Quite simply you cannot. It is widely stated you would need to be at a height of at least 40,000 ft to get even a hint of curvature if earth were round. Commercial aircraft are not allowed to fly this high. They are only allowed to fly just under this altitude. 36,000ft might be typical. In addition, the windows on commercial aircraft are small and heavily curved. Even if they flew high enough for a person to see curvature, it would still not be visible to passengers.
Since temperature inversions are common over water, it is relatively easy to devise experiments in which distant objects beyond the curvature of the earth are visible. Perhaps the most famous are the photographs of the Chicago skyline taken across Lake Michigan, about 60 miles away. The photographer, Joshua Nowicki, does not promote the flat earth, but flat-earthers have used his photographs many times, such as here, as supposed proof that the earth is flat. Flat-earthers do not seem to be aware of just how rare these photographs are. If the earth were flat, then the Chicago skyline would be visible across Lake Michigan nearly every clear day, but it is not. If the earth is spherical, then the hulls of ships ought to disappear as the ships move away from the observer. Since the ship must move many miles away for this to become noticeable, it is difficult to see this with the naked eye.

29. If the Earth were a globe, it would, unquestionably, have the same general characteristics - no matter its size - as a small globe that may be stood upon the table. As the small globe has top, bottom, and sides, so must also the large one - no matter how large it be. But, as the Earth, which is "supposed" to be a large globe, bas no sides or bottom as the small globe has, the conclusion is irresistible that it is a proof that the Earth is not a globe.
“1 In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. 2 And the Spirit of God moved upon the face of the waters. 3 And God said, Let there be light: and there was light. 4 And God saw the light, that it was good: and God divided the light from the darkness. 5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
Rowbotham was a victim of a superior mirage. When flat-earthers hear this, they normally respond by dismissing this as impossible, because mirages supposedly are inverted images, but Rowbotham saw the boat right side up the entire time. However, this confuses superior and inferior mirages. What is the difference? First we must discuss the physics of light a bit.

The earth is surrounded on all sides by an ice wall that holds the oceans back. This ice wall is what explorers have named Antarctica. Beyond the ice wall is a topic of great interest to the Flat Earth Society. To our knowledge, no one has been very far past the ice wall and returned to tell of their journey. What we do know is that it encircles the earth and serves to hold in our oceans and helps protect us from whatever lies beyond.
This balloon footage from a high altitude piggy cam at 121,000 feet shows a flat earth! The flat earth horizon rises to eye level and has no curvature at all. Any slight "curvature" you see is from the lens and the movement of the camera. This is conclusive flat earth proof. When the camera is not moving, you can clearly see that the earth is also not moving. And you can see that the close, small sun is illuminating locally. If you have eyes to see, then see the truth! We are not living on a spinning ball!
29. If the Earth were a globe, it would, unquestionably, have the same general characteristics - no matter its size - as a small globe that may be stood upon the table. As the small globe has top, bottom, and sides, so must also the large one - no matter how large it be. But, as the Earth, which is "supposed" to be a large globe, bas no sides or bottom as the small globe has, the conclusion is irresistible that it is a proof that the Earth is not a globe.
While it's true that unipolar magnets can't exist, this isn't a problem for the Flat Earth. This is because ring magnets, which are shaped like (you guessed it!) a flat disk, are capable of having radial magnetization. In a radial magnet, one magnetic pole is at the center and other is at all points on the edge of the magnet. A magnet like this can be found in loudspeakers, and perfectly replicates what is found on the Earth.
Regarding the "traveling at the same speed" idea. When in an aircraft, two people can toss a ball to each other. From the POV of those Inside, the ball is moving at normal ball tossing speed. From the POV of those outside, the ball is being gently tossed to over 500kph and the catcher catchers it without problem. If this is the "everything on the planet moving at the same speed" idea in action. Which does make it possible to travel in either direction.
195) Astronomers say the magical magnetism of gravity is what keeps all the oceans of the world stuck to the ball-Earth. They claim that because the Earth is so massive, by virtue of this mass it creates a magic force able to hold people, oceans and atmosphere tightly clung to the underside of the spinning ball. Unfortunately, however, they cannot provide any practical example of this on a scale smaller than the planetary. A spinning wet tennis ball, for instance, has the exact opposite effect of the supposed ball-Earth! Any water poured over it simply falls off the sides, and giving it a spin results in water flying off 360 degrees like a dog shaking after a bath. Astronomers concede the wet tennis ball example displays the opposite effect of their supposed ball-Earth, but claim that at some unknown mass, the magic adhesive properties of gravity suddenly kick in allowing the spinning wet tennis ball-Earth to keep every drop of “gravitized” water stuck to the surface. When such an unproven theory goes against all experiments, experience and common sense, it is high time to drop the theory.
53) At places of comparable latitude North and South, the Sun behaves very differently than it would on a spinning ball Earth but precisely how it should on a flat Earth. For example, the longest summer days North of the equator are much longer than those South of the equator, and the shortest winter days North of the equator are much shorter than the shortest South of the equator. This is inexplicable on a uniformly spinning, wobbling ball Earth but fits exactly on the flat model with the Sun traveling circles over and around the Earth from Tropic to Tropic.

