The [Cosmic Distance Ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder) is a chain of measurements; each step yields enough information to unlock the next layer of measurements. This is a metaphor for all sorts of compounding effects, most of the modern world is the culmination of a prolonged bootstrapping process it would be [difficult to restart](https://en.wikipedia.org/wiki/Black_start). Terence Tao has published [a wonderful set of slides](https://terrytao.files.wordpress.com/2010/10/cosmic-distance-ladder.pdf) covering some of this. - Back in ~200 BCE Eratosthenes measured the radius of the Earth with about 8% accuracy, that's stunningly accurate. (And illustrates another point: Eratosthenes did this far before "we" accepted the world was round. Scientific progress is a process of discovering ideas and, equally importantly, of reaching consensus on which to accept) - It's possible to measure the distance from Earth to the Moon in terms of the Earth's radius, knowing the radius of the Earth unlocks a new measurement! - When the ancient greeks first did this they didn't even have an accurate value for **π** yet, the ladder also relies on advances in math. The ladder is a sort of astronomical tech tree. - One of the primary goals of Cook's first voyage [was to observe a venus transit](https://en.wikipedia.org/wiki/1769_transit_of_Venus_observed_from_Tahiti), enabling a measurement of the distance between the earth and the sun! Measurement is expensive! - Earth moves around a fair amount while orbiting the sun, you can use the parallax this induces to come up with a bunch of distances to nearby stars! - The PDF continues and I highly recommend reading it, the details are fascinating.