In 1929 Edwin Hubble showed that distant galaxies are moving away from us, and the farther away they are, the faster they recede — evidence that the universe is expanding.
Why it matters: This discovery led to the Big Bang theory of how the universe began.
A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae — Edwin Hubble (1929) Proceedings of the National Academy of Sciences, vol. 15
Moving the Sun to the centre changed how we see our place in the universe.
Copernicus's book, published in 1543, argued that Earth and the planets orbit the Sun. Galileo's observations and Kepler's laws later supplied the evidence and the mathematics.
Why it matters: It showed that common sense can mislead us — careful observation must decide.
De revolutionibus orbium coelestium — Nicolaus Copernicus (1543)
A thousand years ago, a scholar measured the Earth from a hill near Jhelum.
At Nandana Fort, al-Biruni measured how far the horizon dipped from a hilltop and used trigonometry to estimate Earth's radius at about 6,340 km — close to today's 6,371 km.
Why it matters: With simple tools and clear reasoning, he came within about half of one per cent.
Tahdid Nihayat al-Amakin (Determination of the Coordinates of Places) — Abu Rayhan al-Biruni (c. 1025)