design.md

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`866c7f96` 2026-09-07 Pavel Safronov	1	# Tower — Design
	2
	3	Tower is the setting of a novel. This app exists to serve that: first as an
	4	instrument for the author to see the place and keep it consistent, and second as a
	5	companion piece readers can explore. Where a choice trades spectacle against
	6	coherence, coherence wins — canon here is series-bible material, and the book is
	7	downstream of it.
	8
	9	Tower is a smart cylinder in orbit around Earth, spun for 1 g, with an inner
	10	surface the size of Wisconsin, lit by sunlight piped down its axis and scattered by
	11	a swarm of mirrors it flies itself. It is also a person.
	12
	13	The app has three linked views:
	14
	15	1. Exterior — a 3D render of Tower from outside, with its collector, beam and
	16	orbit. You pick a point on the hull.
	17	2. Map — a 2D unrolled map of the inner surface. You pick a point here too; the
	18	two pickers stay in sync.
	19	3. Interior — a first-person "streetview" from the picked point, with live
	20	controls for time of day and spin phase.
	21
	22	---
	23
	24	## 1. Canon
	25
	26	### 1.1 Dimensions
	27
	28	All numbers derive from two free parameters — radius and length — chosen so the
	29	inner surface matches Wisconsin.
	30
	31	\| Quantity \| Value \| Notes \|
	32	\|---\|---\|---\|
	33	\| Radius \| 50 km \| floor to axis \|
	34	\| Length \| 540 km \| sun cap to far cap \|
`de65ab10` 2026-09-08 Pavel Safronov	35	\| Wall depth \| 1 km \| floor to outer skin, D-044 \|
`866c7f96` 2026-09-07 Pavel Safronov	36	\| Circumference \| 314.16 km \| the world's "width" \|
	37	\| Inner surface area \| 169,646 km² \| Wisconsin total area, 169,635 km² \|
	38	\| Habitable land area \| 169,646 km² \| all of it — the hull is sealed (§1.4) \|
	39
	40	Every square metre of the inner surface is land. There are no windows in the wall,
	41	so nothing is subtracted. *An earlier version sized star slats to leave exactly
	42	Wisconsin's land area of 140,268 km²; D-017 removes them and that second
	43	coincidence with it.*
	44
`1affced2` 2026-09-08 Pavel Safronov	45	The Wisconsin comparison is a fact about area alone, and the honest form of it is
	46	worth stating: the areas match and the shapes do not. Wisconsin spans about
	47	525 km east–west against a circumference of 314, so at 1:1 it cannot be laid on
	48	this surface at all — it overlaps itself by 67%. Feeling the size needs a region
	49	shaped like a corridor rather than a blob, which is what §2.5 is for.
	50
`866c7f96` 2026-09-07 Pavel Safronov	51	### 1.2 Spin and gravity
	52
	53	\| Quantity \| Value \|
	54	\|---\|---\|
	55	\| Angular velocity ω \| 0.014005 rad/s \|
	56	\| Spin period \| 448.6 s = 7 min 29 s \|
	57	\| Rotation rate \| 0.134 rpm \|
	58	\| Rim speed \| 700.2 m/s \|
	59	\| Floor gravity \| 1.000 g by construction \|
	60
	61	Consequences that the app should show or at least respect:
	62
	63	- Gravity falls off linearly with height. g(h) = ω²(R−h). A 1 km peak sits at
	64	0.98 g; the axis is weightless. Mirrors at the axis float.
	65	- Coriolis is mild but real. 2ω = 0.0280 s⁻¹. Walking is unaffected (0.4 % g).
	66	A ball thrown at 30 m/s deflects at 8.6 % g — enough that Tower's sports differ
	67	from Earth's, and enough that thrown objects visibly curve.
`769726ea` 2026-09-08 Pavel Safronov	68	- Nothing in the sky moves, except through the far cap. The wall is sealed
	69	(§1.4), so along 539 of the 540 km there is nothing to wheel past — the far side
	70	of the world overhead co-rotates with you and hangs perfectly still. The one
	71	exception is the far cap window, where the sky is outside and does not
	72	co-rotate: it turns once every 7 min 29 s about a single motionless star on the
	73	axis. The spin is invisible upward and obvious deepward, and only at night.
`866c7f96` 2026-09-07 Pavel Safronov	74	- But everything that falls leans. Rain hangs at a permanent 1.47° slant, smoke
	75	drifts the other way, and a 1 km waterfall lands 133 m off its own cliff (§1.9.2).
