## Down Direction of a Globe

A globe is a scaled-down model of the spherical Earth. It can be used to explain the down direction on the real Earth, which is toward the center of the globe. Flat-Earthers like to confuse the down direction explained by the model with that of the real Earth. In reality, the down direction we experience is different from that explained by the globe, which is only a model of the actual Earth.

## Weighing Scale vs Weighing Balance

A weighing scale measures mass by measuring the gravitational force exerted by the object we measure. Because the magnitude of the force depends on the strength of the gravitational acceleration, the result can vary depending on the location. It is the reason the scale needs to be calibrated after it is moved to another location.

On the other hand, a balance measures mass by comparing the object’s mass to a previously known mass. Because both masses are affected by the same gravitational acceleration, a balance is not affected by the change in gravitational acceleration.

## Ancient Greeks and Spherical Earth

Ancient Greeks have known spherical Earth since at least the 3rd century B.C. According to historian Otto E. Neugebauer, the Greeks discovered Earth is a sphere from the drastic variation in the stars’ positions & motions between Greek settlements of different latitudes.

## Zero-G Aircraft and Duration of Weightlessness

A zero-gravity aircraft flies in a parabolic motion following a free-fall motion to provide a brief weightless environment.

Flat-Earthers claim videos of astronauts in space were faked in zero-gravity aircraft. In reality, a zero-gravity plane can only provide ~25 seconds of continuous weightlessness and cannot be used to film long-take videos in a weightless condition.

## Scientific Theory is not “Only a Theory”

The word “theory” has multiple meanings and can be ambiguous. Its meaning in a scientific context can be different from that in the context of everyday communication. Flat-Earthers erroneously assume we call it a “scientific theory” because it is still unproven. In reality, in the scientific context, a theory is commonly regarded as correct, & we call an unproven supposition a “hypothesis.”

## Moonlight and Inverse Square Law

Light intensity is inversely proportional to the square of its distance. The light we receive from the Moon increases 50000× if we land there. But so does the Moon’s apparent size, spreading the increase in intensity over a larger area.

Flat-Earthers incorrectly claim the inverse square law means the Moon would be too bright if we land there, and they use the argument to “prove” it is impossible to go to the Moon. In reality, the inverse square law applies to a point source. For a large light source, its change in apparent size must also be considered.

## Different Reasons Why a Distant Object is Not Visible

Different reasons can cause a distant object to be not visible:

1. The angular resolution limit of the observer.
2. The visibility limit imposed by the atmosphere.
3. Obstruction by another object, including by Earth’s curvature.

Flat-Earthers incorrectly presumed “a distant ship is not visible only because of Earth’s curvature.” Incorrectly concluded if we can bring the ship back into view, the curve must not exist. In reality, Earth’s curvature is not the only thing that can cause a distant ship to be not visible; other reasons can also cause it.

## Demonstration Using a Glass of Water at the Tiangong Space Station

Chinese astronauts performed science demonstrations at the Tiangong Space Station. One of these demonstrations involves a glass of water and a ping-pong ball. Flat-Earthers saw a photo of the glass of water from the set, and they used it to discredit spaceflight. They claim that if it is really in space, then the water and glass should float.

In reality, the glass was attached to the table, and the water stayed in the glass because of adhesion between the glass and water, not gravity. The surface of the water also bulges and is not level with the glass or the table, as expected in a microgravity environment.

The astronauts used the set to demonstrate buoyancy in space. They pushed a ping-pong ball into the water, and it did not float like on Earth. At the end of the demonstration, they detached the glass from the table and floated it with the water & the ping-pong ball still inside it, and everything behaved as it should in a microgravity environment.

A flame can produce a shadow if the other light source is much brighter, then refracted by the temperature difference, or the flame produces soot, steam, or other combustion products.

Flat-Earthers discovered images of flames not casting a shadow, & when they saw in a photo that a rocket’s exhaust casted a shadow, they used it as “evidence” it was faked. In reality, a flame can produce a shadow under the right conditions. Rockets have opaque combustion products, & in daylight, are lit by the very bright sun.

## Sundial

A sundial tells the time of day from the position of the shadow of its gnomon as cast by the sun. The gnomon is usually raised parallel to Earth’s axis of rotation, so its shadow will always fall to the same line despite the sun’s annual apparent motion.

Aligning the gnomon to the Earth’s axis of rotation allows the sundial to be accurate throughout the year. All of this happens because Earth is a rotating sphere orbiting the sun with a tilted axis.

Flat-Earthers incorrectly claim that sundial works because Earth is flat. In reality, sundials are designed with the knowledge Earth is a sphere. Real-world sundials cannot possibly work if Earth were flat. If Earth were flat, we would have constructed sundials in a very different design.

