Explain why it was previously possible to calculate the diameter of the Earth without going into space?

The method of Eratosthenes and measurements using the shadow of the Sun.

Around 240 BC, a Greek scholar named Eratosthenes managed to measure the circumference of the Earth using just a stick planted in the ground, a bit of basic geometry, and a good dose of observation. Observing in Syene (now Aswan, Egypt) that on the summer solstice day, the Sun shone directly into a well without casting a shadow, while in Alexandria, located further north, he noticed on the same date that his stick cast a slight shadow. He then measured this angle of shadow very precisely, finding it to be approximately one-fiftieth of a full circle (about 7.2°). Knowing the distance between Alexandria and Syene from land surveys, he used a simple mathematical proportion to find the total size of the Earth's circle. The result: just with this clever observation of shadows, Eratosthenes arrived at a circumference of about 40,000 km. That was over two thousand years ago, long before the first rocket! Not bad for a time without satellites, right?

The essential role of geometry and terrestrial triangulation.

Geometry is an old friend of curious scholars: for a long time, it has been used to indirectly measure distances or objects that are impossible to reach directly. To calculate the size of the Earth, scientists like Eratosthenes used methods based on triangulation. The concept is simple: you measure two angles very precisely from two different locations, spaced by a known distance, and with this data, you can reconstruct the size of a third side or even an entire object like the planet! In practice, with just precise angles and a good understanding of triangles, ancient scholars were already able to obtain a surprisingly accurate estimate of the Earth's diameter. It was clever, precise, and frankly impressive, all without leaving the ground.

Astronomical observation and exploitation of lunar parallaxes

Ancient and medieval scholars had figured out a clever way to estimate the size of the Earth without having to travel: by observing the apparent position of the Moon from two different locations at the same time. This is known as the method of lunar parallax. Specifically, if two distant observers look at the Moon simultaneously and note its exact position relative to the stars behind it, they see a slight difference in angle. This angle difference allows them to draw an imaginary triangle Earth-Moon, use a bit of basic geometry, and voilà, they get a good estimate of the distance separating us from our natural satellite. Since this distance is known precisely and we can measure the angle that the Moon occupies in the sky, a simple calculation is then sufficient to obtain the approximate diameter of the Earth. Even without super telescopes or space rockets, this trick allowed for a truly impressive estimate for the time.

Indirect measures: the contribution of navigation and exploration voyages

Before the space age, sailors had already understood that the Earth was a sphere. How so? They simply observed the marine horizon. When a ship sails away, it gradually disappears from bottom to top: first the hull, then the deck, and finally the masts. This phenomenon clearly showed a curved shape.

Then, with the age of great explorations, particularly from the 15th to the 18th century, navigators began to traverse the oceans on a massive scale. Thanks to accurate maps and instruments like the sextant, they estimated their position relative to certain celestial landmarks such as the North Star. These data, combined with their measured distances traveled, indirectly provided a fairly reliable idea of the Earth's diameter on a large scale.

Long voyages also required knowing the circumference of the Earth precisely to correctly calculate routes. By accumulating the records of countless navigators, cartographers, and explorers, the estimates of the Earth's diameter became increasingly precise, even without any rockets or satellites to guide them.

The accuracy achievable without the help of modern space exploration.

Ancient measurements and those conducted until the 18th century had already achieved astonishing accuracy: as early as antiquity, Eratosthenes was able to determine the circumference of the Earth with less than 10% error, simply by measuring shadows on the ground. Over the centuries, thanks to better triangulation techniques, more precise tools like the theodolite, and a better understanding of astronomical phenomena, scientists were able to significantly refine this accuracy. In the 17th century, the French astronomer Jean Picard managed to measure the Earth's radius with a margin of error of barely 0.5%. By the end of the 18th century and into the early 19th century, the error became even more negligible thanks to carefully organized geodetic expeditions across multiple continents. All this, of course, without ever having to leave the Earth's surface!