	76	This inversion is the alien fact the interior view must sell: **the sky is still
	77	and the water is not.**
	78
	79	### 1.3 The sun system
	80
	81	Sunlight enters through the sun-facing end cap and runs the full 540 km axis.
	82
	83	- Collector. A mirror array flies ahead of Tower, gathering sunlight and
	84	concentrating it into the beam. **46,900 km² of aperture — a disc ~244 km
	85	across**, two and a half times Tower's own diameter. It is the dominant visual
	86	element of the exterior view.
	87	- Beam. 10 km in circumference — 1,592 m radius, 7.96 km² cross-section —
	88	carrying 54.3 TW at a concentration of 5,000×, or 6.8 MW/m². It runs the
	89	axis from sun cap to far cap.
	90	- Diffusers. Not lamps — a swarm. See §1.3.1.
	91	- The beam is visible, and it glows by itself. There is real air at the axis
	92	(§1.5), and a 6.8 MW/m² beam Rayleigh-scattering through 3.18 km of it radiates
	93	779 W/m²/sr — **26× brighter than clear blue sky, 25,700× dimmer than the
	94	sun's disc.** It is a brilliantly sunlit cloud extruded into a rod, safe to look
	95	at directly. From the floor 50 km below it is **3.65° wide, about seven
	96	sun-widths**, running the whole length of the sky. This glow is a property of the
	97	beam and the air, not of the diffusers — it would be there if the beam were empty.
	98
`9c891d2e` 2026-09-08 Pavel Safronov	99	#### 1.3.0 Tower never sees the sun
	100
	101	The collector is 244 km across and flies 320 km ahead of the sun cap, which puts
	102	it 41.8° wide in Tower's sky against the sun's 0.53° — **79 times the sun's
	103	diameter**. It sits squarely between Tower and the sun, so it occults it completely,
	104	and the umbra it casts is 120 km across where Tower's 51 km sits inside it. The
	105	eclipse is total, permanent, and self-inflicted.
	106
	107	So nobody aboard can see the sun, for the same reason nobody can see Earth
	108	(§1.4). What they can see, through the sun cap window, is the back of the
	109	collector: 46,938 km² of structure radiating its own waste heat at about 245 K —
	110	infrared, and therefore black. It is the only object in Tower's sky that is neither
	111	Tower nor a star, and it never moves, because the collector flies in formation
	112	rather than spinning. D-050.
	113
`49b0d64d` 2026-09-08 Pavel Safronov	114	Its edge is not black. The face is a mirror pointed away, but the collector is
	115	backlit, and the structure holding 46,938 km² flat stands proud of the mirror
	116	plane at the rim, where the sun strikes it side-on. With the scatter any real mirror
	117	throws at grazing incidence, the edge reads as a thin sun-coloured ring — so the
	118	collector is a hole in the stars with a lit rim rather than an absence, which is how
	119	anything backlit looks. It is not sunlight leaking past the edge: Tower is wholly
	120	inside the umbra, so the sun's disc is hidden from every point in the world. The ring
	121	is lit structure. D-054.
	122
`866c7f96` 2026-09-07 Pavel Safronov	123	#### 1.3.1 The diffuser swarm
	124
	125	The diffusers must be individually invisible, or the sky becomes a row of separate
	126	suns. Two thresholds govern this, and only the second one binds:
	127
	128	- Resolvability. At 50 km, one arcminute is 14.5 m. Anything larger is a
	129	resolved disc with a visible edge.
	130	- Visibility. Being unresolved is not enough — stars are unresolved and we see
	131	them. A mirror preserves radiance, so a mirrorlet aimed at an observer shows
	132	the sun's own surface brightness over a tiny angle. A 1 m mirrorlet reads at
	133	magnitude −13.6, brighter than the full moon. Blending into daylight requires
	134	~1.2 cm, at magnitude −4.0, just under Venus.
	135
	136	Diverting 54.3 TW needs 11.3 km² of mirror, so:
	137
	138	\| \| \|
	139	\|---\|---\|
	140	\| Mirrorlet size \| 1.2 cm, ~1.4 cm², 2 mg \|
	141	\| Count \| 78 billion \|
	142	\| Total mass \| 158 tonnes — the entire sun system \|
	143	\| Mean spacing in beam \| 3.8 m — sparse enough to fly through \|
	144	\| Optical depth over 540 km \| 1.41, so 24 % of light reaches the far cap \|
	145
	146	The swarm cannot be passive — it must fly, always. The axis is not weightless.
	147	At the beam's edge gravity is 0.312 m/s², enough that a foil sinks outward at
	148	0.26 m/s and leaves the beam in 1.7 hours. A drifting swarm dissolves the same
	149	afternoon. Every foil is therefore under continuous individual control. This is not
	150	a flourish on a working system; it is the only configuration that works.
	151
	152	They have absurd power to do it with. Each foil intercepts 979 W of beam.
	153	Harvesting 0.1 % gives it ~1 W on a 2 mg body — 489,600 W/kg, some fifty times
	154	a jet engine's specific power.
