The lexicon

A chart reading and this shelf use about seventy words that nobody is born knowing. Here they are, in plain language, grouped by where you will meet them. Each entry says what the thing is, and where the tradition and the arithmetic part ways, it says that too.

In a chart

Natal chart. A map of the sky at the minute and place you were born: where the Sun, Moon and planets stood along the zodiac, which sign was rising on the eastern horizon, and how the sky divided into houses. The site next door computes it from an astronomical ephemeris, so the positions are the real ones.

Zodiac. The belt of sky the Sun, Moon and planets travel through, divided into twelve signs of thirty degrees each. The signs are a coordinate system first and a set of personalities second.

Ecliptic. The Sun's yearly road across the stars, which is really the plane of the Earth's orbit seen edge on. The planets keep near it, the zodiac is the belt around it, and the strip on the front page and the dome on the tonight page both draw it in gold.

Magnitude. The astronomers' scale of brightness, and it runs backwards: the brighter the star, the lower the number. The brightest stars sit near zero, and the faintest a good eye can catch under a dark sky are about six.

Tropical zodiac. The zodiac measured from the spring equinox, so that Aries always begins where the Sun crosses the equator heading north. This is what Western astrology has used since Ptolemy chose it around 150 AD, and what your chart uses.

Sidereal zodiac. The zodiac measured from the stars themselves. Indian astrology uses it, and it now sits about twenty-four degrees behind the tropical one. The two are measuring different things. Neither is a mistake.

Precession. The slow wobble of the Earth's axis that makes the equinox drift backward through the constellations, one degree every seventy-two years. It is why the constellation Aries has slid into the sign Pisces, and why someone will one day tell you your sign has changed. It has not.

Ascendant, or rising sign. The degree of the zodiac coming up over the eastern horizon at the moment of birth. It changes about every two hours, which is why a reading needs the time of birth and not just the day.

Midheaven. The degree of the zodiac at the top of the sky at the moment of birth, where the ecliptic crosses the meridian. The chart's high noon.

Houses. Twelve divisions of the sky counted from the Ascendant, each assigned a department of life: self, money, siblings, home, and so on. The signs say what; the houses say where.

Placidus. The most common way of drawing the house boundaries, worked out in the 1600s by an Italian monk who presented it as what Ptolemy meant. It divides the sky by time rather than by angle, which is why the houses come out unequal. The chart site defaults to it.

Whole sign houses. The oldest house system: each sign is one house, starting with the sign on the Ascendant. Simpler, older, and now fashionable again.

Aspect. The angle between two planets as seen from Earth. Ptolemy justified the important angles with the musical ratios, so Pythagoras is behind every one of these.

Conjunction. Two planets at the same degree, or near it. Their meanings merge.

Opposition. Two planets 180 degrees apart, facing each other across the sky. Tension that wants resolving.

Trine. Two planets 120 degrees apart, one third of the circle. The easy aspect: the two work together without being asked.

Square. Two planets 90 degrees apart. Friction; the aspect that makes you do something.

Sextile. Two planets 60 degrees apart. A mild, friendly trine.

Quincunx. Two planets 150 degrees apart, five signs on. An awkward angle: the two cannot see each other and have to be managed. Some readers ignore it; the one that started this site did not.

Orb. How far from exact an aspect can be and still count, usually a few degrees. Readers argue about orbs the way cooks argue about salt.

Decan. Each sign cut into three ten-degree faces, an Egyptian idea older than the zodiac itself. The first decan of a sign is the sign at full strength; the second and third are tinted by other planets.

Ruler, or rulership. The planet traditionally in charge of a sign: Mars rules Aries, Venus rules Taurus and Libra, and so on. A planet in the sign it rules is at home.

Exaltation. A sign where a planet is said to be at its best, as a guest of honour rather than at home. The Sun is exalted in Aries, the Moon in Taurus. A ranking from Babylon that Ptolemy kept.

Detriment and fall. The opposite of rulership and exaltation: the signs where a planet is said to be weak or uncomfortable.

Benefic and malefic. Ptolemy's sorting: Jupiter and Venus help, Saturn and Mars hinder, the Sun, Moon and Mercury depend on company. Modern readers soften the words; the sorting is still used.

