how did flat-earth believers explain the horizon?

Bugs Bunny’s take:

If the earth were a flat plane, there would still be a horizon–a clearly defined angle at which there is sky above and ground below. You can have a horizon at a finite distance because of refractive effects–the thicker air at lower altitudes will bend the light rays downward.

Many of the true believer Flat Earthers base their belief in The Bible which mentions the land being flat, having four corners as well as ensuing notions such as the Sun circles the Earth. See the shape of the Earth section here.

Once you adopt a divinely mandated point of view, actual physics doesn’t matter.

  1. A number of years ago, I had a student who corresponded with a president of a flat Earth society. His letters, etc. were absolutely sincere and I had no doubt that he believed it. The flat Earthers may be rare, but they do exist.

  2. I’ve often discussed with students, from elementary through graduate school, the difficulty of proving, from normal every day observations, that the earth is not flat. There are almost no direct observations to be made that will convince you - aside from watching a number of lunar eclipses over many many years, with close attention paid to the rest of the sky, time of year, etc. It’s an astronomer’s work, basically, not your every day observer. Just about every observation you can mention that may support a round earth can be countered by alternative explanations.

Isn’t Eratosthenes’ original method for measuring the Earth’s circumference a reasonable approach for a non-astronomer?

And much easier with modern chronometers… hey, with modern communications you could even co-ordinate your measurements in real-time with your accomplice in the other location… 'course you may have to bounce your coms signals off a satellite to get around that pesky curved earth between you. :slight_smile:

You mean merely stand on the tropic of cancer and sight the sun and then check with someone else standing north of there at the same time? Uh, not for most people. Of course it can be done today, but even doing it would be challenging. My point is that for most people, the Earth IS flat, it looks flat, it would be difficult to find observations that indicate other wise, and it’s even logical to think that it’s flat. After all, how could water, which lies flat in a bowl or pond (even though we now know it’s actually curved) curve around the world? That’s just not reasonable. The world must be flat.

Why wouldn’t stars rise and fall over a flat earth?

Well, given that the measurement Eratosthenes made was inaccurate for a number of reasons:

I think we could deal with the same level of inaccuracy, so, how about on noon of the solstice I stand in Houston, TX (somewhat north of the Tropic of Cancer) and sight the sun, while phoning my friend in Kansas City, KS (about 1000 km not quite due north of Houston) who sights there? I reckon that’s about a High School level experiment (with a bit of planning).

Agreed. In the same way that Newtonian physics are a perfectly decent description for most purposes, and certainly for day-to-day use.

Not really. Just reading over the Flat Earth FAQ itself is pretty much enough to convince me of a round earth; the number of additional fixes and fudges that are needed to make the system work suggest to me that Occam’s razor isn’t well thought of there.

Umm… wouldn’t that have the opposite effect you seem to be imagining? If you stood on an infinite flat plane, and light curved downward, that plane would appear to be a hollow sphere with you on the inside surface.

Eratosthenes didn’t measure shadows in a well to figure out that the Earth is a sphere. That was already well established. He measure shadows in a well to figure out the circumfernece of that sphere.

And that is something that you can’t see.

Christians have gotten many things wrong about science, but the shape of the Earth was never among them. As far back as the 13th century, Saint Thomas Aquinas wrote:

“To the second argument it must be said that there are diverse sciences because things can be known in various ways. For example, the astronomer and the natural philosopher both demonstrate the same conclusion, such as that the world is round; yet the astronomer does so through mathematics, while the natural philosopher does so in a way that takes matter into account. Thus there is no reason why those things treated by the philosophical disciplines through natural reason should not also be treated by another science insofar as they are known by the light of divine revelation.”

The Church never taught that the Earth is flat.

Current flat earthers claim that:

when you see the horizon curved from a plane, it’s a movie being projected on the windows. Or, if you’re parachuting, on your glasses (eat your heart out, virtual reality researchers!).

when you see it from a ship, or just in a very large flatland, it’s an optical illusion.

and if you’ve flown around the world? well, you haven’t. The plane has followed a path close to the edge, but not so close that you’d be able to see the edge, plus anyway there’s the whole movies to be taken into consideration.

Sorry, when I ran into people like that, YouTube didn’t exist yet…

The flat earthers are absolutely, 100% correct on these points. A curved horizon, as viewed from an airplane or ship or farmland, is not evidence of a round earth.

The horizon on a flat earth would be a circle, because the locus of all points equidistant from an observer is a circle. How many times do I have to say this?

These comments puzzle me. I’ve spent a fair amount of time at sea, and the effect always seems extremely obvious. You need decent vision (or binocs), but the observed object can be any ship or boat of normal size. You always see its upper parts (e.g. mast tops) first, then its hull comes into view at a substantially smaller distance.

For an even simpler one, compare the times of local apparent noon (i.e. the time when the sun is highest in the sky). They are obviously different at locations that are east or west of each other - but a flat earth requires that they be the same.

When viewed from a high altitude, the horizon of a flat earth would not curve as it does with a spherical earth.

I believe that there are two uses of “curved horizon” in conflict here. One, that indicates that it curves from in front of us around to behind us - on a horizontal plane, (**Freddy the Pig’**s meaning), and that of **Fear Itself **and others, who are referring to a sense that the horizon, itself, is curved, up and down in a vertical plane. If the world were a flat disc, the horizon would still curve around us, but not up and down. Maybe, if the world were a flat rectangle, the same would happen, if it were large enough. If the world were a sphere, it would do both, particularly if the sphere were small enough. I presume that someone standing on a tiny moon of one of our planets would perceive both types of curvature.

No, it has exactly the effect I’m imagining–a finitely distant horizon. That is an explanation for why we see the tops of ships before the bottoms of ships when they’re at a distance.

Now, you are correct that the same refraction will cause a bowl-like optical effect. Measurements looking for that could be used to disprove that refraction is not significant. And thus the explanation is wrong.

But it’s still an explanation that flat earthers could use.

But the observed horizon doesn’t exhibit any curve in a vertical plane - all the way around you it stays, er, horizontal (well, at least at sea it does - on land, distant terrain may disturb this a bit).

Or, to demonstrate the sphericity of the Earth:

Have a bunch of high school kids all over the planet. They join a conference call. Each one gives an estimate of the position of the sun. For some kids the sun will be rising, for some it will be morning, for some it will be noonish, for some it will be evening, for some the sun will be setting, and for some it will be night.

If the Earth was flat, that would be impossible, all parts of the flat Earth would see the sun in the sky at the same time.

Bonus points if you have one kid above the arctic circle and one below the antarctic circle on the summer solstice.