	155
	156	\| \| \|
	157	\|---\|---\|
	158	\| Down-beam, riding radiation pressure \| 0.85 m/s, 7.4 days \|
	159	\| Back up-beam under its own power at 1.5 m/s \| 15 µW, 4.2 days \|
	160	\| Harvestable from ambient world-light \| 14,400 µW — a 1000× margin \|
	161
	162	So there is no recirculation machinery, at either cap. The foils fly home on
	163	scavenged daylight. Nothing needs to be built for them, and the 1000× power margin
	164	means Tower can afford to move them wastefully — in spirals, shoals and patterns
	165	that cost energy for no reason but preference.
	166
	167	Lighting is a render target, not a schedule. 78 billion independently aimed
	168	mirrors put any intensity on any patch of ground at any time. Dawn, dusk, moving
	169	cloud shadows and seasons are compositing problems, not optical ones.
	170
	171	Aiming resolution is human-sized. A 1.2 cm mirror is diffraction-limited to
	172	4.6×10⁻⁵ rad, so its reflection lands as a 2.3 m spot on the floor 50 km below.
	173	Tower can put a glint on one person that someone five metres away cannot see. The
	174	privacy is set by the wavelength of light, not by any protocol.
	175
	176	Mirrorfall. A failed foil sinks out of the beam and falls. Terminal velocity at
	177	the floor is 0.48 m/s; the descent takes about two days and it lands like a
	178	snowflake. There is a permanent slow glitter coming down out of Tower's sky.
	179
	180	#### Illumination is a dial, not a constant
	181
	182	54.3 TW is Tower's capability, not its setting. Spread over the inner surface it
	183	is 320 W/m² mean, which — with roughly half the surface lit at a time and a
	184	sinusoidal day profile — puts **local noon at ~1,000 W/m², Earth clear-sky
	185	daylight**. But the diffusers are software-controlled, so brightness is genuinely
	186	adjustable in-world across the full range:
	187
	188	\| Setting \| Mean W/m² \| Feels like \|
	189	\|---\|---\|---\|
	190	\| Twilight \| 30 \| permanent overcast dusk \|
	191	\| Temperate \| 200 \| Earth's global mean \|
	192	\| Full \| 320 \| Earth clear-sky noon \|
	193
	194	This should be a control in the app, not a fixed number. It is the most
	195	characterful thing about a world whose sun is a machine.
	196
	197	#### Headroom
	198
	199	5,000× is 11 % of the étendue limit (46,223× at 1 AU). Tower could in principle
	200	push the beam 9.2× further and cook itself; the diffuser network is as much a safety
	201	system as a lighting system. 5,000× was chosen because it is demonstrated
	202	technology — the Odeillo solar furnace reaches ~10 MW/m² on Earth today. Nothing in
	203	Tower's sun system requires physics that does not exist. See `decisions.md` D-003.
	204
`392bb8a7` 2026-09-08 Pavel Safronov	205	#### 1.3.2 Night, and the difference between banked and off
	206
	207	Night cannot be made by aiming the swarm away. The glow above belongs to the
	208	beam and the air, not to the diffusers, so a fully lit beam lights the world
	209	whatever the mirrorlets are doing: 779 W/m²/sr over a strip 3.65° wide running
	210	horizon to horizon is about 0.2 sr, which lands ~100 W/m² on the floor — half
	211	of temperate, and no kind of night. Tower's night is therefore made by **banking
	212	the beam itself, to a thousandth of full: 0.32 W/m²**, a rod you can still find
	213	overhead and a world you can just make out under it (D-046).
	214
	215	Shutting the beam down entirely is a different state and a much worse one. No
	216	airlight, no lit ground, no rod — the only thing left in the world is the sky
	217	through the far cap. §1.9.4 gives the thermal mass about a week of coasting, so
	218	this is a state Tower can survive and not one it can sit in.
	219
	220	\| Setting \| Floor \| What it is \|
	221	\|---\|---\|---\|
	222	\| Dark \| 0 \| the beam shut down. Stars, and nothing else. \|
	223	\| Night \| 0.32 W/m² \| the beam banked to a thousandth \|
	224	\| Twilight \| 30 W/m² \| \|
	225	\| Temperate \| 200 W/m² \| \|
	226	\| Full \| 320 W/m² \| Earth clear-sky noon \|
	227
`da6bcb0e` 2026-09-08 Pavel Safronov	228	#### 1.3.3 The far cap mirror, and where the last quarter of the beam goes
	229
	230	The swarm has optical depth 1.41, so 24% of the beam survives all 540 km — and
	231	it survives collimated, in the beam's own 7.96 km² cross-section. That is the
	232	whole shape of the problem: not 13 TW spread over a 7,854 km² cap, but **1.7 MW/m²
	233	in a 3.2 km spot, about 1,200 suns. An absorber there sits at 2,330 K**.