The Newtonian theory of astronomy requires that the Moon "borrow" her light from the Sun. Now, since the Sun's rays are hot and the Moon's light sends with it no heat at all, it follows that the Sun and Moon are "two great lights," as we somewhere read; that the Newtonian theory is a mistake; and that, therefore, we have a proof that the Earth is not a globe.
9.) As mariners take to sea with them charts constructed as though the sea were a level surface, however these charts may err as to the true form of this level surface taken as a whole, it is clear, as they find them answer their purpose tolerably well – and only tolerably for many ships are wrecked owing to the error of which we speak – that the surface of the sea is as it is taken to be, whether the captain of the ship "supposes" the Earth to be a globe or anything else. Thus, then, we draw, from the common system of "plane sailing," a practical proof that Earth is not a globe.
God speaks to us through the Bible, Creation, and the Holy Spirit, but how does Satan speak to us in order to deceive us into following him and rejecting God? Through the media! The media (TV, movies, music, books, news/ newspapers, internet, video games, etc.) is Satan’s mouthpiece! The media is being controlled by Satan’s human followers. Everything being taught by the media is against God and the opposite of what is taught in the Bible! Think about it. We are being brainwashed by the media to believe that we came from monkeys, the earth took millions of years to form, you can choose your own gender, sex is a free-for-all and is ok outside of a heterosexual marriage (the opposite of God’s design), that its ok to lie, hurt people, and do whatever feels good, as long as it’s what you want to do. The media says, “Do not follow God and the Bible! Do what thou wilt!”. This is Satanic Theosophy being pushed on us through the media.
The astronomers' theory of a globular Earth necessitates the conclusion that, if we travel south of the equator, to see the North Star is an impossibility. Yet it is well known this star has been seen by navigators when they have been more than 20 degrees south of the equator. This fact, like hundreds of other facts, puts the theory to shame, and gives us a proof that the Earth is not a globe.

As the mariners' compass points north and south at one time, and as the North, to which it is attracted is that part of the Earth situated where the North Star is in the zenith, it follows that there is no south "point" or "pole" but that, while the centre is North, a vast circumference must be South in its whole extent. This is a proof that the Earth is not a globe.
On February 24th, YouTubers GlobeBusters (featured in the documentary) uploaded a video entitled "Behind The Curve Documentary Reveals Globe Earth Desperation!" The video explains that the documentary was misleading. It gained 41,000 views in a couple weeks (shown below, left). On February 25th, Jeranism (also featured in the documentary) responded negatively to the film (shown below, right).
These photographs clearly reveal that the hulls of these two ships progressively disappeared as the ships moved farther away. This is consistent with what we would expect if the earth is spherical, but this cannot be explained if the earth is flat. Therefore, this is good evidence that the earth is spherical. The results presented here contradict the many photos on the internet of objects beyond the horizon that supposedly prove that the earth is flat. Those alleged proofs are flawed because they failed to take account of atmospheric refraction due to a temperature inversion. By conducting this experiment when there was no possibility of a temperature inversion, I avoided that complication. The fact that inferior mirages consistently showed up in the photographs prove that there was no temperature inversion, indicating instead that there was a slightly warmer layer of air in contact with the water, with slightly cooler air above.
Mr Hind speaks of the astronomer watching a star as it is carried across the telescope by the diurnal revolution of the Earth." Now, this is nothing but downright absurdity. No motion of the Earth could possibly carry a star across a telescope or anything else. If the star is carried across anything at all, it is the star that moves, not the thing across which it is carried! Besides, the idea that the Earth, if it were a globe, could possibly move in an orbit of nearly 600,000,000 of miles with such exactitude that the cross-hairs in a telescope fixed on its surface would appear to glide gently over a star "millions of millions" of miles away is simply monstrous; whereas, with a FIXED telescope, it matters not the distance of the stars, though we suppose them to be as far off as the astronomer supposes them to be; for, as Mr. Proctor himself says, "the further away they are, the less they will seem to shift." Why, in the name of common sense, should observers have to fix their telescopes on solid stone bases so that they should not move a hair's-breadth, - if the Earth on which they fix them move at the rate of nineteen miles in a second? Indeed, to believe that Mr. Proctor's mass of "six thousand million million million tons" is "rolling, surging, flying, darting on through space for ever" with a velocity compared with which a shot from a cannon is a "very slow coach," with such unerring accuracy that a telescope fixed on granite pillars in an observatory will not enable a lynx-eyed astronomer to detect a variation in its onward motion of the thousandth part of a hair's-breadth is to conceive a miracle compared with which all the miracles on record put together would sink into utter insignificance. Captain R. J. Morrison, the late compiler of "Zadkeil's Almanac;" says: "We declare that this "motion" is all mere 'bosh'; and that the arguments which uphold it are, when examined with an eye that seeks for TRUTH only, mere nonsense, and childish absurdity. "Since, then, these absurd theories are of no use to men in their senses, and since there is no necessity for anything of the kind in Zetetic philosophy, it is a "strong presumptive proof" - as Mr. Hind would say that the Zetetic philosophy is true, and, therefore, a proof that Earth is not a globe..
If you are a Christian and you believe the Bible is the True Word of God, then you need to set aside all previous beliefs and accept what the Bible says, not what the world and the media say. Remember, the Bible tells us that we have an enemy, called Satan. It says in John 8:44 about Satan, that, “He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it.” And also in Revelation 12:9 it describes what happened to Satan and what he is doing on earth: “And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.”
Astronomers are in the habit of considering two points on the Earth's surface, without, it seems, any limit as to the distance that lies between them, as being on a level, and the intervening section, even though it be an ocean, as a vast "hill"-of water!" The Atlantic ocean, in taking this view of the matter, would form a "hill of water" more than a hundred miles high! The idea is simply monstrous, and could only be entertained by scientists whose whole business is made up of materials of the same description: and it certainly requires no argument to deduce, from such "science" as this, a satisfactory proof that the Earth is not a globe.

