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.
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.
In a very casual observation, Polaris appears stationary in the sky because it is very far & we do not live nearly long enough.
Flat-Earthers use that Polaris always appears in the exact location as “evidence” of a stationary Earth. In reality, it does not appear in motion the same way mountains do not appear to move if we move less than an inch.
Because Earth is a sphere moving around the Sun, we can easily see the Milky Way’s center only in certain months. In other months, the Milky Way will be too close to the Sun in the sky, where sunlight will overwhelm and obscure it from view.
Flat-Earthers claim that the Milky Way is visible every night “proves” a flat Earth. In reality, we are inside the Milky Way, and some parts will be visible. However, a specific part of it, like its brighter center, will not be visible every night.
An astrolabe is a device historically used to determine the date and time of day from the positions of the sun or stars. Astrolabe was used from classical antiquity, about the 2nd century BC, until the age of discovery. It was superseded by the more accurate sextant, star charts, and time-keeping devices.
Flat-Earthers claim an astrolabe can work because Earth is flat. In reality, astrolabes are designed using the spherical Earth model. They could not possibly work if Earth is flat.
In astronomy, we use a celestial coordinate system to specify the position of celestial objects, like satellites, planets, stars, etc. Each of the coordinate systems has an origin, which can be the observer (topocentric), Earth (geocentric), the Sun (heliocentric), the Moon (selenocentric), or any other location. We choose a specific coordinate origin according to convenience and suitability for the problem at hand.
Some coordinate systems are described as “geocentric,” and flat-Earthers use it as “proof” of geocentrism, or that Earth is stationary and the center of the universe. In reality, it is just the origin of the coordinate. “Geocentric” here means Earth is the origin of the coordinate system. As we live on Earth, a geocentric coordinate is useful for many purposes.
The “Flat Earth Dome Model” (walter.bislins.ch/FED) is an app created by a fellow flat Earth debunker, Walter Bislin. The app attempts to map observed reality to the flat Earth model. Lines in the app portray how light rays must have traveled if Earth were flat. They demonstrate that light must bend in a physically impossible way if Earth were flat, disproving a flat Earth.
Videos of the app are circulating in flat-Earth communities, and flat-Earthers fail to understand what the app is showing. They incorrectly assume the app “proves” a flat Earth when in reality, it thoroughly disproves the flat Earth model.
Stars are dim. We can only see them if our eyes are adapted for low light. Astronauts in space can see the stars when the sun is blocked from view and avoid seeing any large sunlit objects.
Some astronauts said they did not notice the stars, but others claim they could see them. Flat-Earthers use the conflicting statements to discredit spaceflight. In reality, sometimes astronauts can see the stars, and sometimes cannot, just like us on the ground, depending on the situation.
The sky appears practically identical to all observers on Earth, except that the Earth obstructs the view to a different part of the sky to a different observer on Earth, and daylight obscures the view to most objects in the sky.
The fact that the sky appears identical everywhere on Earth can only be adequately explained if all the astronomical objects are very far from us and that the Earth is a sphere.
Earth’s axis of rotation is tilted. The tilt is relative to Earth’s orbital plane. Therefore, it only affects the apparent positions of objects in the solar system, but not that of the distant stars. Both of Earth’s celestial poles still point to the same location in the sky.
Flat-Earthers fail to understand why the stars are unaffected by Earth’s tilt, and they use it to discredit science. In reality, Earth’s axis of rotation is practically constant in the short term, only not perpendicular to Earth’s orbital plane. As a result, it only affects the apparent positions of the Sun, planets, moon, and other objects in the solar system.
Flat-Earthers like to mention an astronomical event or a specific celestial geometry and insist it cannot possibly occur if Earth is a rotating sphere orbiting the sun. They would incorrectly proclaim it as “evidence” of a flat, stationary Earth.
We can ask them the time of the event and the observer’s location, then use Stellarium or a similar app to simulate the event. If it produces the same result as the actual observation, then they have nothing to complain about. And the problem was just caused by their failure to understand and to create the correct model.
If we look toward the south at night in any location in the southern hemisphere, we will see the same set of stars. We will see the stars rotating around the south celestial pole, in the Octans constellation, near the star Sigma Octantis.
The flat-Earth model cannot explain the phenomenon. Looking at the so-called ‘flat-Earth map,’ we should see another set of stars on a different location in the southern hemisphere. The reason is that the flat-Earth model is a false representation of the Earth.
Planets appear to move near the ecliptic —the line that marks the annual path of the Sun against background stars. From these motions, it is obvious that Venus and Mercury are in motion around the Sun. And the motion of the other planets can only be explained if Earth is in motion around the Sun.
Flat-Earthers claim that Earth is stationary. In reality, from observing the planets’ motion, the most plausible explanation is that all planets, including Earth, are in motion around the Sun.
A nautical almanac is a publication that describes the positions of stars to enable navigators to use celestial navigation to determine their position at sea. Because the positions of stars are not fixed, a nautical almanac needs to be continuously revised.
Flat-Earthers claim that the positions of stars are fixed, and they only circle the celestial pole. In reality, stars’ positions do change, and we can find the historical positions of various stars from old publications of a nautical almanac.
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.
Planets are not stars. On casual observation, both planets and stars appear as bright dots in the night sky. But with careful observation, we know planets are different from the stars.
Flat-Earthers insist that planets are just stars. In reality, we can easily observe many of the planets’ attributes that set them apart from the stars —using minimal efforts and equipment.
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.
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.
Ursa Minor is a constellation in the Northern Sky. It is traditionally important for navigation because of one of its member stars, Polaris, being the north star. Ursa Minor is always visible north of the 18°N and hidden south of 18°S. Polaris is located very close to the North Celestial Pole, and only visible north of the Equator. The visibility of Ursa Minor and Polaris is only consistent if Earth is a sphere.
The pole star Polaris is used to locate the position of the north celestial pole because it is a bright star that is conveniently located very close to it. However, there is no corresponding bright South Star near the south celestial pole. To locate the south celestial pole, we need to observe stars around it.
There is no South Star as useful as Polaris in the north. Flat-Earthers use it to “disprove” the existence of the south celestial pole, which cannot possibly exist if Earth were flat. In reality, the south celestial pole can be easily shown and located. There is simply no bright star near it.