To the Sun and beyond

The measures of the heavens and the Earth have been inextricably intertwined throughout history. Until the seventeenth century, determining geographical position required knowledge of the position and the distance to celestial bodies, measures that in turn depended on the distance between two observing sites on Earth. For example, the first determination of the Earth’s perimeter1, carried out by Eratosthenes of Cyrene (276–194 BC), crucially depended on the actual distance between the cities of Alexandria and Syene. Precision geography was not possible without astronomy or vice versa, and their measurements were always relative to other measurements. Aristarchus of Samos (310–230 BC) concluded that the distance to the Sun2 — roughly the astronomical unit — was 19.1 times larger than the distance to the Moon; a distance that, in turn, he calculated as a function of the Earth’s diameter.