will have dropped 3.2 degrees.

Proof the earth is a sphere.

Want more?

You’re calculating the curvature wrong.

If you’re at a high altitude and looking at the horizon from side to side and seeing a couple hundred miles of horizon it curves 8 inches per mile. Period. Not 8″ per mile squared.

If you’re on the beach and looking forward at a distant object, it is obscured by a curvature of 8 inches per mile squared. Why you ask?

Because to see mile two, you have to look over the 8″ curve of mile one, to see mile three you have to look over both mile one and mile two.

Say you’re 6 foot tall and you’re looking at a tall building 5 miles away. Draw a line from your eyes to the bottom of the building.

At mile one the line is 8 inches lower than your eyes from mile zero.

At mile two the line is 33 inches lower than your eyes at mile zero

At mile three the line is 6 feet lower, i.e. you’re looking at the horizon.

At mile five 16 feet of the building is obscured by the curvature of the earth.

Here's a graphic illustrating that

So when the mountain-climbing flerf looks out and sees the horizon 230 miles away, the side to side curvature is 230 miles times 8 inches or 153 feet.

Flerfs think they should be able to see a 24 inch difference over 3 miles of horizon at the beach, I'm guessing they think they should be able to tell 153 feet difference over 230 miles too.

Flerfs all have a couple things in common: they all overestimate the amount of curvature they think they should see and have unreasonable expectations, and they all underestimate the size of the planet, they seem to think it's a tiny ball.

Where in the hell did all these ignorant people come from all of a sudden?

Before the Internet I assumed most people were intelligent.

Edited 8 April 2018 to specify a source for the material because Quora flagged this saying it was plagiarism.

Source: Person experiences from Curt Thurston.

Why doesn't the person who posted this visit an ocean beach?

Now you can very clearly see the how the surface of the ocean is no longer vsible as it drops below the horizon as the earth curves downward away from you. You don't need to be at 35000 feet looking out over 250 miles. Just 14 miles at sea level. This is one of the many clues the ancient Greeks had that the world was round.

Hi Stuart,

It does and you can see it. . Even more obvious when in an aircraft flying higher, as I have seen.

But still the World is very large, so the amount of curve you can see, even from a high flying aircraft, is still very slight. . To really see a lot of clear curvature, you need to be up even higher, such as the International Space Station. . And yes, they can clearly see the curvature. When you get even further from the Earth’s surface, like on the Moon, then you can see all the curvature, right round to show a complete sphere, assuming there is sunlight on all the Earth you are looking at.

Sometimes we overlook the obvious. Even I am guilty of that sometimes. . . As I type I am watching just such a picture. . I put out my weather report twice a day, I work from the real world, not reports published. So one of the real pictures I watch is a picture of Earth, so I can see how all the Clouds are moving, developing and inter-reacting at different altitudes. . So I do a lot of looking at the whole of Earth, almost live, some pictures are only minutes late. Here is one I use a lot.

This is constantly moving, that is the Sun is stationary, and the World turning, but the effective satellite camera is Geo-stationary, stationary, so always showing the same part of the world. I have selected when the world was full sunlight, except the north pole area. So this is the picture taken at : 17 Feb 2019 - 1715 UTC . Which was 11:15 my time.

The way to detect the curvature is to point your two arms in opposite directions and point at the horizon with both, and your arms will be drooped downward by a small amount that is hard to notice.

There is a curious phenomenon however when flying an airplane at high altitude that reveals the curvature of the earth. When flying at altitude, the controller may point out traffic 12 o’clock, opposite direction, at 1000 feet above your altitude. You may be surprised to spot that aircraft BELOW the “horizon”, but as it approaches, it appears to rise, pass 1000 feed overhead, and then sink back down again toward the “horizon” as it recedes behind. The explanation, of course, is that the horizon is not horizontal, and that the other aircraft, when first sighted, was below the horizontal, but above your altitude following the curvature of the earth.

I have seen this myself. And there was once a mid-air collision (way back in the 1930s?) caused by this illusion. Two planes approaching head-on over a cloud layer. When one plane spotted the other approaching from “below”, he climbed in an effort to avoid it, and thereby smacked into it from below.

