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.
It was created to address the many misconceptions a Round Earther may have about the Flat Earth Theory, and to act as an easily-accessible entry point into the mainstream Flat Earth model. This page is designed to answer some of the questions that many Round Earthers raise when they first arrive. Please check this page before making your first threads in the forums, as it may contain the answers to the questions on your mind.

77.) "Oh, but if the Earth is a plane, we could go to the edge and tumble over!" is a very common assertion. This is a conclusion that is formed too hastily, and facts overthrow it. The Earth certainly is, what man by his observation finds it to be, and what Mr. Proctor himself says it "seems" to be. flat – and we cannot cross the icy barrier which surrounds it. This is a complete answer to the objection, and, of course, a proof that Earth is not a globe.

80) In Chambers’ Journal, February 1895, a sailor near Mauritius in the Indian Ocean reported having seen a vessel which turned out to be an incredible 200 miles away! The incident caused much heated debate in nautical circles at the time, gaining further confirmation in Aden, Yemen where another witness reported seeing a missing Bombay steamer from 200 miles away. He correctly stated the precise appearance, location and direction of the steamer all later corroborated and confirmed correct by those onboard. Such sightings are absolutely inexplicable if the Earth were actually a ball 25,000 miles around, as ships 200 miles distant would have to fall approximately 5 miles below line of sight!
So the old folks were right after all! With all the proofs (don't forget the hard work and continuous effort invested in these by you) that you have given in this book really opened up a new perspective on how things work and how they are projected everyday. Really don't know why these so called "Elites" or whoever or whatever they are, trying to hide and concede the truth from people.The earth's shape, dinosaurs, evolution, the whole misconception about saturn (that it was a planet all along from the beginning, and it being associated with unwanted stuff..) are just to name a few.
147) The ball-Earth model claims the Sun is precisely 400 times larger than the Moon and 400 times further away from Earth making them “falsely” appear exactly the same size. Once again, the ball model asks us to accept as coincidence something that cannot be explained other than by natural design. The Sun and the Moon occupy the same amount of space in the sky and have been measured with sextants to be of equal size and equal distance, so claiming otherwise is against our eyes, experience, experiments and common sense.
28.) 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.
If the Earth were a globe, it would certainly have to be as large as it is said to be - twenty-five thousand miles in circumference. Now, the thing which I have called a "proof" of the Earth's roundness, and which is presented to children at school, is, that if we stand on the seashore we may see the ships, as they approach us, absolutely "coming up," and that, as we are able to see the highest parts of these ships first, it is because the lower parts are "behind the earth's curve." Now since if this were the case - that is, if the lower parts of these ships were behind a "hill of water" - the size of the Earth, indicated by such a curve as this, would be so small that it would only be big enough to hold the people of a parish, if they could get all round it, instead of the nations of the world, it follows that the idea is preposterous; that the appearance is due to another and to some reasonable cause; and that, instead of being a proof of the globular form of the Earth, it is a proof that at 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.

81.) Newtonian philosophers teach us that the Moon goes round: the Earth from west to east. But observation – man's most certain mode of gaining knowledge – shows us that the Moon never ceases to move in the opposite direction – from east to west. Since, then, we know that nothing can possibly move in two, opposite directions at the same time, it is a proof that the thing is a big blunder; and, in short, it is a proof that the Earth is not a globe.
Yes! the earth is flat, just ask the water it will never lie to you , it always seeks its own level, ALWAYS!! That is why we have the term sea level, airplane= air+plane, a thing that flies in the air over a plane. the list can go on and on. I have studied Ancient history for last 30 years read more than 500 books, One must be able to separate what is real and what is not real on many levels to understand truth as it is. remember sound=vibration=frequency on all levels of existence .
If we could - after our minds had once been opened to the light of Truth - conceive of a globular body on the surface of which human beings could exist, the power - no matter by what name it be called - that would hold them on would, then, necessarily, have to be so constraining and cogent that they could not live; the waters of the oceans would have to be as a solid mass, for motion would be impossible. But we not only exist, but live and move; and the water of the ocean skips and dances like a thing of life and beauty! This is a proof that the Earth is not a globe.
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.
141) The “Coriolis Effect” is often said to cause sinks and toilet bowls in the Northern Hemisphere to drain spinning in one direction while in the Southern Hemisphere causing them to spin the opposite way, thus providing proof of the spinning ball-Earth. Once again, however, just like Foucault’s Pendulums spinning either which way, sinks and toilets in the Northern and Southern hemispheres do not consistently spin in any one direction! Sinks and toilets in the very same household are often found to spin opposite directions, depending entirely upon the shape of the basin and the angle of the water’s entry, not the supposed rotation of the Earth.

From the second equation above, the index of refraction at one atmosphere of pressure and a temperature of 310 K (50 degrees F) is 1.000284, while the index of refraction at one atmosphere of pressure and a temperature of 320 K (68 degrees F) is 1.000275. These values yield a critical angle of 89.76 degrees. Hence, when air attempts to pass from 310 K to 320 K air at one atmosphere of pressure, the light will be totally internally reflected if the angle of incidence is greater than 89.76 degrees, or less than about a quarter of a degree from grazing incidence. If the temperature difference is greater, the critical angle will be less; hence the angle from grazing incidence will be greater.

130) From “Earth Not a Globe!” by Samuel Rowbotham, “Take two carefully-bored metallic tubes, not less than six feet in length, and place them one yard asunder, on the opposite sides of a wooden frame, or a solid block of wood or stone: so adjust them that their centres or axes of vision shall be perfectly parallel to each other. Now, direct them to the plane of some notable fixed star, a few seconds previous to its meridian time. Let an observer be stationed at each tube and the moment the star appears in the first tube let a loud knock or other signal be given, to be repeated by the observer at the second tube when he first sees the same star. A distinct period of time will elapse between the signals given. The signals will follow each other in very rapid succession, but still, the time between is sufficient to show that the same star is not visible at the same moment by two parallel lines of sight when only one yard asunder. A slight inclination of the second tube towards the first tube would be required for the star to be seen through both tubes at the same instant. Let the tubes remain in their position for six months; at the end of which time the same observation or experiment will produce the same results--the star will be visible at the same meridian time, without the slightest alteration being required in the direction of the tubes: from which it is concluded that if the earth had moved one single yard in an orbit through space, there would at least be observed the slight inclination of the tube which the difference in position of one yard had previously required. But as no such difference in the direction of the tube is required, the conclusion is unavoidable, that in six months a given meridian upon the earth's surface does not move a single yard, and therefore, that the earth has not the slightest degree of orbital motion."

25.) The surveyor's plans in relation to the laying of the first Atlantic Telegraph cable, show that in 1665 miles – from Valentia, Ireland, to St . John's, Newfoundland – the surface of the Atlantic Ocean is a LEVEL surface – not the astronomers' "level," either! The authoritative drawings, published at the time, are a standing evidence of the fact, and form a practical proof that Earth is not a globe.