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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Star’s Annual Motion

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On each day, we can observe stars to drift about 1° in their motion around the celestial pole. In a month, they will appear to have moved about 30° when observed at the same time in the night. In a year, they will be back to their original positions as the same day in the previous year. The observation is the basis of the solar calendar system we are using today.

Some flat-Earthers claimed that the stars appear the same every night, and they would erroneously conclude that the Earth is stationary. In reality, stars are shifting about 1° every day.

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Coriolis Effect

The Coriolis force is a force that acts on objects that are in motion within a rotating frame of reference. Because the Earth is a rotating sphere, an object traveling unattached to Earth’s surface is affected by the Coriolis force, depending on its speed and direction, as well as its latitude on Earth’s surface.

The rotating motion of the Earth causes the different parts of Earth’s surface to have different linear speeds, depending on its distance from the Earth’s rotational axis. An object moving from a location to another with a different linear speed will be affected by the Coriolis force because the motion of the object is now being observed from another location with a different linear speed/direction relative to Earth’s rotational axis.

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Moving Vehicle Analogy and the Physical Barrier Excuse

We cannot feel the Earth rotating, and to explain the phenomenon, we usually use the moving vehicle analogy. If the vehicle is moving at a constant speed, we will not be able to sense the motion. We cannot feel the motion of Earth because of the same reason.

Flat-Earthers invented the excuse that we are standing on Earth, not inside it, to falsely reject the explanation.

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Ptolemy’s Almagest and Earth’s Axial Precession

Due to Earth’s axial precession, the positions of Earth’s celestial poles shift gradually in a cycle of approximately 26000 years. These days, Polaris is very close to the north celestial pole. However, 19 centuries ago, Kochab is much closer to the celestial pole than Polaris.

Flat-Earthers claim that the stars are always in the same position proves the Earth is stationary. An ancient star chart like Almagest from the 2nd century shows them wrong. The stars today were not in the same place as stars 19 centuries ago.

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Albert Michelson in “Michelson and the Speed of Light”

In 1881, Albert A. Michelson performed an experiment in an attempt to prove the existence of aether. Aether was a hypothesized material that fills the region of the universe. Scientists knew light is a form of wave, and because all other waves require a medium to propagate, they formulated the aether hypothesis, in which light can propagate. However, Michelson’s experiment produced a zero effect.

In 1960, Bernard Jaffe wrote Michelson’s biography in the book “Michelson & the Speed of Light.” Unscrupulous flat-Earthers quoted a lone paragraph from the book out of the intended context, and present it as if Michelson proved the Earth is stationary.

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A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions.

Seismometers are sensitive but they cannot detect Earth’s motion. Flat-Earthers use this to ‘prove’ the Earth is stationary. They are wrong. A seismometer is mounted on Earth’s surface and is already moving at the same speed as the surface itself. It can only detect motion if there is a change in speed —or an acceleration— exerting a force to the seismometer.

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Human Perception of Speed and Acceleration

When aboard an airplane flying at cruising speed, we will not be able to feel that the plane is, in fact, moving at speed of more than 900 km/h. But if the plane changes its speed, turns or changes its altitude, we can easily feel it.

The same thing happens with the motion of the Earth. Due to Earth’s rotation, the surface of the Earth is moving at 1656 km/h near the equator. We never feel it because the speed is constant, or in other words, the acceleration is zero.

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The Variation of Earth’s Gravitational Acceleration

The gravity of Earth varies on the different location on Earth. The variation is caused by the difference in latitude, altitude or depth, and local geology & topography. In most cases, the centrifugal acceleration from Earth’s rotation is also already accounted for in the value of Earth’s gravitational acceleration.

Flat-Earthers claim that according to science, the Earth’s gravitational acceleration is always constant at 9.8 m/s² everywhere on Earth. And they are using the claim to prove Earth is not a rotating sphere. In reality, the magnitude of Earth’s gravitational acceleration varies on the different location on Earth.

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Airplane and Earth’s Rotation

An airplane exists on Earth. It has the same motion as the surface of the Earth, including the Earth’s rotating motion. The motion of the airplane itself is measured relative to the surface of the Earth.

Flat-Earthers think that due to Earth’s rotation, an airplane should not be able to fly east or land. They are wrong. The speed of an airplane is relative to the surface. It retains the motion of the Earth and does not need to chase Earth’s rotation.

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Foucault Gyroscope

In 1851, Léon Foucault used a pendulum to demonstrate the rotation of the Earth. Despite his success, he was not fully satisfied with the pendulum experiment because of the dependency on the sine of latitude, which the public found difficult to understand. He later designed a device which he named ‘gyroscope.’

