There are few certainties in life: death, taxes, and the sun's rising in the east and setting in the west. Or does it rise on the left and set on the right?
Lately, as flat-Earth theorists have become more vocal, it seems as though people are more frequently asking questions of that very nature, and the movement is gaining ground. (Just this week, a flat-earth believer launched himself into space in a homemade rocket in an effort to disprove the round-Earth theory.) These questions go against every logical explanation of what we know to be true about our planet: mainly, that it is sphere-shaped and orbits the sun.
The rise in visibility of flat-Earth theories might be a product of what's rapidly becoming known as the "post-fact/post-truth era" in our society, in which an untruth repeated enough times becomes truth by groupthink. However, these theories existed long before the 2016 election cycle and have outlasted counterarguments from Aristotle, Ferdinand Magellan, NASA, and most rational-thinking humans.
Too often, the discourse about the shape of Earth becomes about proving negatives and centers on explaining that something isn't true rather than proving that it is. Indeed, the burden should be on flat-Earth theorists to explain clearly why their theories are correct and to use science to back those claims.
Fortunately, for every idea on why Earth might be flat, there is physical evidence that proves Earth is definitely globular. Here's a bunch of that evidence, and you don't even need to spend $1 million to launch yourself into space.
1. Watch a ship sail off to sea
Without being in the sky, it is impossible to see the curvature of the Earth. However, you can always see a demonstration of this if you visit a harbor or any place with a wide-open view of the water.
If you are able to watch a ship sail off to sea, watch its mast and flag as it fades off into the distance. You will notice that, in fact, it does not "fade off into the distance" at all; instead, you will see its mast and flag appear to slowly sink. The ship sailed beyond the point at which you would see it. Just to be sure, bring a pair of binoculars with you so that you can see even farther off into the distance.
It's as if you're watching it go over to the other side of a hill. This phenomenon can only be explained by a sphere-shaped planet.
2. Watch a lunar eclipse
Solar eclipses get all the attention, but if you are able to catch a glimpse of a lunar eclipse, you can see evidence that the Earth is, indeed, round. Here's how it works: Earth passes between the moon and sun, so that the sun projects Earth’s shadow onto the Moon in the night sky. You've probably seen a partial lunar eclipse without even noticing it; if the moon looks orange, that's a sign of a lunar eclipse. If you've ever seen a total lunar eclipse, you probably noticed that the shadow did not look like this.
A Lunar Eclipse flat-Earther’s have never seen.
A round shadow crossed over a round object. This does not sound like a thing that would happen if we were on a plane with all of the celestial bodies simply hovering overhead—or, perhaps more assinine, if the sun were orbiting Earth and not vice versa. The last total lunar eclipse took place on January 31, 2018, but it was not visible in most of the United States. Fear not, as you only have to wait a few months for one that will be visible in the Americas on July 27, 2018.
3. Climb a tree
Imagine a vast plane with but one tree smack in the middle. If the earth were flat, your vision would extend exactly as far while standing at the base of the tree as it would when at the top of the tree. However, the farther you climb, the farther your line of sight will extend to the horizon.
That's because parts of Earth that were concealed from view by its curvature are now revealed because your position has changed.
Back to the vast plane. The naked eye can see objects that are millions of miles away in space. Theoretically, with a clear line of sight on a clear night, one would also be able to see bright lights from far-away cities. That this is not possible is further evidence of a round, not flat, Earth.
4. Travel through, or even within, different time zones
According to a 2008 paper in Applied Optics by David K. Lynch, the curvature of the earth becomes somewhat visible at an elevation of 35,000 feet (with a >60° field of view) and more easily visible at an elevation of 50,000 feet. So if you're on the right commercial flight, you might be able to see the curvature of the earth with your own two eyes.
In the event that you're not high enough, though, you can still experience the curvature of the earth another way. For example, if you were to fly all the way around the world, you'd find that it would be nighttime in part of the world and daytime in another part. In that way, the existence of time zones itself is proof that the Earth is round.
Taken another way, you wouldn't even need to travel through different time zones. Time zones are wide enough that you will see the sun rising and/or setting later in the western part of a time zone than in the eastern part. According to the Farmers' Almanac, the sun will rise and set roughly four minutes later for every 70 miles you drive from east to west.
If you wanted to combine this experiment with the previous one, you could note how much more of Earth you can see when you begin your ascent into the air than you can while you are sitting on the tarmac waiting to take off.
5. Watch a sunset
Pick a nice spot from which you can watch a sunset (we'll call this point A). Ideally, you'd have a clear horizon in front of you, and behind you would be some sort of elevated point that you can quickly access (a hill, a building with at least two floors, or perhaps the aforementioned tree; we'll call this point B).
Watch the sunset from point A, and once the sun is out of sight, hurry on over to point B. With the added elevation provided by point B, you should be able to see the sun above the horizon. If Earth were flat, the sun would not be visible at any elevation once it had set. Because Earth is round, the sun will come back into your line of sight.
If you don't have a hill, you could even try lying on your stomach to watch the sunset and then standing up to get a higher line of sight.
6. Measure shadows across the country
Pick two locations that are some distance apart (at least a couple hundred miles from each other and on the same meridian). Grab two sticks or dowels (or other objects) of equal length, two tape measures, and a friend. Each of you will take one stick/dowel/object and one tape measure to your location, stick the object into the ground, and measure the shadow. (For accuracy, you should both take your measurements at the same time of day.)
On a flat Earth, the shadow that is cast by each would be of the same length. However, if you and your friend compare notes, you'll find that one shadow was longer than the other. That's because, due to the curvature of Earth, the sun will hit one part of Earth at one angle and another part of Earth at a different angle even at the same time of day.
This experiment has been around since about 240 B.C., when Greek mathematician Eratosthenes compared the shadows cast in both Syene—now Aswan, Egypt—and Alexandria on the summer solstice. Eratosthenes had learned of a well in Syene where once a year on the summer solstice, the sun would illuminate the entire bottom of the well and tall buildings and other objects would not cast a shadow. However, he noticed that shadows were being cast on the summer solstice in Alexandria, so he measured the angle of the shadow and found it to be an angle of about 7.2°.
7. Google "International Space Station photos"
Seriously, just look at some of the amazing photos you’ll find.