Many people are surprised to learn that you can’t see the Earth’s curvature with the naked eye from the ground—or even from a typical airplane. That’s because Earth is enormous, with a circumference of about 40,000 kilometers, making its curve incredibly gradual. To the human eye, the horizon looks flat simply because the scale is far beyond our everyday perception. At sea level, the horizon is only about 5 kilometers away, and even at cruising altitude (around 10,000 meters), the curvature is s