The best strategy is to focus on a handful of core beliefs, and address those in detail by showing undeniable proof that the evidence presented to support those beliefs is either wrong or is actually only explained by the round Earth. It’s the same strategy as with dealing with believers in Chemtrails, focus your efforts on the core beliefs and resist the temptation to refute all their arguments at once. Flat Earth believers are fundamentally opposed to any form of argument from authority, so you are going to have to show them.

Over the Horizon

Flat Earth claims have changed very little since the 1800s. Modern Flat Earthers are simply repeating claims that were both made and debunked well over a hundred years ago. For example, MMA instructor and YouTube Flat-Earth celebrity Eddie Bravo said in November 2017:

Bravo thinks that if a ship appears to vanish over the horizon, if you were to zoom in on the horizon then the ship would reappear. This is a foundational belief of Flat Earthers, and one of the first things that you will have to address. It dates back to Rowbotham in 1865:

Notice how Bravo’s description of the hull followed by the mast vanishing, then reappearing upon viewing with a zoom lens is very similar to Rowbotham’s. It’s likely because modern Flat Earthers are simply copying Rowbotham’s playbook. For example, Eric Dubay in The Flat Earth Conspiracy:

The fundamental problem with this claim is that it’s simply not true. YouTube videos that claim to show this effect invariably show ships that are simply too far away to observe details with the naked eye. Zooming in on the image makes them bigger and clearer, yes, but it does not restore any portion that was hidden by the horizon. This is something you can verify yourself with a sufficiently powerful zoom, and something that is a powerful practical demonstration for your friend.

Viewing a small boat can be a bit tricky—a more practical, and less error prone, observation can be made of what is behind the boat, the distant mountain. The ideal setup for this demonstration is a mountainous island or headland about thirty to forty miles away, and a viewpoint on a beach, and some nearby place higher than the beach. The coast of Southern California is ideal for this, in particular Catalina Island is visible from beaches accessible to tens of millions of people. The procedure is quite simple: you take a photo of Catalina from the beach, then from a point higher up (like twenty feet above sea level), then again from the highest nearby point, like the top of the cliff in Santa Monica (or ideally from the top of a tall building).

The results are plain to see. When viewed from a cliff above Santa Monica Beach, the two halves of Catalina appear nearly touching (the island is just eighty feet above sea level in the center, with a mountain on either side). When viewed from the beach, the bottom third of the island is now missing, with a huge gap between the two sides. The middle view from halfway up the road shows something in-between the other two. Since we lose sight of a few hundred feet of Catalina simply walking down to the beach, this means it’s behind something. The only thing between Santa Monica beach and Catalina is the ocean. So Catalina is behind the ocean, which means that the ocean, and hence the planet, is curved.

The great thing about looking at a mountain instead of a boat is that there’s no confusion about if the visibility has to do with it being small or over the horizon. It also avoids ambiguity surrounding whether a boat has moved positions due to waves, as mountains tend to be rather fixed in position and elevation. You can see Catalina just the same (weather permitting) with your naked eye; even a cell phone camera can take images that demonstrate this effect.

The challenge here, should you happen to have a Flat Earth friend, is getting them to actually look at the evidence. Hopefully the allure of a trip to the coast might be temping enough. Take a zoom camera, take some binoculars, go to the beach, and look at islands partly hidden by the curve of the Earth.

Where Is the Curve?

A common refrain of the Flat Earther is “where is the curve?” If we live on a ball, they say, should we not be able to see the horizon curving from left to right? The simple answer is “no,” as the ball we live on is very large compared to the amount we can actually see of it. If you fly up into space you’ll see the curve, but at very low altitudes it’s too gradual to see with the naked eye, and at higher altitudes it’s generally obscured by clouds and atmospheric haze.

However, you can see the curve of things that go over the horizon. Lance Caraccioli, who posts on YouTube under the name “Soundly,” has made it his mission over the last few years to collect clear and irrefutable images of this curve.

The Size of the Sun

In the real universe the Earth is a sphere and it orbits the Sun, which is also a sphere. The Sun is really far away and really large, so even though you might be viewing it from places on the Earth that are thousands of miles apart (say, London and New York) the Sun appears exactly the same size in the sky. You can verify this by taking a photo of the Sun at sunrise, noon, and sunset. Roughly speaking when the Sun is overhead for you it’s setting for someone else a quarter of the way around the globe to the east, and rising a quarter of the way round to the west. Noon in New York is sunset in Europe, and sunrise in Hawaii.

The problem for your Flat Earth friend, and the great opportunity for explanation, is that this would be impossible on a Flat Earth. Their idea of the world looks something like this:

In this model, the Earth is flat, and the Sun goes around in a circle above it. It uses some kind of “spotlight” effect to only illuminate the region below it, hence creating day, night, and time zones. The edge of the Earth is a ring of ice (the “ice wall”) and nobody knows if beyond that there’s empty space, an infinite plane of ice, or more flat worlds just a few thousand miles away.

This interpretation obviously has numerous problems. But let’s focus on two that should be the easiest to wrap your head around, and that are very easy to verify with observation.

Firstly, if you look at the Flat Earth model above, there’s nowhere on the surface of the Earth from which you can’t see the Sun. South Africa is shown in darkness, and yet if someone were to be standing there then there’s nothing between the Sun and that person. They should see the Sun, and it should still be quite high in the sky.

