Physical world

2026-09-26

The physical layer of an invented world: star and planet, plate tectonics, climate, rivers, biomes, ecology, resources, settlement, travel and maps. Physical plausibility is cheap to get right and expensive to get wrong: readers and players who know one fact (rivers don't split, deserts sit near 30°) stop trusting everything else. Method and consistency are on worldbuilding fundamentals; how environments shape cultures continues in societies and cultures. Figures are for Earth unless stated; invented examples are marked.

Star and planet

Start from Earth and change one or two parameters deliberately. Each change ripples through climate, life and culture.

The star

Star type	Color	Habitable zone	Worldbuilding notes
F	white	farther out, wider	shorter-lived than the Sun; more ultraviolet
G (the Sun)	yellow-white	~1 AU for Earth-like flux	the known case; main-sequence life around 10 billion years, currently about halfway
K (orange dwarf)	orange	closer and narrower	long-lived and stable; NASA notes K dwarfs as attractive targets in the search for life
M (red dwarf)	red	very close, narrow	the commonest stars; planets in the zone face intense X-ray and UV and frequent flares; close orbits make tidal locking likely

The habitable zone is the range of orbital distances where liquid water could exist on a planet's surface (NASA). Because starlight falls off with the square of distance, the distance that gives a planet the same stellar flux as Earth scales with the square root of the star's luminosity:

d_{\text{AU}} \approx \sqrt{L / L_\odot}

A star with a quarter of the Sun's luminosity puts the Earth-equivalent orbit at about 0.5 AU. Flux is only a start: atmosphere, greenhouse gases and cloud cover move a planet's real temperature a long way.

Rotation, tilt and orbit

Parameter	Earth	What changing it does
Day length	24 h	long days → larger day-night temperature swings, strong daily winds; very long days change sleep, work and farming rhythms
Tidal locking	no	one face always toward the star: permanent day side, night side and a twilight band; weather dominated by day-to-night circulation
Axial tilt	about 23.4°	tilt, not distance, causes seasons (NASA); zero tilt → almost no seasons; a large tilt → extreme seasons (Uranus is tilted about 98°)
Orbital eccentricity	small (distance varies by about 3%)	a strongly elliptical orbit adds a "near" and "far" season that affects both hemispheres at once
Year length	365.25 days	long years → long seasons, few birthdays, calendars based on other cycles
Moons	one large	tides, calendars (months), night light, myth; several moons → irregular tides and complex calendars

Tides

Tides come mainly from the Moon's gravity; the Sun's tide-generating force is about half the Moon's because tidal force falls with the cube of distance (NOAA).
Earth rotates through two tidal bulges each lunar day of 24 hours 50 minutes, so most coasts get two highs and two lows in that time.
Spring tides (largest range) happen when Sun and Moon line up, at new and full moon; neap tides (smallest) at the quarter moons.
Local tidal range depends heavily on coastline shape: funnel-shaped bays amplify it; enclosed seas (the Mediterranean) have small tides.

Worldbuilding use: tide times drive fishing, harbor access, causeway crossings and ambushes. A world with a larger or closer moon has bigger tides (tidal force scales with the moon's mass over distance cubed).

Gravity

g = \frac{GM}{R^2} = \frac{4}{3}\pi G \rho R

For the same average density $\rho$ , surface gravity scales with radius. Mars (0.38 g) and the Moon (about 0.17 g) show the low end. Qualitative consequences to reason through (not precise predictions): a small, low-gravity world holds its atmosphere less well over geological time; high gravity favors stockier land animals and makes falls and flight costlier; low gravity allows taller structures and bigger flyers.

Plate tectonics and landforms

Mountains, volcanoes, rifts and island chains are not scattered at random: they mark plate boundaries. Sketch the plates first and the landforms follow.

