when they inadvertently demonstrate the curvature of the earth is the insight into the psychology of flat-earthers. Like many other conspiracy believers and cult followers, flat-eartherism is a fundamental belief system to them and a community, so flat-earthers cannot allow anything to change their minds. Otherwise, they will lose their sense of identity and group belonging as well as their feeling of understanding and controlling the world around them. As reported in Newsweek,
“Say you lose faith in this thing. What then happens to my personal relationships? And what’s the benefit for me doing that? Will the mainstream people welcome me back? No, they couldn’t care less. But, have I now lost all of my friends in this community? Yes. So, suddenly, you’re doubly isolated,” psychologist Dr. Per Espen Stoknes says in the documentary. “It becomes a question of identity. Who am I in this world? And I can define myself through this struggle.” “If I tried to go…” [flat-earther Mark] Sargent says in the documentary, contemplating the scenario described by Dr. Stoknes. “They would come and say, ‘Don’t, don’t do it.’ So I couldn’t, even if I wanted to.”31
How Do We Know?
One thing we have learned from this widespread skepticism of science and established reality is that we scientists and educators need to do a better job of conveying both the facts of science and the evidence of those facts to people. We need to describe and demonstrate the evidence why we know certain things to be true. As Neil deGrasse Tyson wrote, “The fact that there’s a rise of Flat-Earthers is evidence of two things. One, we live in a country that protects free speech. And two, we live in a country with a failed educational system…. Our system needs to train you not only what to know, but how to think about information and knowledge and evidence. If we don’t have that kind of training, you’d run around believing anything.”32
So how do we know that the earth is roughly spherical in shape?33 How could you tell for yourself without engaging in dangerous stunts like launching yourself in a homemade rocket? To answer these questions, we will not use observations from satellites, spacecraft, or aircraft, because flat-earthers believe that these are all hoaxes and part of a giant conspiracy.
1. Watch ships at sea: Even before the Greeks wrote about the spherical earth, ancient seafarers knew that if you watch a ship sail away to the horizon, the bottom hull of the ship vanishes first, followed by the mast and then the top of the ship (fig. 2.3). If it is sailing toward you, you see the masts first, followed by the hull as it gets closer. This only makes sense if the ship is sailing around the curve of the earth. Flat-earthers also have heard of this evidence, of course, and claim it is an illusion caused by the perspective on different objects. But this is not how perspective works. If an object is far away on a flat surface, it will get smaller, but the lower part will not vanish as it recedes; instead, all of it will get smaller but remain fully in view. This is true even if you go to a harbor and follow the ship using a telescope or binoculars to improve your distance vision. The ship will vanish from bottom to top, not just become smaller.
Figure 2.3. Medieval drawing of a ship at sea disappearing bottom first on the horizon of a curved earth. (Public domain.)
2. Look to the stars: As the ancients noticed, the constellations look different as you travel north and south in latitude on the earth. About 350 BCE, Aristotle was one of the first to record this observation. Traveling from Greece to Egypt, he could see the difference in the skies. As he noted, “There are stars seen in Egypt which are not seen in northern regions.” He realized that the earth was small enough that its curvature was apparent over that relatively short distance, “for otherwise the effect of so slight a change of place would not be quickly apparent.”34 The difference became even more obvious when the first European explorers traveled south of the equator and found a whole new sky full of unfamiliar stars and constellations. The Southern Cross, for example, cannot be seen until you travel south of the Florida Keys, yet it becomes the major constellation of the sky when you are south of the equator. Meanwhile, the Big Dipper, which dominates the night sky above forty-one degrees north latitude, vanishes below the horizon as you head south, so at about twenty-five degrees south latitude in northern Australia, it is gone from the sky.
3. Watch a lunar eclipse: Every few years, we experience a lunar eclipse, where the disk of the full moon is covered by the shadow of the earth. It’s weird to watch the circle of the moon gradually get darker and darker as the edge of the earth’s shadow gradually covers it (fig. 2.4). As first discovered by the ancients and reported by Aristotle, the edge of earth’s shadow is unmistakably curved and becomes even more so as the eclipse approaches totality. Finally, the earth’s shadow covers the moon completely, so the only moonlight you see is from light that has passed around the curve of the earth and through our atmosphere (turning it red), refracting to the middle of the shadow.
Many times, the lunar eclipse is not total, but as the distance of the earth from the moon increases, it casts a slightly smaller shadow and the shining edges of the moon are visible on the edge of the shadow. This is called an annular eclipse, and it shows the entire shadow of the earth as a circle or ring of light around the dark shadow. This would never make sense if the earth were flat. Flat-earthers claim that the sunlight is blocked by the flat circular disk of the earth, but why then does the sun never happen to catch the flat disk of the earth on its edge or at an angle, so the shadow has a shape other than a circle? The only way this is possible is if eclipses happened only at midnight, when the “flat disk” is perpendicular to the sun-earth axis, so the “dark side” of the earth would only see the total eclipse of the moon when it was directly overhead at the stroke of midnight. In fact, lunar eclipses happen at all different times of day and night (although they are not very visible in the daytime).
Figure 2.4. A total lunar eclipse on June 15, 2011, as seen from Budapest, Hungary. The upper left frame shows totality, with the moon entirely covered by the earth’s shadow. Over the next hour, the earth’s shadow moves off to the lower right, and its distinctly curved edge can be seen, showing that the earth casts a curved shadow and therefore must be a spherical shape. By 23:10 Universal Time, the shadow has almost completely vanished and the full moon is visible. (Photo courtesy Wikimedia Commons.)
4. Go climb a mountain: If the earth were flat, you could see huge distances if you had a good enough telescope, so looking across the distance between Miami and New York City, only 1,000 miles or 1,760 kilometers, should be no problem. But if you are standing on level ground, even under the best of conditions with a superpowerful telescope, you can see no farther than about 3 miles (5 kilometers). Any object farther than that disappears below the horizon. Of course, if you climb a tree or even a mountain, you can see a bit farther on a day with excellent clear air and visibility. Standing on a hill 60 meters high, you can see about 50 kilometers. But even from the tallest mountains, no one can see much farther than about 60 miles or 100 kilometers, certainly not the distance from New York to Miami.
Take another example: Mauna Kea volcano on the Big Island of Hawaii is the highest peak in the Hawaiian Islands at 4,205 meters (13,796 feet). On a flat earth with nothing but ocean for many miles, you should be able to see enormous distances on a clear day. On the island of Kauai, only 487 kilometers (303 miles) away, is that island’s highest peak, Kawaikini, at 1,592 meters (5,226 feet). Over such a short distance on a flat earth, someone on Mauna Kea should easily be able to see the top of Kauai, but you can’t, because the earth is curved. Thanks to that curvature, the farthest you can see from Mauna Kea is 374 kilometers (233 miles).
5. Go fly in
