## Equipotential: a Property of the Surface of Water

Gravity causes water to seek the lowest potential, which can be simplified as the position as close as possible to the Earth’s center of gravity. As a result, the surface of the water will have equipotential surface and form an approximately spherical surface which has the same center as the Earth. Every location along the surface has the same potential.

## Atmospheric Refraction

Light waves are not always moving in a straight line. When it passes through a medium of a different refractive index, the waves will deviate. The phenomenon is called refraction and described according to Snell’s Law.

Earth’s atmosphere has variation in air density that depends on the altitude. As the refractive index changes with the density of the medium, light waves passing through Earth’s atmosphere also experience refraction.

## Zooming In On Distant Ships Does Not Disprove Earth’s Curvature

If we can’t see a distant ship, then it is because of one of these reasons:

• Our eyes have limited angular resolution and are unable to resolve the ship at that distance.
• The atmospheric condition is limiting our visibility.
• The curvature of the Earth obscures the ship.

Flat-Earthers are keen to demonstrate that a previously invisible ship at a distance can be made visible by zooming in. They take this fact as ‘proof’ that the curvature of the Earth doesn’t exist. They are wrong. The curvature of the Earth is not the only reason a distant object is not visible.

## Bedford Level Experiment Confirmed the Curvature of the Earth

In 1870, Alfred Russell Wallace successfully demonstrated the curvature of the Earth to answer the challenge put forward by a flat-Earther, John Hampden.

## Long-Range Wi-Fi and the Curvature of the Earth

Wi-Fi is one of the most common ways of getting connected to the Internet. Practically all short-range wireless network use WiFi. Using specialized equipment, Wi-Fi is usable over a very long distance, even over hundreds of miles.

When designing a long-range Wi-Fi link, we must account for the curvature of the Earth.

## Archimedes and the Surface of any Fluid

Archimedes is a scientist from 3rd century BC. He is best known for the Archimedes’ Principle which explains buoyancy. He also contributed to many scientific discoveries and inventions.

But did you know that Archimedes explained the Archimedes’ principle using the spherical Earth model?

## Difference in Entrance Pupil Diameter: The Real Reason Why An Obscured Object Appears When Zoomed In

Ever seen a video where a flat-Earther tries to demonstrate that a distant object that appears to be behind an obstacle can be brought back into view simply by zooming in? He would then conclude the same thing would happen when zooming in to a distant ship, and the reason a distant ship is not visible is not the curvature of the Earth.

Well, no. The real reason why the object in that experiment can be seen after zooming in is that the obstacle is close, and there’s difference in the camera’s entrance pupil size when the zoom changes. For a camera that has very long zoom range —like the Nikon P900—, the difference can be very dramatic.

## Lake Pontchartrain Power Transmission Lines: Evidence of Earth’s Curvature

Lake Pontchartrain is a lake in Louisiana, United States. There are power transmission lines about 24.27 km (15 mi) across this lake. They are practically straight, and each tower is uniform and has the same height, making these towers ideal for observing the curvature of the Earth.

They were first popularized by Soundly who presented evidence of Earth’s curvature using a series of photos and videos of the power lines in June 2017. As of now, the towers and other objects on the lake are probably the most popular Earth curvature tourism spot in the world.

## Angle of a Building and The Curvature of the Earth

Some flat-Earthers claim that if the surface of the Earth is curved, then a building far away from us should look slanted away from us. But we never actually see this, so they conclude the Earth is flat.

Their deduction is somewhat correct, but let’s see if it is realistic to observe the slant of a building that is caused by the curvature of the Earth.

## Surface of Water and Communicating Vessels

Gravity is not the result of a particular shape of an object. It is caused by mass. All objects with mass exert gravity to each other. The larger the mass of an object, the greater the force of gravity.

On Earth, water forms a spherical surface which has the same center as the Earth. It applies to every surface of fluid which is under influence by Earth’s gravity, unaffected by another force. In a 10 cm wide container, the amount of curvature is only 0.0000002 mm. It is the reason the surface appears flat in small containers.

## The Verrazano-Narrows Bridge and The Curvature of The Earth

Accounting for the curvature of the Earth is not usually needed for narrow high-rise building projects. Designers only need to ensure the foundation is flat, and the curvature of the Earth becomes non-factor.

For projects that extend over a long distance, like roads, railroads, canals, etc., they are built along the curvature of the Earth, and specifically accounting for the curvature is usually not needed.

But when the project extends on a long distance, as well as extending upwards, then we have no choice but to take the curvature of the Earth into account. One of such projects is the Verrazano-Narrows Bridge, New York, United States.

## The Little Piggy Balloon Footage Proves Earth is a Sphere, not Flat

Little Piggy is the nickname of a helium balloon launched by IndianaCaver. It reached the altitude of almost 37 km before it finally ruptured and fell to the Earth. Before that, it took several hours of video, and IndianaCaver made these video available on YouTube.

Then, some flat-Earthers cherry-picked bits of the video where the horizon looked flat and presented it as ‘proof’ of flat-Earth. These are widely circulated in various memes and forms, even years after the originals were published.

But, can we use these videos as ‘proof’ of flat-Earth?

## Fisheye Lenses and Earth Curvature

A good majority of pictures of Earth taken from space are being dismissed by flat-earthers merely because they are made using fish-eye lenses. They believe any curvature of Earth shown in those pictures is the result of the fisheye effect, and thus, cannot be used as evidence of Earth’s curvature. They even take a step further and assume a fish-eye lens is deliberately used to take every picture of the Earth taken from a high altitude so that they would have a curvature in it.

These claims are, of course, ungrounded. In some cases, we can conclude the curvature is there, even when a fish-eye lens is being used. We can use this useful property of any photographic lens:

A straight line will appear straight as long as it crosses the center point of the image.

## So, Where’s the Curvature?

A human eye can only visually perceive the curvature of the Earth if we are at a considerable altitude from the surface, which is still beyond the reach of most humans today. A commercial jet airliner is the highest position realistically attainable by most humans today. At such altitude, we can only perceive very slight curvature, and only in an ideal condition.

The basis of flat-Earthers’ belief is that the horizon appears flat. They would say if we cannot see the curvature, then there’s no curvature, and thus, the Earth is flat. They are wrong. Most of us these days cannot travel high enough to see the curve.  The best we can do these days is going on board a commercial jet airliner, which can only go about 11-15 km up, only a fraction of the radius of the Earth.