Earth Axial Tilt

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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.

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Apparent Motion of Planets

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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.

Geocentric Coordinate System Does not Imply Geocentrism

In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects like satellites, planets, stars, etc. The origin of the coordinate can be anywhere, including Earth. If the coordinate system is Earth-centered, we call it a geocentric coordinate system.

Flat-Earthers are often triggered by the term ‘geocentric’. They would search inside astronomy books and scientific journals to find the word ‘geocentric’. If they can find it, they would use the fact as ‘evidence’ of geocentrism, or that the Earth is the center of the universe, and the Sun is revolving around the Earth.

They are wrong. In many cases, the term ‘geocentric’ refers to the origin of a coordinate system. And it has nothing to do with whether the Sun revolves around the Earth or the other way around.

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Astrolabe

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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