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 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 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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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.
Continue reading “Coriolis Effect Misconception & Strawman Argument”
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 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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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.
Continue reading “Geostationary Satellites”
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.
Continue reading “Synodic Day vs Sidereal Day”
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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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.
Continue reading “Star’s Annual Motion”
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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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.
Continue reading “Moving Vehicle Analogy and the Physical Barrier Excuse”
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.
Continue reading “Ptolemy’s Almagest and Earth’s Axial Precession”
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.
Continue reading “Albert Michelson in “Michelson and the Speed of Light””
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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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.
Continue reading “Human Perception of Speed and 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.
Continue reading “The Variation of Earth’s Gravitational Acceleration”
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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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” 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.
Continue reading ““Behind the Curve”: the Ring Laser Gyroscope Experiment”