Which ancient observation supported the conclusion that Earth is spherical?

• A. The circular shape of Earth's shadow during a lunar eclipse
• B. The changing of seasons
• C. The phases of Venus
• D. The motion of comets

Answer: (A)

Observing the shadow Earth casts on the Moon during a lunar eclipse reveals Earth’s shape. When Earth sits between the Sun and the Moon, its shadow appears consistently circular, regardless of the Moon’s position in the sky. A sphere naturally produces a round shadow from any angle, so this regular circular shape strongly supports a spherical Earth. A flat Earth would not guarantee a circular shadow in all configurations and would produce shapes that could vary with geometry. The other options—changing seasons, the phases of Venus, or the motion of comets—reflect broader orbital dynamics and timing, but they don’t directly indicate Earth’s shape.