Humans knew Earth is a sphere since at least the 3rd century B.C., far before the first spaceflight and before NASA was founded. Flat Earthers have no choice but to slander NASA to defend the flat-Earth belief, but there is no need to rely on NASA to know that Earth is a sphere.
The Coriolis force is a force that acts on objects that are in motion within a rotating frame of reference. Because the Earth is a rotating sphere, an object traveling unattached to Earth’s surface is affected by the Coriolis force, depending on its speed and direction, as well as its latitude on Earth’s surface.
The rotating motion of the Earth causes the different parts of Earth’s surface to have different linear speeds, depending on its distance from the Earth’s rotational axis. An object moving from a location to another with a different linear speed will be affected by the Coriolis force because the motion of the object is now being observed from another location with a different linear speed/direction relative to Earth’s rotational axis.
Due to its almost +90° declination, Polaris practically cannot be seen from the south of the equator, and the fact is consistent only with the spherical Earth model. There is a prevalent myth in the flat-Earth community that claims Polaris is sometimes visible from the south of the Equator. The origin of the myth was an erroneous interpretation of a news article by flat-Earther Samuel Rowbotham, 1½ centuries ago.
The Sweden Solar System is the largest scale model of the Solar System in the world. We can use it to get a better sense of the sizes and distances in the Solar System.
Flat-Earthers like to claim that we never see a scale model of the Solar System, and use it to “prove” the failure of science to explain the world. The Sweden Solar System proves them wrong, and also gives us an idea about the problem of making a scale model of the Solar System.
In 2006, astronaut Tim Peake from the UK was tasked to perform some physics demonstration on the ISS for the National Space Academy. Tim performed the demonstrations in front of a gridded background specifically made for the purpose.
One of Tim’s video appeared during the former US president George H.W. Bush’s visit to NASA’s Mission Control Center. Flat-Earthers discovered the footage and quickly claim that it used a green screen background for CGI purposes. Such allegation was only from their ignorance and the eagerness to accuse others of lying. Tim’s videos are published in the National Science Academy’s web site as is, proving the background in the videos was merely an ordinary background, not a green screen for CGI.
Auckland, New Zealand and Madrid, Spain are a near antipodal pair. Both are located at the diametrically opposite points to each other on Earth’s surface. Therefore, we can fly from Auckland in any direction and arrive in Madrid after traveling a roughly similar distance, regardless of the direction taken.
Any flight between an antipodal pair has a similar duration even though the different flights have different directions. This fact is only consistent with spherical Earth and cannot possibly be explained if Earth is flat.
Radio propagation is the behavior of radio waves as they travel from one point to another. As a form of electromagnetic radiation, radio waves are affected by reflection, refraction, diffraction, absorption, polarization, and scattering.
Flat-Earthers assume all radio waves are identical. They invented various “explanations” with the erroneous assumption. In reality, radio waves with different frequencies have different properties and different ways of propagation.
There is a legendary diagram of spherical Earth from flat-Earthers that was made using AutoCAD involving the Pythagorean theorem, flaunting the fact that it is was created “using AutoCAD 2016 with 15-digit precision”™. The diagram is not wrong, but it does not account for the height of the observer and atmospheric refraction. And therefore, it is unsuitable for determining the obstruction of a distant object due to Earth’s curvature.
The numbers in the diagram will give us a much higher number than it should be. If a distant object is visible, but according to the diagram it should not, then flat-Earthers will incorrectly conclude that the Earth is flat.
A page on NASA’s website is at the center of the attention of flat-Earth followers. It is a page created by Fred ‘Mr. Eclipse’ Espenak, explaining the Saros cycle. The first paragraph from the page:
The periodicity and recurrence of eclipses is governed by the Saros cycle, a period of approximately 6,585.3 days (18 years 11 days 8 hours). It was known to the Chaldeans as a period when lunar eclipses seem to repeat themselves, but the cycle is applicable to solar eclipses as well.
After they discovered that the said ‘Chaldeans’ was a civilization from 25 centuries ago, they got excited and jumped to the conclusion that NASA used ancient technology to predict eclipses. But, like other similar cases, this tells us more about the flat-Earthers themselves than about NASA.
