## Astrolabe

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 It was superseded by the more accurate sextant, star charts, and time-keeping devices.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.

## Geocentric Coordinate System Does not Imply Geocentrism

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.

## Earth Axial Tilt

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.

## Apparent Motion of Planets

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.