Determining the Visibility of a Star From Its Declination and the Observer’s Latitude

We can determine if a star is visible from a specific location using the declination of the star and the latitude of the observer, subject to other conditions like observer’s topology, the magnitude of the star, weather conditions, etc. It is possible to do this because Earth is a rotating sphere.

If the Earth is flat, every star would have been visible all night from every location. We don’t see the same stars every night because some of them are below the horizon and obscured by the Earth.

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The Varying Continent Sizes in Different Images of Earth from Space

The amount of Earth’s surface we can see at once from a location in space depends on our distance to the Earth. The closer the observer, the smaller the amount of visible Earth’s surface. Conversely, the farther the observer, the larger the amount of Earth’s surface visible to them. But no matter how far the observer, they would not be able to see the entire hemisphere.

Flat-Earthers discovered that there are the differences in the sizes of continents in the different images of the Earth, and used the fact as ‘evidence’ of misconduct. They are wrong. Such differences in continent sizes are present because the images show the Earth from different distances and different field of view.

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Ships Disappearing Over the Horizon and the Various “Explanations” Invented by Flat-Earthers

Due to Earth’s curvature, ships traveling over an ocean disappear from the bottom up. This fact is one of the first evidence to confirm the Earth is a sphere, and one of the first facts of which flat-Earthers had to invent various “explanations” for.

Some of the popular “explanations” are: refraction, perspective, zooming reveal distant ships and visibility limitations. None can explain away the fact.

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Circumpolar and Non-Circumpolar Stars

A circumpolar star is a star, as viewed from a given latitude on Earth, that never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars stay up there in the sky, even during the day.

Flat-Earthers claim the Earth is stationary because the same stars are always visible in the sky. They are wrong. Only circumpolar stars are always in the sky. There are others that are not circumpolar. Some are only visible during certain times in a year.

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Astrolabe is an astronomical instrument for measuring the altitude of the sun or stars, and to determine the solution of various problems in astronomy, time, and navigation. Astrolabe was used from classical antiquity, about 2nd century BC,  until the age of discovery where it was superseded by the more accurate sextant, star charts, and time-keeping devices.

Flat-Earthers claim that astrolabes can only work because the Earth is flat. They are wrong. Astrolabes were designed using the spherical Earth model. To use an astrolabe, a good understanding of the spherical Earth model is required.

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The Dip of the Horizon

There are two kinds of the horizon:

  • Astronomical horizon: the horizon at the eye level.
  • True horizon: the line that visually divides the Earth and the sky.

Because the Earth is a sphere, the true horizon always lies below the astronomical horizon, or the eye-level. The angle between them is the dip of the horizon. The higher the observer, the larger the dip of the horizon.

Flat-Earthers claim there’s no dip of the horizon. They are wrong. It is not hard to observe the drop of the horizon and prove the curvature of the Earth.

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Crow’s Nest on Ships

A crow’s nest is a structure in the upper part of the ship, especially old-fashioned ones. It is used as a lookout point and positioned high above to increase visibility over the curvature of the Earth.

On the deck of a ship 4 m (13 ft) above the surface of the ocean, an observer can spot a 20 m (66 ft) high ship from at most ±25 km (16 mi). On the other hand, from a 35 m (115 ft) high crow’s nest, an observer will be able to spot the same ship from ±40 km (25 mi) away.

On modern ships, the role of a lookout is replaced by radars. And for the same reason, a radar is positioned in the upper part of a ship.

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The Reason We Cannot See Earth’s Curvature When Standing on a Beach

We can’t directly observe and subjectively perceive the curvature of the Earth from a position close to the surface. And this matches the expectations in the spherical Earth model.

Flat-Earthers often take the fact we can see the curvature by standing on a beach as ‘evidence’ the curve doesn’t exist. They are, once again, wrong.

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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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Zooming In On Distant Ships Does Not Disprove Earth’s Curvature

If we can’t see a distant ship, then it is because of one of these reasons:

  • Our eyes have limited angular resolution and are unable to resolve the ship at that distance.
  • The atmospheric condition is limiting our visibility.
  • The curvature of the Earth obscures the ship.

Flat-Earthers are keen to demonstrate that a previously invisible ship at a distance can be made visible by zooming in. They take this fact as ‘proof’ that the curvature of the Earth doesn’t exist. They are wrong. The curvature of the Earth is not the only reason a distant object is not visible.

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Sunset at Burj Khalifa

Having the total height of about 830m, Burj Khalifa is the tallest building in the world. It is so tall that we can observe the sunset at the base of the building, then rush to the upper floors and watch the same sunset again for the second time on the same day.

This phenomenon can only happen if the Earth is a sphere. On a higher position, we will observe the sunset later than when we are closer to the ground.

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