Appeal to ridicule is a fallacy that presents opponents’ arguments as absurd, ridiculous, or humorous, making the opponent and their arguments the object of a joke.
Flat Earth lacks a scientific basis, and flat-Earthers have no choice and will usually end up committing the appeal to ridicule at some point. On the other hand, real scientific arguments are done without ridiculing arguments from others.
Due to Earth’s curvature, a distant but still visible object is slightly leaning away from an observer. However, the angle is minuscule and undetectable to any observer.
After being shown photos showing the obstruction of a distant object due to Earth’s curvature, flat Earthers would often shift the topic by pointing out that the tilt due to Earth’s curvature is not apparent in those photos. In reality, the tilt is too small and not visible to our eyes.
Flat-Earthers discovered a map titled “Air Map of the World” from 1943, explaining the flight routes during the time. Because the shape is identical to the fictional “flat Earth map,” they incorrectly claimed that it is a flat-Earth map. In reality, it is an azimuthal equidistant map that has distortions, a flattened shape of the spherical Earth on a flat surface, and does not depict the true shape of the Earth.
The exact nature of the map is even explained in a very detailed manner in the map’s description. Unfortunately, in the flat-Earth community, the map is passed around in a very low resolution, and therefore, the explanations become unreadable.
Various pictures showing satellites and space debris around the Earth are visualizations. They are not real photos taken from cameras. These visualizations were created by drawing the objects with sizes far larger than their actual dimensions. If they were drawn to scale, then they all will be too small and cannot be seen in the visualizations.
Flat-Earthers fail to realize that these are visualizations. They think the pictures are presented as real photos. Because we never see satellites like that, they concluded that it was a deception. In reality, these pictures are simply visualization, not actual photos.
Polar areas have a lower temperature than areas closer to the equator because the same amount of solar radiation is dispersed over a larger area, and the surface of ice and snow reflects more sunlight than darker surfaces.
In the spherical Earth model, the distance to the Sun from the poles and the equator is practically the same because the sun is much farther than the distance between any two locations on Earth. Because of that reason, flat-Earthers then claim that the temperature in polar and equatorial areas should be the same. In reality, the distance to the sun is not the only factor that can determine temperature.
Showing a photo or video from the Apollo Moon landings, then rhetorically asking “Who took or recorded this image?” is a common behavior exhibited by many flat-Earthers, as if there is no plausible answer, and only a conspiracy theory can possibly answer it.
In reality, there are simple answers to those questions. And it is very easy for anyone to find the answers.
Offshore wind farms consist of a series of large turbines having a similar shape and size. In some cases, they are spread over a very large area of water. They are suitable landmarks where we can easily observe the effect of the curvature of the Earth.
If the turbines are far enough, the bottom parts will not be visible and the blades can look as if they are submerged. The phenomenon is only possible if Earth is a sphere.
The stolen concept fallacy is committed when someone requires the truth of something that they are at the same time trying to disprove. In other words, concept A is used to deny concept B, although A depends on B.
Many conspiracy theories are the results of a stolen concept, including many flat Earth claims. Usually, flat-Earthers would use a scientific concept to disprove science itself. On the other hand, the concept itself depends on science.
Red Bull Stratos was a high-altitude diving project involving Felix Baumgartner. In 2012, Felix Baumgartner flew 39 km (24 mi) into the stratosphere in a helium balloon before free falling in a pressure suit and then parachuting to Earth. Flat-Earthers invented various excuses to dismiss the resulting footage from the project.
Some people are confused that the crescent moon can be visible at night, even though during the time, the Moon is closer to the Sun than Earth, and the part of the Earth is facing away from the Sun.
Many flat-Earthers have the same visualization difficulties, but they take it a step further and use it as ‘evidence’ as a ‘failure’ of science to explain the fact. In reality, the phenomenon can be readily explained in the spherical Earth model.
In images taken using a fisheye lens, a straight line will remain straight if it crosses the center of the image. We can use this attribute to determine if a line is straight in reality.
In any fisheye videos taken from a high altitude, there should be plenty of moments where the horizon crosses the center of the image, and we can use those to determine that the horizon line is curved in the real world.