Uniform Circular Motion

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The motion of a specific location on Earth due to Earth’s rotation is a uniform circular motion. We can separate it into a uniform linear motion and a centripetal acceleration toward Earth’s center. From the reference frame of an observer on Earth, the latter results in a constant centrifugal acceleration, which is felt simultaneously with Earth’s gravitational acceleration.

We often use the uniform linear motion of everyday objects to explain the effect (or the lack of thereof) of uniform motion of Earth’s rotation. Flat-Earthers incorrectly reject it just because “it is not a circular motion unlike Earth’s rotation” In reality, a uniform linear motion is one component of a uniform circular motion. And these explanations are correct and still apply.

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Airplanes Do Correct for the Coriolis Effect

Airplanes do correct for the Coriolis effect. They constantly make adjustments to counter forces that try to push them away from the planned route, including the Coriolis force.

Flat-Earthers claim the Coriolis effect does not appear to affect airplanes, and they use it to “prove” that Earth is stationary. In reality, the Coriolis effect does affect airplanes, and we do have to correct for it. But the effect is tiny, and we can safely ignore it in flight planning.

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Moving Vehicle Analogy: Inside vs Outside of the Vehicle

The moving vehicle analogy is often presented to explain how Earth’s rotation does not seem to affect an object’s motion. Similarly, a ball released inside a moving vehicle will not fall rearward because it retains its inertia.

Flat-Earthers offer the moving open-air vehicle as if it is the “correct ” analogy. In reality, with their analogy, the air moves relative to the vehicle, which exerts an additional force that does not exist in the fact being explained. Their analogy fails to represent the fact being explained.

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Hill Sphere

The Moon orbits the Earth —not the Sun— because the Moon lies well within Earth’s Hill sphere. Hill sphere is the region around Earth where Earth —instead of the Sun— dominates the attraction of satellites.

Flat-Earthers noticed the Sun exerts a stronger pull to the Moon than Earth does to the Moon. They incorrectly concluded that according to science, the Moon should orbit the Sun, then proceeded to use it to discredit science. In reality, they did not account for the centrifugal acceleration resulting from the motion around the Sun, which is one of the factors that determines Earth’s Hill sphere.

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Flying a Drone Inside a Moving Vehicle

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If we fly a drone inside a vehicle that is moving in a constant speed and direction, the drone will hover normally because it preserves the motion it gets from the vehicle. It will not abruptly move toward the rear of the vehicle.

Flat-Earthers claim that it should not be possible for drones to hover if Earth is rotating. In reality, drones follow Newton’s first law of motion and will preserve their inertia. It is easy to demonstrate the phenomenon by flying a drone inside a moving vehicle. The fact that drones can hover still in mid-air is not evidence of stationary Earth.

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Airplanes and Earth’s Rotation

An airplane exists on Earth. It has the same motion as the surface of the Earth, including Earth’s rotating motion. The motion and speed of an airplane are measured relative to Earth’s surface.

Flat-Earthers claim that an airplane should not be able to fly eastward or land. In reality, an airplane is moving relative to the surface. It has Earth’s motion as a component of its motion and does not need to chase Earth’s rotation.

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Rock Balancing

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Rock balancing is an activity in which rocks are balanced on top of one another. It is possible to do it if Earth is rotating because the speed is constant and the rocks are affected by a constant net acceleration from gravity and Earth’s rotation.

Flat-Earthers use rock balancing as “evidence” that Earth is stationary. In reality, it is possible to balance objects on a moving platform, as long as it is moving in a straight line with a constant speed, and there is no force —like wind, affecting it.

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Negative Parallax

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Stellar parallax is the apparent shift of closer stars against more distant stars due to the orbiting motion of Earth. Negative parallax, or stars going the wrong way— occurs due to measurement errors placing the nearer star as the farther one and vice versa.

Stellar parallax directly proves Earths’ motion around the sun, and flat-Earthers use negative parallaxes to dismiss it. In reality, negative parallaxes are just statistical uncertainties.

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Earth’s Rotation in Videos Taken From Space

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The Earth rotates only once every 24 hours. To notice the rotation of Earth in a video taken from space, we need to speed up the video so that it appears fast enough for us to notice it. We also need to make sure the video was taken from a spacecraft where its motion does not dominate over the movement of Earth itself.

Flat-Earthers claim that videos taken from space were faked because the Earth does not appear rotating. In reality, Earth rotates far too slowly to be casually seen without increasing the speed.

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Airy’s Failure

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Airy’s Failure was an experiment performed in 1871 in which Sir George Biddell Airy failed to confirm the aether theory by measuring stellar aberration. The phenomenon of stellar aberration, in itself, was the first direct evidence of Earth’s motion around the sun.