Boundary	Motion	Landforms	Real example
Divergent (ocean)	plates pull apart; new crust forms	mid-ocean ridge, volcanic islands on the ridge	Mid-Atlantic Ridge, Iceland
Divergent (continent)	continent splitting	rift valley, long lakes, volcanoes	East African Rift
Convergent: ocean under continent	oceanic plate subducts	deep trench offshore, volcanic mountain range inland, big earthquakes	Peru–Chile Trench and the Andes
Convergent: ocean under ocean	one oceanic plate subducts	trench plus a curved volcanic island arc	Mariana Trench and islands
Convergent: continent–continent	neither subducts; crust crumples	highest ranges, broad plateau, few volcanoes	India–Asia collision → Himalaya
Transform	plates slide past sideways	fault lines, offset rivers and ridges, earthquakes, few mountains	San Andreas Fault
Hotspot (not a boundary)	a plate moves over a fixed plume	a chain of volcanoes getting older in one direction	Hawaiian chain

Rules of thumb:

Mountain ranges are long and linear or arcuate, running parallel to the boundary that made them; they are not isolated cones scattered on a plain (single volcanoes excepted).
Young ranges are high and jagged; old ranges are low and rounded because erosion wins over hundreds of millions of years.
Plates move a few centimeters a year (the Mid-Atlantic Ridge spreads at about 2.5 cm/yr; the East Pacific Rise at over 15 cm/yr, per the USGS). Nothing changes on human timescales except by earthquakes and eruptions.
A subduction coast has a narrow shelf, a trench offshore and mountains close to the sea; a passive margin (the trailing edge of a continent, like the US Atlantic coast) has a wide shelf and a broad coastal plain.

Climate

Global circulation

Solar heating is strongest near the equator. The three-cell model describes the resulting average circulation in each hemisphere:

Cell	Latitudes	Surface winds	Consequences
Hadley	0–30°	trade winds blow toward the equator from the east (north-east in the northern hemisphere, south-east in the southern)	rising air at the equator (the ITCZ) → rain; sinking air near 30° → subtropical highs, clear skies, deserts
Ferrel	30–60°	westerlies	changeable weather; storms travel west to east; a "zone of mixing"
Polar	60–90°	polar easterlies	cold, dry sinking air at the poles; storms where polar and mid-latitude air meet near 60°

The Intertropical Convergence Zone (ITCZ) follows the Sun seasonally, which creates wet and dry seasons in the tropics (monsoons are the extreme case, amplified by large landmasses).
The great hot deserts (Sahara, Arabian, Kalahari, Australian, Sonoran) cluster near 30° north and south under the subtropical highs.
Faster rotation or a larger planet changes the number and width of cells; a tidally locked world has a completely different pattern. Treat the three cells as an Earth result, not a law.

Rain shadows

A rain shadow: wet windward slopes, dry leeward lowlands

Moist air forced over mountains cools as it rises and drops its water on the windward side. Descending on the leeward side it warms by compression and dries, producing warm, dry downslope winds (foehn, chinook) and a dry rain shadow. Real examples: the Great Basin of Nevada and Utah (behind the Sierra Nevada and Cascades), eastern Patagonia (behind the Andes), the Tibetan Plateau and Tarim Basin (behind the Himalaya).

To place one: find the prevailing wind for that latitude (trades or westerlies), put the wet side facing it, the dry side behind.

Ocean currents and coasts

Current type	Where	Temperature	Climate effect
Western boundary currents	west side of ocean basins (off continents' east coasts)	warm, narrow, fast (Gulf Stream, Kuroshio, Agulhas)	warm, humid coasts; moisture for storms
Eastern boundary currents	east side of ocean basins (off continents' west coasts)	cold, broad, slow (California, Humboldt, Benguela, Canary)	cool coasts, fog, stable dry air; coastal deserts in the subtropics (Atacama, Namib); upwelling brings nutrients → rich fisheries

Mid-latitude west coasts receive the westerlies straight off the ocean: mild, wet, small seasonal range (maritime: Britain, the Pacific Northwest). East coasts at the same latitude get wind that has crossed the continent: hotter summers, colder winters. Between roughly 30° and 45° on west coasts, Mediterranean climates (dry summers, wet winters) form where the subtropical high dominates in summer and the westerlies in winter.