19.) Every man in his senses goes the most reasonable way to work to do a thing. Now, astronomers (one after another – following a leader), while they are telling us that Earth is a globe, are cutting off the upper half of this suppositious globe in their books, and, in this way, forming the level surface on which they describe man as living and moving! Now, if the Earth were really a globe, this would be just the most unreasonable and suicidal mode of endeavoring to show it. So that, unless theoretical astronomers are all out of their senses together, it is, clearly, a, proof that the Earth is not a globe.
We know from the flood account that God opened the “windows of Heaven” in order to flood the whole earth with water and destroy the wicked. It is interesting that windows today are made of glass, and you can only open something that is closed and solid. God did not use all the water above though, because it says in Genesis 8:2 that the “the fountains also of the deep and the windows of heaven were stopped, and the rain from heaven was restrained.” God shut that windows and stopped the flow of water from above when there was finally enough water to cover the tallest mountain and the tallest tree. That means there is still water above the earth.
1) The horizon always appears completely flat 360 degrees to the observer, regardless of how high you go up. Any curvature you think you see is from curved airplane windows or Go Pro cameras and fisheye lenses (which NASA loves to use). The reality is that the horizon never curves because we are on an endless plane. On a globe with 25,000 miles in circumference you would see a noticeable disappearance of objects the further they are as they would be leaning away from you and dropping below the constantly curving horizon!
120) The etymology of the word “planet” actually comes from late Old English planete, from Old French planete (Modern French planète), from Latin planeta, from Greek planetes, from (asteres) planetai “wandering (stars),” from planasthai “to wander,” of unknown origin, possibly from PIE *pele “flat, to spread” or notion of “spread out.” And Plane (n) “flat surface,” c. 1600, from Latin planum “flat surface, plane, level, plain,” planus “flat, level, even, plain, clear.” They just added a “t” to our Earth plane and everyone bought it.
38.) When the Sun crosses the equator, in March, and begins to circle round the heavens in north latitude, the inhabitants of high northern latitudes see him slimming round their horizon and forming the break of their long day, in a horizontal course, not disappearing again for six months, as he rises higher and higher in the heavens whilst he makes his twenty-four hour circle until June, when he begins to descend and goes on until he disappears beyond the horizon in September. Thus, in the northern regions, they have that which the traveler calls the "midnight Sun," as he sees that luminary at a time when, in his more southern latitude, it is always midnight. If,
21.) Man's experience tells him that he is not constructed like the flies that can live said move upon the ceiling of a room with as much safety as on the floor: – and since the modern theory of a planetary earth necessitates a crowd of theories to keep company with it, and one of them is that men are really bound to. the earth by a force which fastens them to it "like needles round a spherical loadstone," a theory perfectly outrageous and opposed to all human experience, it follows that, unless we can trample upon common sense ane ignore the teachings of experience, we have an evident proof that the Earth is not a globe.