I’m not sure what math is being used to calculate that the “earth should curve 35,000 feet,” but if it means that the earth should appear 35,000 feet lower than the person when it’s 230 miles away, that’s a bit of an odd assumption. Consider what Mount Everest would look like if you viewed it from 230 miles away. Would it look like it’s 35,000 feet above you? No, it would like like it’s rising less than an inch off the horizon. I think this is what the image is getting at.

If you mean that the horizon should look like a 35,000 foot curve, then I’m not sure what that even means (what part of the curve is 35,000 feet a measurement of?), or where that number came from.

If the earth was a basketball, Mt Everest would be the size of a grain of sand.

The groves on the basketball would be deeper than the deepest ocean. The dimples would be rougher than the roughest terrain on earth.

I have never been on Mount Everest. I can tell you that I have been to 45000 feet flying from JFK to Bahrain in a Boeing 747SP. Not only is the curvature clearly visible but the sun came up and went down after only a couple of hours. I don’t recall seeing it at the altitude of Mount Everest because it is not as pronounced. I see it in retirement watching ships sail away. I see it when using a cheap Davis sextant kinda for fun. I see it when I flush the toilet or drain the sink. I saw it by flying over the North Pole on a compass and by arriving on the other side of the world. I saw it in the gyro alignment of the old mechanical Litton 51 or was it 52…..Inertial Navigation Systems. I saw it through my telescope observing planets.

I'm going to ignore all the complications in this situation, and take the question at its face.

We are given two assertions; i) we can see 230 miles from the top of Mount Everest, and ii) the Earth should curve 35,000 feet over that distance. i) is potentially questionable, because it depends on the state of the atmosphere. For one there is likely to be fog, clouds, etc., and there are also important optical effects due to the change of index of refraction with air temperature. ii) is easy to estimate, based on the known radius of the Earth, but I haven't calculated myself. Anyway, let's assume that both of these assumptions are approximately true.

Finally, the 'argument' is that the horizon remains at 'eye level', which is contrasted with the supposed largeness of a 35,000 ft drop that we should certainly be able to see...

But should we? 35,000 ft is about 6.6 miles. By trigonometry, a height of 6.6 miles at a distance of 230 miles is 1.6 degrees (Sine[theta] = height/distance). 1.6 degrees is a rather small angle, and I don't think that most people are precise enough in their estimation of what line of sight is orthogonal (90 degrees) to the radius of the Earth to notice a shift of their reference 'level' by 1.6 degrees, without using some tools to help them.

The assumption hidden in this 'comon-sense' agrument was that the effect, if we really can expect it at all, should be big and visible, but actually it isn't. It is important to understand your assumptions, and then to put your conclusion in context.

People are always looking for a “curve” in the wrong places.

IF THE EARTH WERE FLAT and there was a flat ocean like the pacific filling a basin on the surface of an otherwise FLAT Earth, you could be in a small boat miles from any dry land and see nothing but water. Water in all directions … fore, aft, left, right like you were on a circular lake. The horizon would go all around you as a curve; and that’s exactly what you see on the earth. So it means nothing about the shape of your surface.

The curve in question is going away from you in all directions. The horizon is moving away from you in all directions as you gain altitude. From the top of a mountain, looking at the horizon doesn’t tell a thing … unless you realize where you’re looking to see it … like downward.

• if you were looking horizontally, you wouldn’t be seeing any horizon. That’s the drop/curve that’s been known for centuries.
• the reason they built tall lighthouses when it was hard to do
• the reason they built ships with a crows nest and sent some kid up there in a storm
• the reason a truck driver can see farther down the road than someone in a sports car.
• the reason height allows you to see over a curve in the road sooner/farther

I hear so many comments that “the horizon is always at eye level” when in fact, it nearly never is unless you’re treading water.

This is yet another whacky assertion by the flat Earth crowd who call it proof that we have all been lied to, the Earth isn't spherical, all pictures from space are fakes, etc, etc...

There are tons of YouTube videos stating that:

• "The horizon always stays at "Eye Level" no matter how high one goes"
• "Even on Mount Everest the horizon looks exactly the same as seen from the ground"
• "When flying in a plane, the horizon always rises to eye level"

Other answers to this question (and the webpage from Metabunk below) have shown that the horizon is about 3 degrees lower when viewed from the top of Mount Everest.