Köppen climate groups

Wladimir Köppen first published his classification in 1884 (revised 1918 and 1936; Rudolf Geiger's changes in 1954 and 1961 give the Köppen–Geiger version). It classifies by temperature and precipitation, which is exactly what a worldbuilder needs.

Group	Name	Defining criterion	Typical location
A	Tropical	coldest month ≥ 18 °C	equatorial belt; rainforest (Af), monsoon (Am), savanna (Aw)
B	Arid	annual precipitation below a threshold that rises with temperature (and depends on whether rain falls in summer or winter); desert (BW) under half the threshold, steppe (BS) between half and all of it	subtropical highs near 30°, rain shadows, continental interiors
C	Temperate	coldest month between 0 °C (or −3 °C) and 18 °C; at least one month above 10 °C	mid-latitudes; Mediterranean (Cs), humid subtropical (Cfa), oceanic (Cfb)
D	Continental	coldest month below 0 °C (or −3 °C); at least one month above 10 °C	continental interiors of the northern hemisphere; includes taiga (Dfc)
E	Polar	every month below 10 °C	tundra (ET), ice cap (EF); high latitudes and high mountains

Altitude mimics latitude: temperature falls with height, so a tropical mountain can climb from rainforest to tundra.

Water and rivers

Rivers are the most common map error. The physics is simple: water flows downhill by the steepest available route until it reaches the sea or a basin with no outlet.

Rule	Why	Common map mistake
Rivers merge, they don't split	tributaries join as they descend; the network is a tree whose trunk is at the sea	a river forking into two that each reach the sea
Splitting happens in deltas and on alluvial fans	low slope and sediment make channels divide near the mouth (distributaries)	distributaries in the uplands
Rivers start in high ground and run to the coast	rain collects in valleys	rivers running along ridges or between two seas across a mountain range
Rivers flow away from the nearest divide	the drainage divide is the ridge line	rivers from both sides of a range flowing the same way
Short, steep rivers on the side where mountains are near the coast; long, gentle ones on the other side	slope	equal rivers on both sides of an off-center range
A lake usually has one outlet	the lowest point of its rim	a lake draining in two directions
Endorheic basins have no outlet; water leaves by evaporation, so lakes are salty (Caspian, Great Salt Lake, Dead Sea)	about 18% of Earth's land drains inland	a closed basin with a fresh-water lake and no explanation
Rivers widen downstream	more tributaries, more water	a river that narrows toward the sea
Rivers meander on flat land and cut valleys and gorges in hills	energy and slope	ruler-straight lowland rivers

The Casiquiare, which links the Orinoco and Amazon basins, is a famous real exception; use such things sparingly and make them notable in-world.

Biomes

Biomes follow climate (Robert Whittaker's classic diagram plots them against mean annual temperature and precipitation). Put the climate first and the biome follows.

Biome	Köppen	Climate drivers	Typical life and human use
Tropical rainforest	Af	hot all year, heavy rain spread through the year (typically 1,750–3,000 mm), under the ITCZ	extreme biodiversity; nutrient-poor, leached soils; shifting cultivation; travel by river
Savanna	Aw	hot, long dry season alternating with a wet one as the ITCZ moves	grassland with scattered trees; large grazing herds and predators; pastoralism; fire
Hot desert	BWh	subtropical high, rain shadow or cold current	sparse life; oases and river valleys hold people; caravan trade; salt
Mediterranean	Cs	dry hot summers, mild wet winters; west coasts about 30–45°	scrub and hardy trees; olives, vines, wheat; sea trade; wildfire
Temperate forest	Cfb, Cfa, Dfb	reliable rain, four seasons	deciduous or mixed woods; cleared for farming; timber
Temperate grassland (steppe, prairie)	BSk, Dfa/Dfb edges	too dry for trees, cold winters in continental interiors	deep fertile soils; horses and herding; nomadic empires; later breadbaskets
Taiga (boreal forest)	Dfc, Dfd	long cold winters, short summers	conifers, furs, timber; thin population
Tundra	ET	warmest month under 10 °C; permafrost	no trees; caribou or reindeer; hunting and herding

Ecology

Trophic levels: producers (plants, algae) → herbivores → carnivores → top predators. Energy passing up each step is a small fraction of the level below. The "10% rule" is attributed to Raymond Lindeman's 1942 paper "The trophic-dynamic aspect of ecology", though Lindeman did not call it a law and the efficiencies he cited ranged from about 0.1% to 37.5%.