Flat-Earthers claim that the “failure” in “Airy’s Failure” is the failure of proving the motion of Earth. In reality, the experiment aimed to confirm the aether theory. The “failure” was in proving the aether theory. The experiment relied on the already accepted knowledge that Earth is in orbit around the sun.

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Earth’s Axial Precession

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Axial precession is the change in Earth’s rotational axis. It is a slow change with a cycle of 25772 years. As a result, the role of pole star will pass from one star to another.

Contrary to flat-Earthers’ assertions, due to Earth’s axial precession, Polaris has not always been the North Star and will not remain as the North Star. The star Gamma Cephei will replace Polaris’ place in 2000 years as the North Star.

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Helical Motion of Planets

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The Solar System is in motion around the Milky Way galaxy, while the planets in the Solar System are in orbiting motion around the sun. As a result, from an observer outside the Solar System, planets will appear to move in a helical motion. Someone created an animation of this helical motion; it went viral and had a lot of media attention.

Flat-Earthers noticed the animation and use it to reinforce their arguments. They claim if the animation is correct, then stars should appear moving & star trails should be impossible. In reality, the animation does not show us the vast distances to the stars.

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Eötvös Effect: Direct Evidence Earth is a Rotating Sphere

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Eötvös effect is the change in perceived gravitational acceleration when moving eastward vs. westward. An object will weigh slightly more when moving westward than when it is stationary or moving eastward.

The slight difference in weight due to the Eötvös effect can be measured in an experiment on a flight. The Eötvös effect is direct evidence that the Earth is a rotating sphere.

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Apparent Motion

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Apparent motion is the appearance that celestial objects move due to the motion of Earth. If all the objects that appear in the sky are moving similarly, then it is far more likely that the movement is only apparent and caused by the motion of the observer.

Flat-Earthers claim that the motion of celestial objects “proves” that Earth is stationary, and the celestial objects are moving. By the same “reasoning,” we can conclude that the train we are in is stationary, it is just all the objects outside, as luck would have it, are moving in a very similar fashion.

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Coriolis Effect Misconception & Strawman Argument

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The Coriolis effect acts on an object that is launched to another location closer to or farther from Earth’s axis of rotation. The object retains the inertia from the surface it was launched from, but it no longer matches what is required to keep it moving in a straight line, according to an observer on the surface.

On the equator, Earth’s surface is moving at a speed of 465 m/s. Flat-Earthers assert that if that is the case, then an object launched upward should move away from the observer at the same speed. It does not happen, and they use it to disprove the Coriolis effect and Earth’s rotation in general. In reality, the Coriolis effect is not what they describe.

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Stellar Aberration

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Stellar aberration is the apparent shift of stars about their actual positions, depending on the direction Earth is moving in its orbit around the sun. It occurs because the speed of light is finite; it takes time for light to reach the observer.

Stellar aberration was discovered in 1727 by James Bradley. It was the first direct proof of heliocentrism, that Earth is in orbit around the sun.

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Stellar Parallax

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Stellar parallax is the apparent shift of the position of a nearby star against the background of distant stars. It is the result of Earth’s orbital motion around the Sun. It is tiny and difficult to observe. Successful measurement of stellar parallax was done only after the 19th century.

Some flat-Earthers claim that stellar parallax has never been successfully observed, and they use it as ‘evidence’ Earth is stationary. In reality, stellar parallax has been successfully measured in 1838 and is now used as the basis for measuring stellar distances.

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Geostationary Satellites

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If a satellite is placed in an orbit 35786 km above the equator, it will be in motion at the same speed as the Earth’s rotation. As a result, the satellite will appear practically motionless from an observer on Earth’s surface. Many communication satellites are in this orbit so that a satellite receiver does not need to track the satellite continuously.

Flat-Earthers often take the fact that satellites are in motion, and most satellite dishes have a fixed direction as ‘proof’ the receivers cannot be pointing to satellites. In reality, it is possible to place a satellite in a geostationary orbit to make it appear in a fixed position in the sky relative to an observer on Earth.

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Synodic Day vs Sidereal Day

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In 24 hours, the Earth makes slightly more than one full rotation due to its motion around the Sun. We call it one day or one synodic day. On the other hand, a sidereal day is the period of one rotation relative to distant stars.

Flat-Earthers claim that if the Earth is in orbit around the Sun, after six months, at the same hours, then the day would turn into night and night would turn into day. In reality, it would have been true if we are using sidereal day, not synodic day, or just “day” in our everyday language.

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Coriolis Effect in Sports

Many sports involve launching balls into the air. Launched balls are projectiles and will be deflected by a tiny amount due to the Coriolis effect from Earth’s rotation.

Athletes never account for the Coriolis effect, and flat-Earthers use it to “disprove” Earth’s rotation. In reality, the deflection from the Coriolis effect in sports is very tiny. It does not mean the Coriolis effect does not exist.

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