The most straightforward way to check the shape of the Earth is to look at it. There’s one small problem, though. To see the shape of the Earth as a whole, you need to be far enough away from it. For most of human history, this has not been possible. It was only with the advent of the space age that our technology has allowed us to send a human being, or a camera, more than a few kilometres from the surface.
The earliest photo of the whole Earth from space was taken by NASA’s ATS-3 weather and communications satellite in 1967. The photo was taken from geostationary orbit, some 34,000 kilometres above the surface of the Earth, and shows most of the western hemisphere, with South America most prominent. And you can see quite clearly that the Earth is round. It looks spherical, for the other landmasses that we know are there are hidden around the other side, and there is foreshortening of the features near the edges which matches our experience with spherical objects.
A more famous image of Earth taken from space is the Blue Marble image, captured by the Apollo 17 astronauts on their way to the Moon in 1972. This photo was taken from a distance of about 45,000 kilometres.
This image is clearer and it’s arguably easier to see the spherical shape of the planet. Both these photos were taken with the full hemisphere lit by the sun.
The Apollo 13 astronauts, in their ill-fated flight, captured a different view, with part of the Earth in darkness because the sun was not behind them.
Here it is even easier to get a feeling for the round, spherical shape of Earth, because our experience with the way light falls on round objects helps our minds make sense of the curved shadow.
Well, that’s pretty definitive. But what else do we have?