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Introduction
Introduction
The idea that Earth is round originated over 2,000 years ago. Ancient Greek philosophers and scientists used careful observation and reasoning — long before telescopes or satellites — to conclude that the Earth is spherical. This discovery was a major milestone in scientific thinking and laid the groundwork for modern astronomy and geography.
1. Early Views of Earth
1. Early Views of Earth
Many ancient peoples believed Earth was flat.
The Greeks were among the first to propose that Earth is a sphere — not flat — based on natural observations and logic.
2. Greek Evidence That Earth Is Spherical
2. Greek Evidence That Earth Is Spherical
Ancient Greeks used everyday observations to show that Earth is round:
• Ships on the Horizon
• Ships on the Horizon
When a ship sails away, its hull disappears first and the mast stays visible longer. This can only happen if Earth’s surface curves.
• Lunar Eclipse Shadow
• Lunar Eclipse Shadow
During a lunar eclipse, Earth casts its shadow on the Moon. The shadow is always round, which can only be produced by a spherical object.
• Changing Stars With Latitude
• Changing Stars With Latitude
Different constellations are visible at different latitudes. As a person travels north or south, the stars they see change, indicating a curved surface.
3. Greek Thinkers Who Contributed
3. Greek Thinkers Who Contributed
Pythagoras
Pythagoras
Believed that the sphere is the most perfect shape and applied that idea to Earth.
Aristotle (384–322 BC)
Aristotle (384–322 BC)
Provided multiple lines of evidence (ship horizon effect, lunar eclipse shadows, star visibility changes) showing that Earth is spherical.
Eratosthenes (c. 276–195 BC)
Eratosthenes (c. 276–195 BC)
Used basic geometry and shadows to estimate Earth’s circumference to within a few percent of the correct value — all without modern instruments.
4. How Eratosthenes Measured Earth’s Circumference
4. How Eratosthenes Measured Earth’s Circumference
On the summer solstice in Syene (now Aswan, Egypt) the Sun stood directly overhead so vertical objects cast no shadow. At the same time in Alexandria, a stick cast a measurable shadow. By comparing the angle of the shadow between cities and knowing the distance between them, Eratosthenes calculated Earth’s circumference at about 40,000 km — a remarkable achievement using simple math and observation.
5. Why This Discovery Matters
5. Why This Discovery Matters
Established Earth as a spherical planet, permanently changing how humans viewed their world.
Set the stage for later astronomical models and navigation.
Demonstrated that observation + reasoning is the foundation of science.
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