Overview
This lecture discusses evidence demonstrating that Earth is round, including observations, historical experiments, and scientific reasoning from ancient to modern times.
Observational Evidence for a Round Earth
- The ground appears flat to our senses, but large-scale observations show Earth is curved.
- From a field in Chicago, you can't see distant mountains because Earth's curvature blocks your view.
- Boats disappear bottom-first over the horizon, showing a curved surface.
- The distance to the horizon increases as your elevation increases; from a mountain, you can see further than from ground level.
- Taller individuals can see slightly farther due to their increased viewpoint.
Historical and Scientific Proofs
- Ancient Greeks observed the Earth's curvature over 2000 years ago.
- Aristotle noticed Earth's round shadow on the Moon during a lunar eclipse and that different stars are visible from different latitudes.
- Eratosthenes measured the Earth's curvature by comparing shadow angles in two cities at noon during the summer solstice and used the data to estimate Earth's size.
Modern Confirmation
- Simple experiments today, like comparing sunset times by phone across locations, also confirm Earth's curve.
- Photographs taken from space clearly show Earth as a sphere.
Key Terms & Definitions
- Horizon line — The apparent line where the sky meets the Earth, marking the limit of your visual field.
- Summer solstice — The day of the year when the sun reaches its highest point, resulting in the longest daylight period.
- Lunar eclipse — An event where Earth's shadow falls on the Moon, often showing Earth's round shape.
Action Items / Next Steps
- Try the stick shadow experiment at noon with a friend at a different location.
- Observe a ship or sunset and note how visibility changes with distance and elevation.