Category: Things that are, in fact, the case

The list below is believed complete, though membership – like everything here – is subject to revision.

• The age of the Earth – Kelvin cooled the planet from first principles and got twenty-four million years. His own former assistant had the right answer in 1895, and let it go.
• Charles Babbage – Designed three calculating engines and completed none. Held Newton's chair eleven years, never lectured. The Times got his birth year wrong in his obituary.
• Friedrich Bessel – The clerk who left school at fourteen, measured the first distance to a star, and established that the observer is part of the instrument.
• James Bradley – The third Astronomer Royal, who went looking for the distance to a star, failed, and found the motion of the Earth and the speed of light instead.
• Celestial navigation – One sight does not say where you are. It says you are somewhere on a circle three thousand miles across, and the craft is everything after that.
• Circle – The set of points at one fixed distance from one point. A curve, not the region it encloses, and the definition several doctrines here decline to read.
• The CMB dipole – The warm side of the sky: a 3.4 millikelvin asymmetry caused entirely by our own motion, and used to calibrate the instruments that measure it.
• Deflection of the vertical – Down does not point at the centre of the Earth. The plumb line leans towards whatever rock is nearby, by a few seconds of arc, and the lean is a map.
• The dol – A real unit of pain. One dol was two just-noticeable differences of heat, the scale ran to ten and a half, and in 1947 a woman in labour reached the top.
• The double-slit experiment – The experiment that settled whether light is a wave, and is now chiefly cited for a claim about consciousness resting on one word being read as ordinary English.
• Earth – The third planet, and the one thing every other article here is about: an oblate spheroid since antiquity, flattened and taken personally ever since.
• Eclipse – Announced to the second, centuries ahead, on a named strip of ground. The one term that cannot be computed is how fast the Earth will be turning.
• Fata Morgana – Stacks a distant coast into towers, stretches a lighthouse into a column, hangs a ship above the sea. It cannot move any of them sideways.
• John Flamsteed – First Astronomer Royal. Forty-three years on a star catalogue, a refusal to print it unfinished, and a bonfire of the three hundred copies printed for him.
• Foucault's pendulum – A weight on a wire that reports the Earth's rotation without reference to the sky, at a rate that goes exactly as the sine of the latitude.
• Gas pressure needs a container – The objection that an atmosphere cannot sit against a vacuum unheld. The premise is right; the lid is not. The container is gravity, and it leaks anyway.
• General relativity – A theory of gravitation containing no force of gravity, built out of the fact that a falling man feels nothing at all.
• GPS – The most thoroughly relativistic machine in ordinary civilian use, whose clocks were detuned before launch, and which is regularly offered as evidence that relativity is unnecessary.
• Gravity – The weakest of the four forces and the worst-measured quantity in physics: sixteen laboratories have determined its constant and they do not agree.
• Hairy ball theorem – There is no way to comb a sphere flat, and the reason is a single number. Odd-dimensional spheres comb perfectly, which is the half nobody mentions.
• Edmond Halley – He paid for the Principia, invented the isoline map, and is remembered for a comet he did not discover, under a name given it after he died.
• Heliocentrism – The Earth goes round the Sun: correct, and believed for very nearly three centuries before anyone could show it.
• Edwin Hubble – He proved the universe is bigger than our galaxy, got every distance seven times too short, and refused to the end to call it expanding.
• The isotropic antenna – The reference that antenna gain is measured against. None has been built, and the standard defining it calls it hypothetical in the definition.
• Lord Kelvin – Fixed the bottom of the temperature scale and the age of the Earth. He got one of them right, and never said the sentence he is famous for.
• Henrietta Swan Leavitt – Employed at thirty cents an hour to measure star brightness on glass plates, she found the relation that tells us how far away everything is.
• The longitude problem – Not a question about where you are but about what time it is somewhere else. It was solved twice, by two methods that went to sea together for a century.
• Nevil Maskelyne – Fifth Astronomer Royal, remembered as the man who cheated John Harrison. He ran the trial in question, and it beat his own method by three to one.
