Most people who were unfortunate enough to receive a public education think that ancient people believed the earth was flat. Indeed, I thought that myself until I started studying the history of science. If you do the same, you will learn that the earth was known to be spherical even in ancient times. In fact, the shape of the earth was so obvious to ancient Greek scholars that when Archimedes mathematically derived his Law of Buoyancy in roughly 250 BC, this was his second postulate:
The surface of any fluid at rest is the surface of a sphere whose centre is the same as that of the earth.
In other words, bodies of water are spherical because the earth is spherical. This was necessary for him to demonstrate what causes objects to float or sink.
Because the spherical shape of the earth was so well-known among ancient Greek scholars, Eratosthenes decided to measure the distance around the sphere in roughly 240 BC. He heard that there was a city called Syene in Egypt in which the sun was directly overhead at midday on the summer solstice. He traveled to Syene to make sure this was correct. On that date, he waited for the sun to reach its highest point in the sky and then looked down a deep well. He saw that its sides cast no shadow onto the water at the bottom of the well. That told him the sun was directly overhead. If there had been a pole sticking straight out of the ground, it would not have cast a shadow. The next year, he traveled to a city called Alexandria. When the sun reached its highest point in the sky on the same date, he measured the length of the shadow of a pole that was sticking straight up out of the ground. With the height of the pole, the length of its shadow, and the distance between Syene and Alexandria, he determined the circumference of the earth.
This brings me to the topic of this post. A YouTube personality, Scimandan, decided to get his listeners help him do a modern version of the experiment. Essentially, he told them to wait until the sun was highest in the sky on the Northern Hemisphere’s summer solstice and do what Eratosthenes did – measure the length of the shadow cast by a vertical object. The image above shows me doing this with a level. I sent him my latitude and longitude, the height of the level, and the length of its shadow, as did 1,013 others from all over the world (both hemispheres). He compiled the results and shows them in this video.
He displays three graphs that clearly show the earth is a globe. First, he uses trigonometry to calculate the sun’s angle of elevation for each person’s result and then graphs that based on the person’s latitude. The black dot shows where my data point is on the graph:
He then shows what the results should look like for both a spherical earth and a flat earth:
As you can see, aside from a few outliers, the data correspond to what you expect for a spherical earth, and not what you expect for a flat earth. More importantly, he calculates the ratio of the shadow length to the object’s height for all the results, and graphs them according to latitude. Once again, the black dot shows where my data point is on the graph.
The green curve is what you expect for a spherical earth, while a flat earth should show a linear relationship. Obviously, the data are only consistent with a spherical earth.
Near the end of the video, he chooses three data points that occur roughly along the same longitude. He then uses those points to do what Eratosthenes did: calculate the distance around the sphere. Those three data points give a value that is within 4% of the known circumference of the earth.
As this crowd-sourced experiment shows, you don’t have to rely on modern technology or photos from space to understand the shape of the earth. Ancient Greek scholars could figure out that it was spherical because careful observations on earth demonstrate it.
This input is just intended for you as food for thought. I love the science books you have authored. I recommend them to others. I purchase your options whenever I can. I am a bit bothered by a comment in your latest Proglogion. I hope you will ponder this comment I have to make. I am a public school educated Christian teacher. All four of our children went through the public system. They are successful and happy career and family people who lead their families with a strong faith-centered approach. I also teach home school kids on occasion now that I am retired. I encourage families to do whatever education plan works for them. I never say critical comments on those who choose home school. In fact, I help them, as a volunteer, whenever I can. I know those families who choose public schools for their children’s education may face extra challenges. But for them to be leaders and shine for Jesus in that setting is admirable. Our grandchildren do that. Others I know do that. For that reason, I find your recent opening statement of “Most people who were unfortunate enough to receive a public education think that ancient people believed the earth was flat” to feel very offensive and as if you have mass grouped all of us in a negative way. As I taught school, I was delighted to be able to be an example of a Christian teacher just by the way I lived my life and taught students in my classroom using the golden rule as a guide our interactions together. I know I was able to make a difference to many over the years. It wasn’t the easiest route to take to go out among the troubled and those needing lots of guidance, but I was pleased to have that opportunity. Please continue your wonderful authoring of science materials, and please try to not be critical with mass judgements when Christians choose do their work among those who need Jesus the most. It would have been easy to get your point across without the words “unfortunate enough to receive a public education.” Otherwise, the article was interesting. Thank you.
I appreciate your openness and honesty. To be just as honest, however, I really do think that anyone who gets a public education here in the U.S. is unfortunate, since there are significantly better options available. Now, that doesn’t mean I dislike public schools. I actually support them. I have friends who are public school teachers, and each year, they post their list of classroom needs on Amazon. I often purchase things for them from that list. I am also happy to answer questions from public school teachers. In fact, I taught elementary educations students, most of whom ended up teaching in public schools (see here and here). Nevertheless, I can say without hesitation that there is a better choice for pretty much every student in public school. Sometimes, that choice is a private school. More often than not, that choice is homeschooling. Since students in public school are being taught by the least effective educational model available, they are, in my opinion, unfortunate.
In regards to the subject of this post, they are particularly unfortunate. Most private school curricula and home school curricula do not teach the myth that ancient people thought the earth was flat. That myth is found predominantly in public school curricula.
Thanks for your response. I have traveled all over the nation doing presentations at over 40 National Science Teacher conventions. I can attest to the fact that the educational system is different in all parts of the country….some better, some worse. And none perfect. And teachers often have a bigger case load of students than is optimal. Some students will thrive no matter where you put them…others struggle when things cannot be adapted enough. As a teacher though, I have felt so much positive support from my students and their parents (Christian or not)…and then criticism has always come (in a general way, like yours, not specifically to me) from the Christian community. Honestly, that hurts since I picture myself going out in a “mission field” of sorts and making a difference. So that is my wish that Christians would do what they need to and want to do in educating their family–I’m not critical of their choices, but please DO NOT be critical to those of us trying to make a difference in the public sector. Not everyone has the ability or finances to teach their children. We teach everyone in public education and we care about them all. Thank you for your part in helping all people trying to do science education also.
Very interesting! I always love it when you post, I’ve learned so much from you over the years 🙂
Thanks! I am hopefully going to start posting more often.