Question

Earth's Shape. It took thousands of years for humans to deduce that Earth is spherical. For each of the following alternative models of Earth's shape, identify one or more observations that you could make for yourself and that would invalidate the model. a. A flat Earth b. A cylindrical Earth (which was actually proposed by the Greek philosopher Anaximander (c. \(610-546\) B.C.) c. A football-shaped Earth

Short answer

Observations like how ships vanish, star visibility changes, and Earth's shadow during lunar eclipses suggest a spherical Earth over flat, cylindrical, or football shapes.

Step-by-step solution

Observations for a Flat Earth

One key observation to invalidate a flat Earth model is the way ships disappear on the horizon. As a ship sails away from the observer on a flat Earth, it would appear smaller until it disappears due to distance. However, in reality, ships appear to vanish hull-first, suggesting a curvature such as that of a spherical Earth.

Observations for a Cylindrical Earth

To refute a cylindrical Earth model, consider the phenomenon of varying star visibility depending on your location on Earth. On a spherical Earth, as you travel north or south, different sets of stars become visible because the Earth's curvature blocks some stars and reveals others. This wouldn't occur if the Earth were cylindrical, as the visible stars would remain constant along its curved side.

Observations for a Football-Shaped Earth

For a football-shaped Earth, observe the consistent shadow Earth casts on the Moon during a lunar eclipse. The shadow is always round, signifying a spherical shape. If Earth were shaped like a football, the shadow would appear elongated or differently shaped, depending on its orientation.

Key concepts

Flat Earth theory

The Flat Earth theory is an ancient belief that the Earth is flat rather than spherical. This idea, despite overwhelming evidence to the contrary, has persisted into modern times in some circles. However, numerous observations can easily debunk this theory. For example:

• As ships sail into the distance on a vast body of water, they disappear from view hull first, not just getting smaller until they vanish as one might expect on a flat surface. This effect is due to the Earth's curved surface.
• The phenomenon of day and night wouldn't occur as it does if Earth were flat. On a flat Earth, all areas should experience sunrise and sunset at the same time, which contradicts our everyday experiences.
• An airplane traveling in a straight line over a flat Earth theoretically wouldn't have to adjust its course to maintain a consistent altitude, yet in reality, pilots do adjust their course to align with Earth's curvature.

These simple observations demonstrate that the Earth cannot be flat.

Spherical Earth evidence

The prevailing scientific model is that Earth is an oblate spheroid, which means it's mostly spherical but slightly flattened at the poles. One compelling piece of evidence supporting this is the observation of Earth's shadow on the Moon during a lunar eclipse. The shadow remains consistently round, no matter the angle of the eclipse.
Another strong line of evidence comes from circumnavigation. Explorers can, and indeed have, traveled around the Earth in all directions, confirming its sphere-like shape. Additionally, satellite imagery taken from space unmistakably shows Earth as a sphere.
Gravity offers even more proof. The gravitational pull remains consistently directed towards the center of Earth, resulting in a uniform effect experienced globally, signifying a roughly spherical shape.
All these authentic observations affirm Earth's roundness.

Lunar eclipse observations

Lunar eclipses provide insightful evidence about Earth's shape. During a lunar eclipse, Earth aligns perfectly between the Sun and the Moon, casting a shadow across the lunar surface. One of the most telling observations during these events is the shadow's shape.
Regardless of how Earth is oriented during the eclipse, the shadow is always round. If Earth were flat or cylindrical, there would undoubtedly be occasions when its shadow appeared as a line or other non-circular shape rather than a circle.

• The uniform round shadow suggests a spherical Earth.
• This consistent result is independent of Earth's position relative to the Sun and Moon, further cementing the idea of a spherical Earth.

These observations provide crucial evidence refuting non-spherical Earth theories.

Historical models of Earth

Throughout history, various models of Earth have been proposed, reflecting humanity's evolving understanding.
The Greek philosopher Anaximander suggested a cylindrical Earth, entertaining the idea that Earth was like a column tumbling through space. This model fails under scrutiny; for example, varying star visibility with latitude disproves it, as the stars visible to us change distinctly from one pole to the other.
Other early models posited Earth as a flat disc; however, as people began to recognize the implications of longitude and latitude, along with exploration that enabled global circumnavigation, the spherical model gained acceptance.
Each progression in these models was driven significantly by new observational evidence contradicting earlier misconceptions, guiding society closer to the truth of Earth's actual shape.

Astronomical observations

Astronomy has greatly contributed to our understanding of Earth's shape. By observing celestial bodies and their interactions, we gain insights that affirm the spherical model of Earth.
Travelers navigating by stars noted that different constellations become visible or invisible as they moved north or south. This is consistent with the curvature of Earth, as a curved surface would block or reveal stars that a flat plane would not.
Additionally, the concept of gravity further supports spherical Earth. Gravitational models, which rely on mass and the center of gravity, make practical sense only with a spherical Earth because this shape naturally allows for gravitational equilibrium and uniform weight distribution.

• Satellite images and GPS technology rely on spherical models for accuracy, demonstrating sophisticated evidence from modern astronomy that further refute a flat Earth.
• The rotation of celestial bodies, including Earth, creates predictable patterns of night and day that align with a spherical Earth.

These astronomical observations continually validate a spherical Earth, dismissing alternative shape theories.