A spinning gyroscope keeps a constant axis of rotation in space, so it should slowly rotate with respect to an observer attached to the rotating Earth. The challenge was technical; it would need to have minimal friction, and it has to be able to spin for a sufficient duration so that the precession due to Earth’s rotation can be observed.

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“Behind the Curve”: the Ring Laser Gyroscope Experiment

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“Behind the Curve” is a documentary showing behind the scenes on what is happening in the flat-Earth community in the United States just before a flat-Earth conference. In a section, the film shows us how a flat-Earther did an experiment involving a ring laser gyroscope, which proved the very thing he tried to disprove: Earth’s rotation; and that he was unwilling to accept the result.

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Ring Laser Gyroscopes are Sensitive Enough to Measure Earth’s Rotation

A ring laser gyroscope (RLG) is an instrument for measuring the change in orientation and rotational velocity. It is sensitive enough to measure Earth’s rotation easily.

Flat-Earthers claim that there is no instrument able to measure Earth’s rotation. Such claim arose from their ignorance. Ring laser gyroscopes —which are installed in some airplanes and ships— can easily detect and measure Earth’s rotation.

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Horizontal Motion of Smoke

Smoke rises due to convection current. The solid particles emitted from a combustion process are carried by the hot air that rises because of its lower density than the surrounding air. The horizontal motion of smoke itself is affected by the movement of air surrounding it.

Flat-Earthers claim that a rising smoke without horizontal motion “proves” a stationary Earth. They are wrong. The horizontal motion of smoke is an effect of the horizontal motion of air relative to the smoke, and the motion of the object emitting the smoke is inconsequential.

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Frame of Reference

A frame of reference consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system and standardize measurements. Speed or velocity is relative to a frame of reference. It is possible that two speeds/velocities are measured relative to a different frame of reference and thus cannot be directly compared.

Flat-Earthers would often compare two different speed measurements like an airplane moving at 900 km/h (560 mph) and Earth’s surface near the equator at 1674 km/h (1040 mph). They would conclude the airplane should not be able to catch up with Earth’s surface and would not be able to land if the Earth is rotating. They are wrong. The speeds are measured relative to the different frame of reference, and therefore, cannot be compared directly.

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Old Star Charts and Earth’s Axial Precession

Due to Earth’s axial precession, the positions of Earth’s celestial poles shift gradually in a cycle of approximately 26000 years. These days, Polaris is very close to the north celestial pole. However, 2000 years ago, Kochab is much closer to the celestial pole than Polaris. In the next 2000 years, Errai will replace Polaris as our pole star.

Flat-Earthers claim that the stars are always in the same position proves the Earth is stationary. They are wrong. By observing old star charts from a few centuries ago, the shift of celestial poles can be determined. It proves stars are not always in the same position as today, and that the Earth is rotating.

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Earth’s Atmosphere Rotation

Earth’s atmosphere is part of the Earth. It practically follows Earth’s rotating motion like the rest of the Earth. Due to various factors, Earth’s atmosphere has relatively small and uneven movement relative to the surface. We call it ‘wind.’ The fastest wind ever recorded is about 408 km/h, relative to Earth’s surface.

Many misconceptions about Earth arise from the wrong idea that only Earth’s solid body is rotating, but the atmosphere is not. If it is true, then we should always feel the wind with the speed of 1674 km/h near the equator, or more than 4× the fastest wind ever recorded. We don’t feel such a wind, and thus we know our atmosphere is moving at practically at the same speed as Earth’s surface.

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The Lack of Motion Blur in Earth Photos

The surface of the Earth close to the equator is moving at the speed of about 1670 km/h due to Earth’s rotation. To take a sharp picture of the Earth with the resolution of 10000×10000 from a stationary position in space, it would require the shutter speed faster than 2.7 seconds. It is not difficult at all to take a sharp picture of the entire Earth without perceivable motion blur.

Flat-Earthers claim that it should be impossible to take a sharp picture of the Earth from space due to the speed of Earth’s rotation. To them, the fact that a photographer cannot take a sharp photo of a speeding race car from the sidelines (they actually can) tells us it should be impossible to take a sharp picture of the Earth moving at 10× the speed.

They are wrong. Motion blur is caused by the angular speed of the object relative to the camera, not from its absolute speed. The vast distance required to take a photo of the Earth results in a very low angular speed, making it not difficult at all to take sharp images of the Earth.

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