How in blue blazes do flat-earthers think that they can tell that small amount of a difference by looking at a photo with no indication of the slight pitch or tilt of the camera? Or by eye, without the benefit of a very expensive surveyors instrument? It's just plain silly.

Earth curvature refraction experiments - debunking flat/concave Earth

Earth's Curve Horizon, Bulge, Drop, and Hidden Calculator

The meme is wrong. An observer does see the curve.

Even if the Earth were flat, the neighboring mountains wouldn’t be at “eye level” because they aren’t as high as Everest. You’d still be looking down, if only slightly.

But the Earth is not flat, so when you see something on the horizon, it is lower than it would be if the world were flat. The trouble is, you have nothing “flat” to compare it to so you just assume that the horizon is at “eye level”, but if you were to take a leveling telescope up with you and set it to be completely horizontal to the earth, it would look out over the horizon by a large margin (depending on exactly how far away the horizon actually is).

I do a regular volunteer coastguard watch. From our station the horizon is about 6 or 7 miles away. We have powerful telescopes and can see ships approaching Falmouth Harbour.

We also have an AIS facility (Automatic Identification System) which enables us to identify ships approaching on the computer screen a long time before we see them. Sometimes we see that it is a tall ship, and very occasionally when visibility is good our first sighting is of its sails appearing on the horizon.

If it is fully rigged the first sail we see is the Moonraker. As it gets closer we get a view of the next sail, the Skysail, then the Royal followed by the Gallants. Our view is ever lower down the masts.

It is much as though the ship is climbing the hill towards us as we see lower sails appearing over the ‘top of the hill’. Through the telescopes we see them over the sharp edge of the horizon.

Of course as far as the crew of the ship is concerned they are not ‘climbing a hill’. They are sailing across a horizontal sea. I can manipulate the AIS system to show their course, a straight line heading in my direction. I go back to the telescope and I can see the ship is ‘climbing higher up the hill’ and I can see the Topsails and soon the Mainsails. Finally the ship is on the horizon and my telescope shows the whole ship, hull and all.

I have also seen a submarine surfacing, though much closer, but the experience is much the same — periscope appears first, then the conning tower, then the hull. But, maybe oddly, I have not been under the delusion that the tall ship is actually an ancient version of the submarine, slowly, sail by sail, surfacing. What I know is that the course that I have plotted on the AIS is actually a line that curves away from me and down over the horizon, and what I see is its sails, course by course, peeking one by one over the horizon at me as it comes closer.

Through the telescope I can see Plymouth, 60 miles away. Actually, I can’t, what I can just see is the tops of the hills on either side of the Tamar where Plymouth is. Very occasionally a Navy helicopter takes a casualty to Derriford Hospital there, and every time it appears to crash into the sea before it gets there. But I don’t fret, because it’s just gone out of sight over the horizon.

Such a bad question and such a bad graphic. If you’re arguing the earth is flat from the top of Everest you would be able to see the entire earth or as far as you could see before the atmosphere obscured your vision.

The horizon is always below eye level. Not much below eye level because of the great distance involved but it doesn’t matter if the earth is flat or round if you’re looking exactly tangentially to the point where you’re standing your line of sight would leave the planet without ever making contact unless something tall was in the way. In the case of a sphere, tangents only meet the surface once and the case of a flat earth parallel lines never meet at all.

The horizon is actually caused by the curvature of the earth. Without the gradual drop off the earth would go on indefinitely simply becoming hazier and hazier instead of dropping out of sight.

The math in the equation is really wrong.

The logical explanation is that he does see the effects of the curvature. If the Earth were flat, the horizon of this - or any - observer would be at the edge of the disc (or at an infinite distance if this hypothetical disc were infinite in size). Because the Earth is a rough sphere and curves ‘down’ in all directions, any observer can only see a certain distance depending on his elevation. In this case quite far because he’s on top of a tall mountain. But there is still a limit - as the text dutifully states. Anything at a larger distance will only be visible if it has a certain elevation itself. The further out, the taller it needs to be.

However, 30k feet is still only a very tiny distance ‘up’ compared to the size of the planet which is 42 million ft. in diameter. So any visible lowering of the limb of the Earth (actually seeing the horizon below ‘eye level’) caused by his elevation, is still more or less negligible. I’m sure a very precise optical system could measure it if it weren’t for the atmosphere hiding the actual horizon from view most of the time.

