Ice cores preserve layers of snow, trapped air bubbles, dust and water isotopes. Together these record past temperature, greenhouse gases and atmospheric conditions, in some cases back 800,000 years.
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In 1854 Snow mapped cholera deaths in Soho, found them clustered around the Broad Street pump, and used interviews and records to explain apparent exceptions.
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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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Sailors compared local time with the time at a fixed point such as Greenwich. They got Greenwich time from a marine chronometer or from the Moon's position, a method called lunar distances.
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The usual explanation is that early clocks copied the shadow of a sundial in the northern hemisphere. The link is plausible but not firmly documented, and there is a notable objection.
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The 24-hour day comes from ancient Egyptian and Greek practice, while 60 minutes and seconds come from the Sumerian and Babylonian base-60 system, kept for its convenient divisibility.
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The most widely accepted explanation is that $ evolved from handwritten abbreviations for the Spanish peso, with the S written over the P. Other theories exist but have weaker support.
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Red barns came from cheap, farm-made paint tinted with iron oxide (rust). Red stayed popular because it was inexpensive and protective, not for symbolic reasons.
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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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Today the Moon's distance is measured by timing laser pulses bounced off reflectors left on its surface, accurate to about a millimeter. Ancient astronomers got a rough figure from eclipses.
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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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DNA comparisons, protein tests and shared anatomy all point the same way: humans sit inside the great apes, and chimpanzees and bonobos are our closest living relatives.
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Pasteur showed microbes cause fermentation and spoilage, and Pasteur and Koch then linked specific bacteria to anthrax and other diseases by isolating them and reproducing illness in animals.
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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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Astronomers split starlight into a spectrum and read the dark or bright lines, which act as a unique fingerprint for each chemical element.
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