Retrograde. A planet appearing to move backward against the stars for a few weeks or months. It is a trick of perspective as the Earth overtakes or is overtaken, and the Ptolemy essay has a machine that draws it. Mercury retrograde is the one everyone has heard of.

Element and mode. Each sign is fire, earth, air or water, and cardinal, fixed or mutable. Four elements times three modes make the twelve.

Ephemeris. A table of where the planets are, day by day. The site next door uses an astronomical one, checked against the Swiss Ephemeris, so your Saturn is where Saturn was.

Life path number. Your birth date added down to one digit. It comes from a system assembled in the United States in the early 1900s under Pythagoras's name. He had nothing to do with it and would have liked it anyway.

Among the ancients

Kosmos. The Greek word for order, and the name of this site. Pythagoras is said to have been the first to use it for the universe, meaning: the universe is an arrangement, not a heap.

Monochord. One string over a sounding box with a movable bridge. Pythagoras's instrument, and the first on this shelf: stop the string at a half, two thirds or three quarters and you get the octave, the fifth and the fourth.

Music of the spheres. The Pythagorean claim that the planets, moving at speeds in the same ratios as the musical intervals, sound a chord we cannot hear because we have never heard its absence. Kepler tried to write the notes down.

Tetractys. The triangle of ten dots, one plus two plus three plus four, that the Pythagoreans swore oaths on. Every numerology that reduces a number to a digit descends from it.

Demiurge. Plato's craftsman in the Timaeus, who makes the universe by arranging a mess according to the best pattern. Not a creator from nothing; an arranger.

World soul. Plato's claim that the universe is alive and has a soul, with the sky as its visible body. The Stoics kept it and gave it a physics.

Spindle of Necessity. The image at the end of Plato's Republic: the heavens turning on a spindle in the lap of Necessity, the stars one way and the seven wanderers the other. The instrument in the Timaeus essay.

Pneuma. The Stoic word for breath, meaning the living tension that runs through everything and holds it together. At its finest it is reason, the logos, which is also God, which is also fate.

Sympatheia. Feeling together. The Stoic claim that because the universe is one body, nothing in it is isolated, and the sky and the earth move together the way the Moon and the tides do. The foundation of every horoscope, and the word Huygens reached for when his clocks fell into step.

Logos. Reason, word, the ordering principle. For the Stoics, what the universe thinks with.

Hermetica. Greek dialogues from Egypt, second and third century AD, in which Hermes Trismegistus explains the universe. Claimed to be older than Moses; shown in 1614 not to be.

Emerald Tablet. A page of oracle talk that surfaces in Arabic around 800 AD and became the founding text of alchemy. Its second line is as above, so below.

As above, so below. The claim that the pattern of the heavens and the pattern of a life are one pattern at two scales. The premise of astrology stated as a law, and the question this whole site circles.

Epicycle and deferent. Ptolemy's circles: a planet rides the small circle, the epicycle, whose center rides the big one, the deferent, around the Earth. It draws retrograde motion correctly, because the epicycle is secretly the Earth's own orbit.

Almagest and Tetrabiblos. Ptolemy's two books: the mathematics of where the planets will be, and the four books on what it means. He did not think they were two subjects.

Harmonices Mundi. Kepler's 1619 book, The Harmony of the World, where he found his third law of planetary motion and the chord the planets sing. The site next door is named after it.

Synchronicity. Jung's 1952 word for two events connected by meaning rather than by cause: the scarab at the window. Not merely at the same time; at the same time and meaningfully so.

Archetype. Jung's term for the patterns a mind is born with: the mother, the hero, the shadow, the trickster. Cousins of the planetary gods, and he knew it.

Unus mundus. One world. Jung's name, borrowed from the Hermetic writers, for the single order that psyche and matter are both expressions of.

In the physics

Fine tuning. The observation that several numbers in physics, measured and typed into the equations by hand, sit in narrow ranges outside of which stars, carbon or chemistry would not exist. The dials in the fine-tuning essay let you move four of them.

Anthropic principle. Brandon Carter's 1973 name for the point that we should not be surprised to find the universe compatible with observers, since otherwise we would not be here to be surprised. The weak form is nearly a tautology. The strong form says the universe had to turn out this way, and most physicists reject it.

