Selenelion: The Phenomenon Where Both the Sun and Moon Are Visible During a Total Lunar Eclipse

A selenelion occurs during a lunar eclipse when the sun and moon are observed above the horizon. Atmospheric refraction bends light rays and lifts the image of the sun, and the moon typically up to 0.6°, so both can appear above the horizon.

Flat-Earthers assert that a selenelion should not be possible if Earth is a sphere due to the fact during a lunar eclipse, the sun and moon are 180° apart. In reality, a selenelion is possible because Earth’s atmosphere refracts light.

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Umbra, Penumbra, and Antumbra in an Eclipse Diagram

Light generally travels in a straight line. We can use this property of light to determine the umbra, penumbra, and antumbra part of a shadow by drawing lines from the edges of the light source toward and past that of the occluding object.

Flat-Earthers regard the lines in an eclipse diagram as a violation of the fact that a light source emits light in every direction. In reality, the lines are there only to determine the boundaries of the shadow’s regions in an eclipse diagram.

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Sunlight Creates Umbral Shadow Smaller than the Object Casting the Shadow

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The sun is a huge light source. It is larger than anything on Earth. As a result, the umbral part of a shadow formed by sunlight will always be smaller than the object casting the shadow.

Flat-Earthers assert that a shadow cannot be smaller than the object casting the shadow. They would use that to “disprove” the scientific explanation about the occurrence of a solar eclipse. In reality, it is not difficult to demonstrate that the umbral part of a shadow can be smaller than the object casting the shadow, especially if the light source is the sun.

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The Saros Cycle and Saros Series

A page on NASA’s website is at the center of the attention of flat-Earth followers. It is a page created by Fred ‘Mr. Eclipse’ Espenak, explaining the Saros cycle. The first paragraph from the page:

The periodicity and recurrence of eclipses is governed by the Saros cycle, a period of approximately 6,585.3 days (18 years 11 days 8 hours). It was known to the Chaldeans as a period when lunar eclipses seem to repeat themselves, but the cycle is applicable to solar eclipses as well.

After they discovered that the said ‘Chaldeans’ was a civilization from 25 centuries ago, they got excited and jumped to the conclusion that NASA used ancient technology to predict eclipses. But, like other similar cases, this tells us more about the flat-Earthers themselves than about NASA.

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Lunar Phase: Earth’s Shadow has Nothing to do with It

It can be surprising that many people believe that the lunar phase is caused by the Earth casting its shadow on the surface of the Moon. Flat-Earthers are no exception. But they take it one step further and use the misconception do “disprove” the fact that Earth is a sphere. They are wrong. The lunar phase is the shape of the sunlit part of the Moon seen from the observer.

On the other hand, the phenomenon where the Earth casting its shadow on the surface of the Moon is called the lunar eclipse. Moon phase occurs all the time and undergoes a monthly cycle, while a lunar eclipse happens only a couple times in a year.

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Some flat-Earthers like to observe celestial phenomena and the positions of celestial objects. Sometimes, they insist such events cannot possibly happen if the Earth is a sphere orbiting the Sun, and took the wrong conclusion that the Earth must be flat.

We can try asking them the time of the occurrence and the position of the observer, then use Stellarium or similar applications to simulate it. If the result matches the observation, then they have nothing to complain about. Their confusion was only the result of their ignorance.

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Sunrays are Practically Parallel, but not Perfectly Parallel

The Sun emits sunrays to every direction. But as the Sun is very far, the sun rays that reach us are practically parallel. To an observer on Earth, the rays of light coming from the Sun form a maximum angle of about 0.53°. They are practically parallel, but not perfectly parallel.

Flat-Earthers often question the fact we say sun rays are parallel, but in any diagram of an eclipse, they are drawn at an angle. They merely confuse practicality with perfection. Sunrays are practically parallel, but they are not perfectly parallel.

