How we know

Radiocarbon dating measures how much carbon-14 remains in once-living material. The decay is steady, so the remainder indicates time since death, after calibration.

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Most specialists attribute the extinction of the non-bird dinosaurs 66 million years ago to the Chicxulub asteroid impact, backed by iridium, a crater, and global ejecta layers.

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Around 240 BC, Eratosthenes compared the Sun's shadow angle at Alexandria with the overhead Sun at Syene, then scaled up the distance between them to get Earth's circumference.

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Nobody has sampled the core. Its iron-nickel makeup is inferred from earthquake waves, density, meteorites, magnetism and lab experiments, while the lighter elements remain debated.

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In 1838 Friedrich Bessel published the first reliable stellar parallax, for 61 Cygni, using Earth's orbit as a baseline. Henderson and Struve made rival claims.

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Cavendish used a lead-sphere torsion balance to measure tiny gravitational pulls, then compared them with the Earth's pull to find its density: about 5.48 times that of water.

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Scientists read pigment-bearing structures called melanosomes in exceptionally preserved fossils. This gives fairly good evidence for some patterns, but less certainty about true hues.

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Evidence built up over two centuries: fixed mass ratios in chemistry, the kinetic theory of gases, Brownian motion, scattering experiments, and finally instruments that detect single atoms.

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Scientists rely on randomized trials where possible, and otherwise build a case from timing, strength, consistency, mechanism and dose-response while ruling out confounders and reverse causation.

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P-hacking is running many analyses or tweaking choices until a result looks statistically significant, then reporting it selectively. This inflates false positives.

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Random assignment balances known and unknown differences between groups, so differences in outcomes can be attributed to the treatment. The design has costs and limits.

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Scientists date meteorites and Earth's lead isotopes using radioactive decay, giving an age of about 4.54 billion years, because Earth's own earliest rocks no longer survive.

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