The simplest experiment would be to take a good camera tripod with a spirit level or a GPS system to help you level it and make sure the camera is as horizontal as you can get it, zoom in at the horizon and make a 360° panorama. If the horizon is below the centerline of the picture in all directions, the horizon is actually below ‘eye level’ and you're observing the curvature of the Earth.

Well, I figure/maintain that you don’t even need to ascend Everest to see the curvature of Earth, but that you can do so even from Earth’s surface!!

How come? - Well, assuming you’re on a vast plain with no hills or you’re standing in a boat on the sea/lake and very far from the nearest shore, then by looking at the apparently straight-lined horizontal horizon ahead of you and following it AROUND with your eyes by TURNING your body until you’re right back facing the same direction as you started, what you’ll have seen can be clearly nothing but an enormous 360 degrees CIRCLE with you yourself at its very centre!

Now if you do the same exercise from progressively higher locations then that circle (or actually an even larger one of over 8k miles diameter) will appear smaller and smaller, until eventually at a height of just a few thousand miles the completely circular ‘horizon’ will have contracted in apparent size to such an extent that it can be seen without even shifting your gaze.

And at a further ‘height’ of about a million miles that circle will have further diminished in size to appear no larger than does the moon to us here on earth.

Well, have I convinced you or will you claim that my argument is specious? ;)

Can you draw the horizon on the walls of a circular room, as if you are on the top of a mountain, and see the “curvature”?
You will end drawing a straight line!

Yes: since you are in the center of the circle every point of the horizon has to be at the same height from the ground, so looking around must be the same in every direction. So you will have a straight line, and you can think that you are inside a “flat circle”, unless you understand other basic proofs of the curvature.

Things changes when you have to look at the floor too see the horizon, but this is FAR higher than a mountain, and probably higher that ISS orbit (is not so far from our planet as you may think)

The peak of Mount Chimborazo and not Everest, is the farthest point on the Earth from its centre or the closest point to Space on the Earth surface and for all intent and purposes has the highest elevation from the Earth's Center. The peak of Everest is simply the highest point above sea level (a latitude dependent phenomenon) or the point on the earth surface having the thinnest atmosphere (thickness or thinness of the atmosphere also depends on latitude).

If the earth were flat, we would still need to take into account the equatorial bulge, meaning the peak of Chimborazo and not Everest would have the highest elevation above sea level and thinnest atmosphere as well.

It is the bulged spherical shape of the earth that makes the Chimborazo - Everest conundrum possible.

To answer the main question

Draw a circle with a compass having a radius of 100mm (10cm).

Using your protractor and a ruler, draw a circular sector with an angle of 3 degrees and extend the radii of the sector above the circle circumference. Then using your ruler draw a tangent to the midpoint of the sector’s arc such that it connects to the two extended sector radii. If you have done this correctly your tangent should be roughly 5.3mm long when measured from one radius to the other.

You’ll find out that even though the circumference is a closed curve, 3 degree arcs (that are about 5.2mm long) would appear as a straight line, almost indistinguishable from the 5.3mm tangent drawn using a ruler.

The radius of the earth is 3963 miles and the radius of the horizon around mount Everest is 210 miles; the ratio of earth radius to the horizon, around Mount Everest peak is 18.87 and the ratio of the circular radius on our paper sketch to the tangent is also 18.87.

I hope this helps.

If the earth was smooth, ie, a perfect sphere, and there was no atmosphere, the horizon would be 208 miles away when viewing from the height of Everest (29030 feet). This is about 3 degrees around the earth, which means that the horizon would be approximately 3 degrees below horizontal. In these conditions you could tell the earth was curved.

In practice, the Earth is too lumpy and it is too hazy to be clear about what is what. Take a look at the actual photo used in the question. Haze. It is obvious that you aren’t looking 230 miles! You might see a few tens of miles on a day of exceptional visibility. You would not see the geometric horizon. You see closer mountains, clouds and haze. This makes it very hard to detect curvature. It certainly won’t be obvious.

For comparison, the ISS is 50 times higher than Everest and the “horizon” is about 20 degrees below “horizontal”. A circle of the Earth about two thousand miles across can be seen. Haze and mountains aren’t a problem. The curvature is obvious.