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The so-called “Experiment” Done by Flat-Earthers

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We often see flat-Earthers create a ‘miniaturized physical model’ of a real object and present them as ‘proof’ in support of flat-Earth and against spherical Earth. Their M.O. is to observe if the model remotely resembles the actual object in a particular way. If it does, then it is enough for them to conclude the model describe how the real object works.

Conversely, if a real object cannot be miniaturized and still have the same behavior as the original object, they would conclude that the behavior of the original object doesn’t exist.

They would wrongly call creating such models “doing an experiment”.

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Orbital Inclination: The Reason an Eclipse Does Not Occur Every Month

The orbit of the Earth and that of the Moon are not perfectly aligned in the same plane. The orbital plane of the Moon is inclined 5.145° from the Earth’s orbital plane. This is the reason an eclipse does not happen every month.

Sometimes we see flat-Earthers claim that an eclipse should happen every month. But it does not, and they take that as ‘evidence’ of the failure of modern science to explain the occurrence of an eclipse. Their mistake is that they don’t account for the orbital inclination.

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The Saros Cycle and Prediction of Eclipses

A long time ago, Babylonians carefully maintained records of the occurrence of eclipses and used these records to predict future eclipses. To honor that, in 1691, Edmund Halley named the interval in a cycle of eclipse using a Babylonian unit of time: the “Saros.”

NASA explained the Saros in a web page titled Eclipses and the Saros, and the unscrupulous flat-Earthers were quick to devise a conspiracy theory. They invented the scenario that NASA —a space agency with billions of dollars of a budget— are somehow using ancient technology to predict the occurrences of an eclipse. They are wrong. NASA does not use the Saros Cycle to predict eclipses.

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Atmospheric Refraction

Light waves are not always moving in a straight line. When it passes through a medium of a different refractive index, the waves will deviate. The phenomenon is called refraction and described according to Snell’s Law.

Earth’s atmosphere has variation in air density that depends on the altitude. As the refractive index changes with the density of the medium, light waves passing through Earth’s atmosphere also experience refraction.

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Earthshine Proves the Moon Obscures the Sun During a Total Solar Eclipse

During a total solar eclipse, the Moon is right between the Earth and the Sun. Thus, the near side of the Moon does not receive any sunlight. But while it is dark, it still gets some light reflected by the surface of the Earth. This phenomenon is called Earthshine.

Some flat-Earthers argue that a solar eclipse is not caused by the blocking of the Sun by the Moon, but by another, mysterious celestial body. The reason is that eclipses are incompatible with their beliefs about the motion of the Sun and the Moon.

Earthshine proves that they are wrong.

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Brightness of a Total Lunar Eclipse

“Looking at the pictures of the progression of a lunar eclipse, the Moon turns red, but there’s no apparent change of brightness of the Moon as it progresses from full moon to total eclipse. Therefore, a total lunar eclipse cannot be the result of the Earth casting its shadow on the surface of the Moon.”

Some flat-Earthers —presumably never seen a total lunar eclipse in person before— actually claim that.

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Lunar Phase and Lunar Eclipse

Apparently it is a very common misconception that the phases of the Moon is caused by the Earth’s shadow. Not only among flat-Earthers, but the general public as well. The difference is that flat-Earthers will usually jump to conclusion, and have some outlandish interpretation about this.

In reality, the phases of the Moon happen because we see a different sunlit part of the Moon at different time. Not because it is over the shadow of the Earth.

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Not to Scale: Illustration of the Solar System

Practically every illustration showing the Sun, Earth, and Moon is drawn not to scale. The reason is that the sizes of celestial bodies are far too small compared to the distances between them.

Flat-Earthers sometimes use this fact to point out the alleged “failure of modern science” to describe celestial bodies, or even “a mean to deceive us all.” However, the real reason is just that it is practically impossible to create a model of the solar system in the actual scale, using static images, that still effectively describes the objects it is trying to explain.

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