Consequences worth building in:

Apex predators are rare. If a region supports a million tonnes of plants, it supports far fewer tonnes of deer and far fewer again of wolves. A land crawling with dragons needs an enormous food base, a magical energy source, or very few dragons.
Carrying capacity: the population an environment can sustain indefinitely. Populations overshoot and crash; famine cycles and migration follow.
Big animals need big ranges. A large predator's territory can be hundreds of square kilometers.
Food chains are short (rarely more than four or five levels) for the same energy reason.
Islands produce odd results: dwarfism in large animals, gigantism in small ones, flightless birds, vulnerability to invaders.

Domestication

Jared Diamond's Guns, Germs, and Steel (1997) argues that only a handful of large mammals were domesticated because a candidate must pass every test (his "Anna Karenina principle"):

Criterion (Diamond)	Fails it
Diet: cheap to feed	large carnivores
Growth rate: matures quickly	elephants, gorillas
Breeds in captivity	species needing elaborate courtship or space
Disposition: not too dangerous	grizzly bears, zebras (hard to tame)
No tendency to panic	gazelles, many deer
Social structure: herds with hierarchy the human can top	solitary, territorial species

Critiques: geographers and anthropologists (James Blaut; the essays in Questioning Collapse, 2010) accuse the book of environmental determinism and of underplaying human agency and institutions. Use the criteria as a checklist for invented beasts of burden (why do people ride these and not those?), not as a theory of history.

Resources

Where resources occur shapes where people settle, what they fight over and what they trade.

Resource	Where it tends to occur	Settlement and trade effects
Fertile soil	river floodplains and deltas (silt renewed by floods), volcanic soils, loess, grassland soils	dense farming populations, early states, flood control, irrigation politics
Fresh water	rivers, springs, lakes, wells; oases in deserts	the first constraint on every settlement
Copper	commonly porphyry deposits in subduction-related volcanic arcs (the Andes are the largest concentration)	early metalworking; long trade routes
Tin	almost always associated with granite; much from stream (placer) deposits downstream of lodes	rare and localized, so Bronze Age powers depended on long-distance tin trade
Iron	widespread; bog iron in wetlands was the main source in the Viking Age	iron is common, so iron tools democratise; the limit is fuel and skill
Coal	sedimentary basins formed from ancient swamp vegetation	fuel for smelting and later industry; coalfields become industrial regions
Salt	sea-salt pans on dry coasts, rock salt, salt lakes in endorheic basins	essential for people, livestock and preserving food; taxed and fought over
Timber	forests; scarce in steppe, desert and Mediterranean hills after clearance	ships, fuel and building; deforestation drives imports
Stone	local geology	building style follows it: timber, mudbrick, limestone, granite

Settlements

Every real town sits where it does for reasons. Give yours at least two.

Factor	Examples
Water	a river, spring or well; the first requirement
Food	fertile land within a day's walk; fishing grounds
Crossing points	fords, bridges, ferries: roads converge there, so markets do too
Confluences	two rivers meeting: two trade routes meeting
Harbors	sheltered bays, river mouths, lagoons; deep water close to shore
Defense	hills, river bends, islands, peninsulas, marsh-protected ground
Trade routes	passes, oases, portages between rivers, the head of navigation (the highest point ships can reach upriver)
Resources	mines, salt, timber, quarries, pilgrimage sites
Administration	a crossroads chosen as a capital, a fort that grew a town

Spacing: pre-modern farm villages cluster within walking distance of their fields, and market towns tend to sit about a day's return trip apart for the farmers who use them. Big cities need a large food hinterland or cheap water transport (river or sea) to feed them.

Travel times

Pre-modern travel rates, sustained over days, from the ORBIS model of the Roman world (Scheidel, Meeks and Weiland, Stanford, 2012). Treat these as rough historical averages, not maxima: terrain, weather, season, roads and logistics move them a lot.