• Min Min light – The light that follows travellers across western Queensland. Shown in 2003 to be a mirage of a car 300 km away, and still officially unexplained.
• Moon – The Earth's satellite, and its measuring instrument: one face turned to us, and the wrong reading of its density once hollowed out the planet.
• Mountweazel – A false entry planted in a reference work to catch plagiarists. Named after one of its own instances, and it proves the one thing that is not an offence.
• The myth of the flat Earth – That medieval Europe thought the world flat is a Victorian invention, assembled from a novelist's fiction and put to work in the quarrel over evolution.
• The Nautical Almanac – Published every year since 1767, and the reason longitude starts at Greenwich. Every figure in it was computed twice, by two men forbidden to confer.
• Nautical mile – A minute of the world, rounded off: the unit named for a minute of latitude, which had to stop being one before it could be used.
• Newton's bucket – The last of five arguments for absolute space, now quoted alone and in support of the position it opposed.
• The Ninth Bridgewater Treatise – The ninth volume of a series of eight. Nobody commissioned it; Babbage paid for it, and made his case on an engine built to break its own rule on cue.
• Null Island – 0°N 0°E, where there is no island. It was named, and given a square metre of land, so that data which failed to locate would land somewhere visible.
• Occam's razor – The principle of parsimony, named for a man who did not write the words, did not originate the idea, and was not the person the phrase was coined about.
• Orbits – A fall that keeps missing. The space station drops 4.32 metres a second, and over the 7.66 km it covers meanwhile, its own orbit drops 4.32 metres.
• Pi – 3.14159…, the ratio of a circle's circumference to its diameter, which is the one length nobody draws a circle with. Also written as half of tau.
• The Pioneer anomaly – A sunward drift on both Pioneers that outlasted the missions and stayed unexplained for thirty years. It was waste heat: the craft were pushing themselves.
• The Sagnac effect – The fringe shift from sending light both ways round a rotating loop, offered as the disproof of relativity, and in fact a direct measurement of the relativity of simultaneity.
• Schrödinger's cat – A thought experiment devised in 1935 to show a reading of quantum mechanics absurd, now the emblem of the reading it was built to ridicule.
• Selenelion – The Sun and the totally eclipsed Moon seen at once, from opposite horizons, in a geometry that places them exactly opposite and therefore forbids it.
• Special relativity – Two postulates and their consequences, named after the wrong half of itself: not that everything is relative, but that the speed of light is not.
• Sphere – The set of points at one fixed distance from one point in space. A surface, not the solid it bounds, and no part of it will lie flat.
• Stellar aberration – The annual tilt of every star by the same 20.5 arcseconds, found by a man hunting parallax, and now cited against the motion that causes it.
• Stellar parallax – The annual shift demanded as proof of the Earth's motion in 1543, not found for 295 years, and there the whole time.
• Stigler's law of eponymy – No scientific discovery is named after its original discoverer – a law its author credited to somebody else, making it an instance of itself.
• Sun – The nearest star, and the one this encyclopedia keeps calling as a witness: never quite where it is seen, and gone from the horizon before it looks it.
• Surveying – Plane surveying ignores the Earth's curvature up to about a hundred square miles. That is a tolerance with a number on it, not an opinion about the shape.
• Tau – The true circle constant: circumference over radius, twice pi, and the one that makes a quarter turn come out as a quarter. 6.28318…
• The telescope – Refused a patent in 1608 for being too easy to copy. Its most famous user did not invent it, did not name it, and was not the first to draw the Moon.
• 30 February 1712 – The only 30 February known to have occurred. Sweden converted to the Gregorian calendar gradually, lost track during a war, and got out by adding a day.
• Time – The second was once a fraction of the Earth's day. Since 1967 the day has been measured against a caesium atom, and the Earth has been losing.
• Vacuum – Defined by what is not in it, which makes it the one thing in physics described by a list. The list has been declared complete three times.
• Wave–particle duality – The finding that light and matter behave in ways the words wave and particle were built to keep apart. The difficulty is in the words.

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