We don't actually know how big — or how heavy — the universe is

Bryson opens by pointing out how astonishingly late and difficult it was for humans to even measure basic facts about the universe. Figuring out Earth's weight, for instance, took until the 1700s, when Henry Cavendish used a delicate torsion balance with lead spheres to measure gravitational attraction precisely enough to calculate the planet's mass — a fiddly experiment conducted alone, obsessively, by a brilliant but painfully reclusive scientist who barely spoke to other people.

Similarly, estimating the distance to stars and the age of the universe required strings of indirect inference stacked on top of each other, each step vulnerable to error, revised repeatedly across centuries. Bryson uses these stories to make a broader point: many "facts" we take for granted about the scale of the cosmos are the product of extraordinarily patient, unglamorous measurement by individuals working with primitive tools, often getting the number roughly right through more luck and persistence than genius.

Takeaway: the confident numbers in textbooks usually hide centuries of clumsy, hard-won measurement.