Mode	Sustained km per day	Notes
Ox cart	12	heavy freight
Porters, heavily loaded mules	20
Walking, armies on the march, pack animals, mule carts, camel caravans	30	the default for people on foot
Routine private vehicle	36	with convenient rest stops
Accelerated private vehicle	50
Routine horseback	56
Rapid military march, no baggage	60	short-term only
Fast carriage (state post, couriers)	67	with changes of animals
Continuous horse relay	250	messages only; an upper bound
River boat downstream	commonly about 65	varies by river
River boat upstream (towed)	about 15	slow and costly
Sea (a mostly-sea route, Carthage to London, July)	about 138	wind and season dominate; the same trip in January, avoiding the Atlantic, took about 93 days at 36 km/day

Worldbuilding consequences: water is faster and far cheaper than land for bulk goods, so coasts and rivers are where trade and cities concentrate; winter closes seas; a kingdom 1,000 km across is a month's journey on foot, so central control is weak and news is weeks old.

Maps

Scale and projection

Pick a scale before drawing (a scale bar, and how many days' walk it represents). Check every distance against the travel table.
A whole planet cannot be flattened without distortion. Mercator (conformal) keeps local shapes but inflates areas toward the poles; equal-area projections keep areas but distort shapes. For a region of a few hundred kilometers, ignore projection; for a world map, choose one and note it.
Latitude matters for climate: mark the equator or at least a latitude range on the world map, then check deserts, forests and ice against it.

Coastlines

Real coastlines are fractal: their measured length grows as the ruler shrinks (Benoit Mandelbrot, "How Long Is the Coast of Britain?", Science, 1967). Practical results:

Coasts should have detail at every scale (bays within bays), not smooth blobs.
Coast character follows geology: fjords in glaciated mountains, rias in drowned river valleys, straight sandy coasts with barrier islands on passive margins, cliffs where hard rock meets the sea.
Noise-based generators produce plausible coastlines quickly (see game worlds), but still need a human pass for rivers and mountains.

Labeling conventions

Convention	Why
Water features in italic (traditionally blue)	separates water from land names at a glance
River names follow the river's curve	readers link the name to the feature
Region names spaced out across the region	shows extent without borders
Size and weight of text by importance	a visual hierarchy: capitals and seas largest
Few fonts (one serif, one italic)	clarity
Don't label everything	empty space is information: wilderness, the unknown

Common mistakes

Mistake	Why it's wrong	Fix
Rivers that split or connect two seas	water flows downhill and merges	redraw as a tree draining to one outlet; put splits only in deltas
Rivers flowing away from the sea into mountains	uphill flow	start rivers in high ground
Desert next to rainforest with no barrier	no climate cause	add a mountain rain shadow, latitude change or cold current
Mountains in random blobs or a lone ring	tectonics makes long ranges at boundaries	sketch plates first
Uniform climate across a continent	latitude, altitude and coast position vary	apply the three-cell model and Köppen groups
Every coast the same	geology and currents vary	mix fjords, cliffs, beaches, deltas
Travel at car speeds	armies and caravans are slow	use the travel table
Huge inland cities with no river	food and water can't reach them	put big cities on navigable water
Too many apex predators	the energy pyramid can't support them	fewer, larger ranges, or a special food source
Seasons from distance to the sun	seasons come from tilt	set an axial tilt
Two moons with Earth tides	extra moons change tides and calendars	decide their effect or make one moon small and distant
Resources everywhere	then nobody trades or fights for them	concentrate scarce ones; let geology choose

Templates

Planet sheet

PLANET:
STAR: type (F/G/K/M), luminosity vs Sun:
ORBIT: distance (AU), year length, eccentricity:
DAY LENGTH:             TIDALLY LOCKED? y/n
AXIAL TILT:             SEASONS:
MOONS: number, size, period, tide effect:
GRAVITY (vs Earth):     ATMOSPHERE:
OCEAN / LAND RATIO:
PLATES: sketch attached (boundaries marked by type)
CIRCULATION: cells, prevailing winds by latitude
MAJOR CURRENTS: warm / cold, which coasts
CLIMATE BANDS: Köppen groups by latitude
DIFFERENCES FROM EARTH (and their ripples):
  1.
  2.

