How big is the Sun? How could you figure it out?

According to LessWrong, the ancient Greeks figured out how big the Sun is through sheer geometric ingenuity and careful observation. They spotted Earth's curved shadow during a lunar eclipse, proving the planet is round. Then they estimated Earth's size by watching ships disappear over the horizon. Eratosthenes later refined this estimate using the angle of shadows cast at different latitudes on the same day—and came within a fraction of a percent of the actual value. From these observations, the Greeks reasoned that the Sun must be both incredibly distant and incomprehensibly large. It's a stunning example of how a chain of simple observations and logical deductions can answer what seems like an impossible question about the cosmos.

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