## Gravitational Acceleration vs Force of Gravity

Earth pulls all objects downward by the same gravitational acceleration of 9.8 m/s². But the force of gravity exerted by Earth on an object still depends on its mass. An object with a greater mass has a greater force of gravity (also called weight).

Flat-Earthers claim that if greater mass means a greater force of gravity, a bowling ball in a vacuum should fall faster than a feather. In reality, they confuse acceleration with force. Both fall at the same speed because the gravitational acceleration is the same on both, but their forces of gravity are different.

## Free Fall

Objects fall down because of gravity. Flat-Earthers deny gravity & wrongly claim they fall down because of density. For a demonstration, we can ask any flat-Earthers the following simple physics problem:

“A rigid object is released at the height h above the ground, what is the object’s velocity right before it hits the ground?”

Flat-Earthers will not be able to solve this free-fall problem without using the gravitational acceleration g=9.8 m/s². Some will call it by a different name, showing that it is just a “branding” problem. Practically all the time, they will not involve density to solve it, if they can solve it at all.

## Archimedes’ Principle and Gravity

Archimedes’ principle states that any object, totally or partially immersed in a fluid, is buoyed up by force equal to the weight of the fluid displaced by the object. Meanwhile, Newton’s law of universal gravitation states that every particle attracts each other with force directly proportional to their masses & inversely proportional to the square of their distances.

Flat-Earthers like to characterize gravity as if it was “invented” to “replace” Archimedes’ principle & that the two are competing theories. In reality, these are two different theories that explain different phenomena. Gravity does not explain anything that Archimedes’ principle explains and vice versa. Gravity does not replace Archimedes’ principle; both are valid and in use today.

## Countering Gravity

Gravity is not the only force. Other forms of force exist. Multiple forces can affect an object at the same time. Forces can counteract gravity, making an object move against the direction of gravity. These cases do not disprove gravity.

Objects can move against the direction of gravity, and flat-Earthers use it as “evidence” that gravity does not exist. In reality, there is at least a force other than gravity affecting the object, in the opposite direction from gravity, with a larger magnitude. Continue reading “Countering Gravity”

## Point Source

A point source is a source or reflection of light with a smaller angular size than the angular resolution of the observer. It is visible as a bright dot if bright enough against a dark background but not resolvable, or no detail is discernible.

Flat-Earthers claim the ISS should not be visible like a distant plane is not visible. In reality, aircraft lights can be seen from 60 km in a dark sky but are not resolvable. We can see them because they are point sources on a dark background.

## Density and Balance Experiment

Objects fall down because of gravity. Flat-Earthers deny gravity & wrongly claim they fall down because of density. We can disprove flat-Earthers’ claim by setting up two different weights of the same material (thus, the same density) against each other using a balance. We can see the objects’ densities are not why they fall down.

## How Satellites Can Orbit and Stay Up There

Earth’s gravity pulls things toward Earth. Satellites can stay up there because they are orbiting. They are moving at the right speed and right direction to counter the pull of gravity.

Flat-Earthers claim that satellites should fall down if gravity pulls toward Earth. In reality, we do not just launch satellites to space. We also give them the correct speed & direction, so they neither fall toward Earth nor keep going farther away.

## Directions of Up and Down

“Down” is the same direction as Earth’s gravitational acceleration, toward Earth’s center. “Up” is the opposite, or away from Earth’s center. This applies to all observers on Earth.

Flat-Earthers like to question & ridicule about what would occur on the people “down” there. They just do not understand that to the people on the other side of Earth, “down” is still toward Earth’s center, like it appears to everyone else.

## Theories of Gravity

Gravity is a natural phenomenon by which all things with mass or energy are brought toward one another. Today, gravity is explained by two theories: Newton’s law of universal gravitation and Einstein’s general relativity.

Flat-Earthers treat the existence of the two theories as a supposed conflict, and they use it as “evidence” of wrongdoing. In reality, these are two separate theories that explain the phenomenon of gravity. Einstein’s is more accurate yet more complex. In contrast, Newton’s is simpler but less accurate. Despite having been superseded, Newton’s gravity continues to be used as an excellent approximation of the effect of gravity in most applications.

## Cult of Personality Accusations

Flat-Earthers like to accuse that science accepts ideas due to the cult of personality toward the people behind the ideas. In reality, science accepts ideas because of their merits, not because of the identity of the people behind the ideas.

Isaac Newton is considered one of the most influential scientists ever. But we have no problem saying that some of his ideas were just plain wrong and not scientific.