How do we know the Earth isn’t flat? For thousands of years people have known the Earth is round. Ancient civilizations used the stars and shadows to figure it out. Mariners confirmed it by sailing around the globe. And when the space age began, we saw it with our own eyes — the Earth is round! A NASA scientist explains how we’ve known for centuries that our planet is a sphere. Indirect evidence of the Earth’s spherical shape has existed for a long time, but the photographic proof was lacking until well into the 20th century. The ancient Greeks believed the Earth was round and calculated its circumference with remarkable accuracy, while observers inferred our planet’s spherical shape as it cast a curved shadow on the Moon during lunar eclipses. With the advent of aviation, photographers could reach altitudes from which they could record the Earth’s curvature. With sounding rockets and then spacecraft returning photographs from ever-greater distances from the planet, we could begin to see the Earth first as a full disk, then as a smaller and smaller blue oasis against the emptiness of space. Through these images, we gained a better understanding of Earth’s, and therefore of our own, place in the universe.
An article in the May 1931 issue of The National Geographic Magazine described how a photograph taken from an airplane east of the Andes mountain range in South America provided evidence for the Earth’s curvature. Capt. Albert W. Stevens, an officer in the U.S. Army Air Corps and an aerial photographer, took the image on Dec. 30, 1930, while flying at an altitude of 21,000 feet over Villa Mercedes, Argentina. The Andes Mountains, 287 miles away, and although taller than the plane’s altitude, lay below the sensible horizon, marked by the white horizontal line in the photograph. The Earth’s curvature explains this phenomenon, as described in the diagram accompanying the photograph. The Earth’s curvature is also visible laterally in the photograph, although the effect is subtle as the image encompasses only 1/360 of the Earth’s circumference.
In a joint program between the U.S. Army Air Corps and the National Geographic Society, Capt. Stevens, joined by Capt. Orvil A. Anderson, took the first photograph from a high-altitude balloon, clearly showing the Earth’s curvature. On Nov. 11, 1935, the pair took off from the Stratobowl near Rapid City, South Dakota, aboard the helium-filled Explorer II balloon, and ascended to a then world record altitude of 72,395 feet. The photograph showed the troposphere-stratosphere boundary and the actual curvature of the Earth and demonstrated the potential for long-range reconnaissance from high-altitude balloons.
On Oct. 24, 1946, more than 10 years before the launch of the first artificial satellite Sputnik, scientists at the White Sands Missile Range in New Mexico placed a camera on top of a captured German V-2 ballistic missile.
