To an observer on the ground, an airplane at 36000 ft will start going over Earth’s curvature at a distance of 400 km. However, we are also limited by atmospheric visibility. In an extremely best-case scenario, we can only see up to 240 km.
We can easily see ships going over Earth’s curvature, and flat-Earthers like to demand us the same thing for airplanes. In reality, at the typical distance a plane does that, it is far out of visibility range. But if it is very low, like when approaching a runway to land, it is possible to see the plane going over the curvature of the Earth.
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With careful planning, we can easily observe Earth’s curvature from an airliner. To “explain” the observation, flat-Earthers invented the excuse that it is caused by the distortion from the shape of the airplane’s window.
In reality, all camera lenses are curved, too, yet most of these lenses produce normal, rectilinear images. The fact that the shape of the window is curved does not mean looking through the window will produce a curved horizon.
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A frame of reference consists of an abstract coordinate system and physical reference points that uniquely fix the coordinate system and standardize measurements. Speed or velocity is relative to a specific frame of reference. It is possible that two speeds/velocities are measured relative to a different frame of reference and thus cannot be directly compared.
Flat-Earthers like to compare two different speed measurements like an airplane moving at 900 km/h (560 mph) and Earth’s surface near the equator at 1674 km/h (1040 mph). They would conclude the airplane should not be able to catch up with Earth’s surface and would not land if the Earth is rotating. In reality, both the speeds are measured relative to a different frame of reference, and therefore, cannot be compared directly.
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Airplanes do correct for the Coriolis effect. They constantly make adjustments to counter forces that try to push them away from the planned route, including the Coriolis force.
Flat-Earthers claim the Coriolis effect does not appear to affect airplanes, and they use it to “prove” that Earth is stationary. In reality, the Coriolis effect does affect airplanes, and we do have to correct for it. But the effect is tiny, and we can safely ignore it in flight planning.
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An airplane exists on Earth. It has the same motion as the surface of the Earth, including Earth’s rotating motion. The motion and speed of an airplane are measured relative to Earth’s surface.
Flat-Earthers claim that an airplane should not be able to fly eastward or land. In reality, an airplane is moving relative to the surface. It has Earth’s motion as a component of its motion and does not need to chase Earth’s rotation.
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Pilots are taught to work with the fact that Earth is a sphere. It is part of the education to become a pilot. Their job depends on the knowledge that Earth is a sphere.
Flat-Earthers wrongly claim that pilots are taught to fly based on a flat Earth. We can easily disprove these claims by reading some of the books these pilots use in their education.
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During cruising, an airplane is practically level. It is flying perpendicular to the direction of gravity. The aircraft is still flying following Earth’s curvature and continuously changes its orientation. However, not only its orientation changing but also the direction of gravity, having the same amount of change. As a result, a spirit level on a flight will not show us the difference in orientation of the airplane due to its motion following the curvature of the Earth.
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The cruising altitude of a jet airliner is about 33000-42000 ft or 10000-12800 m. It is very tiny compared to the radius of the Earth of 6378 km. Therefore, the distance measured at the cruising altitude does not differ much compared to the surface distance.
Some flat-Earthers claim that if Earth is a sphere, the distance traveled by a jet airliner would have been multiple times the surface distance. They are wrong. The air distance at 33000 ft is only 0.16% farther compared to the surface distance.
On many flights, they would give away leveling devices to all passengers for free, so that everyone would be able to observe the dip of the horizon and proves Earth is a sphere. And as a nice side effect, to quench our thirsts, too!
Such devices are called ‘bottled water’. Using these simple ‘instruments’ on a flight, we can demonstrate that Earth is a sphere.
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