Region sheet

REGION:                 LATITUDE RANGE:
SCALE: 1 cm = __ km = __ days on foot
TERRAIN: mountains (why here?), plains, coasts
PREVAILING WIND:        RAIN SHADOW SIDE:
CLIMATE (Köppen):       SEASONS:
RIVERS: sources -> mouths (tree, no splits)
LAKES: outlet / endorheic (salty?)
BIOMES:
KEY RESOURCES and where:
SCARCE RESOURCES (imported from):
SETTLEMENTS: name | why here (2 reasons) | size
ROUTES: roads, rivers, passes, sea lanes, travel days
HAZARDS: floods, quakes, storms, droughts, fires
SEASONAL CONSTRAINTS: closed passes, winter seas

Map-making checklist

[ ] Scale bar and days-of-travel conversion
[ ] Latitude marked; climate checked against it
[ ] Plates sketched; ranges follow boundaries
[ ] Prevailing winds drawn; rain shadows placed
[ ] Cold and warm currents; coastal deserts if any
[ ] Rivers: high ground -> sea or closed basin
[ ] No river splits except deltas and fans
[ ] Lakes have one outlet or are salty
[ ] Biomes follow climate
[ ] Coastlines detailed at several scales
[ ] Settlements have 2+ reasons to exist
[ ] Big cities on navigable water
[ ] Travel distances checked against the story
[ ] Labels: water italic, rivers along course
[ ] Deliberate blank areas (unknown lands)

References

NASA Space Place: What causes the seasons? (opens in a new tab): axial tilt, not distance, causes seasons
NASA Science: The habitable zone (opens in a new tab): definition; dependence on star type; M-dwarf flares and radiation; K dwarfs
NOAA: What causes tides? (opens in a new tab): Moon vs Sun tidal force, inverse cube of distance
NOAA: The lunar day (opens in a new tab): 24 h 50 min and two tides per lunar day
USGS: This Dynamic Earth, Understanding plate motions (opens in a new tab): boundary types, landforms, spreading rates
Atmospheric circulation (Wikipedia) (opens in a new tab): Hadley, Ferrel and polar cells; trades, westerlies, ITCZ, horse latitudes
Rain shadow (Wikipedia) (opens in a new tab): mechanism and examples
Boundary current (Wikipedia) (opens in a new tab): warm western and cold eastern boundary currents, upwelling
Köppen climate classification (Wikipedia) (opens in a new tab): group definitions and thresholds; history
Endorheic basin (Wikipedia) (opens in a new tab): closed basins, about 18% of land, salt lakes
Distributary (Wikipedia) (opens in a new tab): where rivers split (deltas, alluvial fans)
Biome (Wikipedia) (opens in a new tab): Whittaker's temperature–precipitation classification
Lindeman, R. L. (1942), "The trophic-dynamic aspect of ecology", Ecology 23 (opens in a new tab): origin of trophic efficiency estimates
Ten percent law (Wikipedia) (opens in a new tab): the rule of thumb and Lindeman's 0.1–37.5% range
Jared Diamond, Guns, Germs, and Steel (W. W. Norton, 1997): domestication criteria
Anna Karenina principle (Wikipedia) (opens in a new tab): Diamond's six criteria summarized
Guns, Germs, and Steel (Wikipedia) (opens in a new tab): critical reception, Blaut, Questioning Collapse
Porphyry copper deposit (Wikipedia) (opens in a new tab), Tin (Wikipedia) (opens in a new tab), Bog iron (Wikipedia) (opens in a new tab): where metals occur
Scheidel, Meeks and Weiland, ORBIS: The Stanford Geospatial Network Model of the Roman World (2012) (opens in a new tab): travel rates by mode, river and sea speeds
Mandelbrot, B. (1967), "How Long Is the Coast of Britain?", Science 156 (opens in a new tab): coastlines as fractals