This is usually explained as some kind of strange function of perspective, however all perspective does is make far away things look smaller.

This brings us directly to the next point: the Sun does not vary in apparent size through the day. Consider someone standing below the Sun at noon on the equator, that means it’s 3,000 miles above them (estimates vary but make no significant difference to the problem). Now if someone is 4,000 miles away on the surface of this Flat Earth, towards the edge of the cone of sunlight, that makes the actual straight-line distance to the Sun 5,000 miles.6

This means that through the day the Sun (on a Flat Earth) is between 3,000 and 5,000 miles distant. A general law of perspective is that if something is twice as far away, then it appears to be half the size. You can verify this yourself by taking a photo of something like a car from ten paces away, then from twenty paces away. This law does not change as things get bigger and the distance increases. Try it with a house at one mile and two miles, or a mountain at ten miles and twenty miles.

But since the Sun (and the Moon for that matter) stay exactly the same apparent size all day long, then that means it’s not getting significantly closer or further away. Since it’s moving across the Earth then that means its distance must be many multiples of the size of the Earth itself—so it must be millions of miles away. Therefore the Earth cannot be flat.

This can be difficult to communicate. Most people are not really that comfortable with basic geometry. Try to keep it as simple as possible. It’s best done by pointing to the Flat Earth map. Show that the Sun would be closer at noon, and then far away at sunset. Get them to verify for themselves that it does not change size—I explain a few techniques on Metabunk.7 As always, a practical demonstration beats a verbal explanation.

Ground Truth of the Stars

Perhaps one small reason for the ease with which some people get sucked down the Flat Earth rabbit hole is the decline of popular astronomy because of light pollution. Ancient asronomers made detailed observations of the Sun, the Moon, the stars, and the planets thousands of years ago, and it was very apparent to anyone with a dark-enough night sky that the Earth appears to sit in the middle of a sphere of stars that rotates around it. This sphere (called the celestial sphere) appears to be very far away. The relative positions of the stars in the constellations do not noticeably change, even if you move thousands of miles away.

The motion of the Earth makes the celestial sphere of stars appear to rotate about the same axis as the Earth. You can see this in time-lapse photos if you point your camera to the North. The stars all trace out circles around a point close to the North Star, which we call the northern celestial pole. This observation works out just fine on the globe Earth, and you can even roughly shoehorn it into a Flat Earth model by imagining it as a dome of stars rotating over the Earth.

Where the Flat Earth model doesn’t hold to scrutiny is in the southern hemisphere. In the real world the southern hemisphere (Australia, South America, Southern Africa, Antarctica) behaves as a mirror image of the northern hemisphere. The stars appear to rotate in circles around the southern celestial pole.

But how would this work on a Flat Earth? If the stars were on a dome, then the stars in the South, when viewed from Australia, would not appear to rotate around a southern point, but instead would be whizzing by in nearly straight lines at very high speeds.

As always, in an ideal situation you would get your friend to observe this for himself. Taking a time-lapse of the stars circling the northern celestial pole is relatively easy. You can even do it with a phone app like NightCap which has modes to automatically record the paths of the stars. The northern celestial pole can be easily photographed from North America. The southern pole can still be detected, even though it’s well below the southern horizon, you can still point your camera south and see the star trails as they arch up and over the celestial pole—totally unlike what they would do if they were on a dome.

There’s an easier way than going out into the cold and fiddling in the dark with your camera. You can simply use a night sky simulator app to get the same view from the warmth of indoors. My favorite is Stellarium, which is both free and very comprehensive. It allows you to set up a view from anywhere on the Earth at any time and see what the sky looks like from there. You can also speed up time, so you can position the camera in Australia, look south to see the stars rotate around the southern celestial pole, and look north to see the trails arc up over the northern celestial pole.

The objection that will be raised here is, “Why would you trust a computer program over your own eyes?” The answer is that Stellarium has never been wrong. Millions of people use it all around the world. None of them have even reported that the image of the sky that Stellarium generates is any different to what they actually see.

This is a concept known as “ground truth”—the verification of distant observations or models by actual observations on the ground. Stellarium has a lot of ground truth, so you can use it with confidence that what you are seeing is what you would actually see if you were to go to that particular place and time yourself.

It’s not just Stellarium either. There are many other programs, and many easy phone apps like Pocket Universe that allow you to point your camera up at the sky and they tell you what they see (allowing you to form your own ground truth). These apps also allow you to change your time and location with a few taps.

Flat Earth Summary and Resources

Key Points to Convey to Your Friend

• The modern Flat Earth theory is only around 150 years old. We’ve known the Earth was round for over 2,000 years.

• Most Flat Earth points are just repeating what was written and debunked in the 1800s.

• You can go to the beach and see things like boats and islands vanish behind curve. Zooming in does not bring them back.

• If you find a long straight causeway or line of pylons you can see them curve up and over the horizon.

• The Sun does not visibly change size through the day. It also sets below the horizon. Both are impossible on a Flat Earth.

• The stars form a “celestial sphere” around the Earth that acts the same in Australia as it does in the US. On a Flat Earth it would be totally different.