Cosmological constant. The energy of empty space, which pushes the universe apart. It is smaller than theory predicts by a factor with a hundred and twenty zeros, and if it were not, nothing would ever have gathered into a galaxy.

Multiverse. The proposal that there are vast numbers of universes with different constants, and we find ourselves in a rare one where living is possible. A lottery with so many tickets somebody had to win.

Block universe. The reading of relativity in which every moment, past and future, exists as a location in a four-dimensional block, and the present is only where your attention happens to be. Philosophers call it eternalism.

Light cone. The region of space and time a signal from here and now can reach, or could have come from. Drawn as two lines at forty-five degrees. Everything outside them is elsewhere, and cannot be now for everyone at once.

Relativity of simultaneity. The finding that two people moving relative to each other disagree about which distant events are happening at the same time, and neither is wrong. The light cone essay has a diagram you can walk through.

Simulation hypothesis. Nick Bostrom's 2003 argument that one of three things is true: civilizations never reach the technology to simulate their ancestors, or they never bother, or you are almost certainly in one.

Panpsychism. The view that mind, in some faint form, is a feature of matter all the way down, rather than something that switches on in brains. Taken seriously again by philosophers who could not find where else to put consciousness.

Among the shapes

Fractal. A shape that is rough at every scale, so that a piece of it looks like the whole. Mandelbrot's word, from the Latin for broken. Coastlines, ferns, lungs and clouds are fractal across a range of scales; the Mandelbrot set is fractal forever.

Mandelbrot set. The shape you get by squaring a number, adding a constant, and repeating, and colouring each constant by whether the numbers stay put. One line of arithmetic with an infinitely detailed edge.

Murray's law. The rule that when a pipe splits, the cubes of the daughters' radii add up to the cube of the parent's. It holds for blood vessels, airways and tree limbs because it is the least work a branching system can get away with.

Chladni figure. The pattern sand draws on a vibrating plate, settling where the plate is still. Each note has its own figure. Chladni showed them in 1787; Napoleon paid to have them explained.

Node. A place that does not move while the rest vibrates: the still lines on a Chladni plate, the fixed points on a plucked string.

Voronoi cells. The figure you get when things grow outward from scattered points at the same speed until they meet: a giraffe's patches, a dragonfly's wing, dried mud, the voids between galaxies. Named for Georgy Voronoi, 1908.

Reaction and diffusion. Turing's 1952 mechanism for spots and stripes: two chemicals, one that makes more of itself and one that stops it, spreading at different speeds. Enough to paint a leopard, a zebra and a pufferfish.

Phyllotaxis. The arrangement of leaves, seeds and florets on a plant. At the golden angle no seed ever lines up behind another, and the two families of spirals come out as neighbouring Fibonacci numbers.

Golden angle. 137.5 degrees, the angle that divides the circle in the golden ratio. Nudge it a fifth of a degree either way and a sunflower head falls apart into spokes.

Fibonacci numbers. 1, 1, 2, 3, 5, 8, 13, 21, 34, 55: each the sum of the two before. They count the spirals on a sunflower and a pinecone for a real reason, and they do not describe the nautilus, whatever the posters say.

Logarithmic spiral. A spiral that keeps the same shape as it grows, so that each turn is a fixed multiple of the last. The nautilus, the hurricane and the galaxy are all logarithmic spirals, for three different reasons and with three different ratios.

Surface tension. The skin on a liquid, made by the molecules at the surface being pulled inward by their neighbours. It is why a drop is a bead and a bubble is a ball, and it only wins while the thing is small: for water, under about three millimetres.

Hydrostatic equilibrium. The state of a body whose own weight has pulled every part of it as close to the centre as it can get, which makes a sphere, or a flattened one if it spins. Rock gives in at roughly four to six hundred kilometres across. Written into the definition of a planet in 2006.

Isotropy. The same in every direction. Where nothing picks a direction, the circle is what is left, which is the whole of the round essay in a word.

Conway's Life. A grid with three rules, written by John Conway in 1970, from which gliders, guns and a working computer have been built. The smallest universe anyone has made, and the instrument in the simulation essay.