	234
	235	So the spot is a mirror (D-048), and only the spot: it is a thousandth of the
	236	cap's area, which is what leaves the other 99.9% free to be window (§1.4). A mirror
	237	is the cheap answer rather than the expensive one — at 99.9% it absorbs **1,665
	238	W/m²**, about 1.2 Earth solar constants, so the thing that would have glowed white
	239	is merely warm.
	240
	241	It also recovers the energy. The residual goes back up the column, which makes
	242	the optical path 1,080 km instead of 540 and utilisation 1 - e^(-2τ) = 94%
	243	rather than 76%. What survives even the return trip leaves through the beam
	244	aperture toward the collector, which sends it back again; in the limit the whole
	245	54.3 TW lands on the inner surface. **This is why full brightness is 320 W/m² and
	246	not 243** — the figure in §1.3 assumes the entire beam is eventually used, and with
	247	the cavity closed at both ends it is.
	248
	249	Aiming light at a region to warm it is a separate lever and stays one (§1.9.4). The
	250	mirror is about not wasting the beam; the desert is about where the weather goes.
	251
`2bb3e0de` 2026-09-08 Pavel Safronov	252	#### 1.3.4 The day, and why dawn has a direction
	253
	254	Tower's day is 24 hours, chosen for the people living under it — nothing
	255	physical offers one, since the spin is 7.5 minutes and has nothing to do with light.
	256
	257	Dawn cannot be a dimmer, and it cannot be the swarm either. §1.3 gives the glow
	258	to the beam and the air, so the sky, the airlight and the rod overhead all brighten
	259	together the moment beam power comes up; there is no way to have one end of the axis
	260	light before the other. But the ground is lit by the diffusers, and they can put
	261	the light wherever they like. So Tower brings the day up as a **front running
	262	deepward from the sun cap, 540 km in 40 minutes — 225 m/s**, near Tower's own rim
	263	speed, and slow enough to stand and watch arrive.
	264
	265	The result is a morning nothing on Earth has: a fully lit sky over dark ground,
	266	with a line of daylight coming toward you along the world. §1.9.4 says Tower has no
	267	latitude. The cycle gives it longitude — 40 minutes of it, sun cap to far cap.
	268
	269	Dusk runs the same way, so the light withdraws in the direction it arrived and the
	270	sun-cap end always has the shorter day by a hair. Nothing forces that; the
	271	alternative was a dusk that retreats, and a consistent direction of time reads as
	272	designed rather than as an oscillation.
	273
`866c7f96` 2026-09-07 Pavel Safronov	274	### 1.4 Sealed hull, and the cap windows
	275
	276	The cylinder wall is opaque and unbroken. No slats, no windows, no view out
	277	along any of the 540 km. From anywhere on the inner surface, the sky is land, haze
	278	and the beam — nothing else. See D-017.
	279
	280	The only views out are at the end caps, and they are peculiar:
	281
	282	- A cap is a wall, not a floor. Gravity there points radially outward, which is
	283	tangent to the cap's surface, so nothing rests on it — you would slide to the
	284	rim. Each cap is a 50 km vertical cliff standing at the end of the world, and
	285	its windows sit at its base, at eye level for anyone standing on the floor.
	286	- The sky through them does not sweep — it turns. A cap window looks along the
	287	spin axis, which is the celestial pole. Stars rotate about a fixed centre, one
	288	full turn every 7.5 minutes, with one motionless star at the middle. **A pinwheel
`769726ea` 2026-09-08 Pavel Safronov	289	sky.** It is the only moving sky anyone in Tower ever sees — and because the axis
	290	is horizontal from the floor, it wheels about a point on the horizon rather
	291	than overhead.
	292	- The far cap is glazed whole — a 100 km window looking into deep anti-sun
	293	space, not a wall with windows at its foot (D-045). Its structure survives as the
	294	window's mullions, on the same polar tessellation the checker used and at the
	295	same cell pitch, so the one object in the world with a known scale still is one:
	296	the rule now reads against the sky instead of against a plate. The sun cap
`9c891d2e` 2026-09-08 Pavel Safronov	297	faces the collector rather than the sun, and is glazed too (D-050): §1.3.0
	298	retires the "heavy glare management" this section used to demand of it, because no
	299	direct sunlight has ever reached it. Through it, a 244 km black disc, stars round
	300	the edge of it, and the beam blazing out of the aperture at its centre.
`769726ea` 2026-09-08 Pavel Safronov	301	- By day the window is simply blue. §1.5 puts the far cap 100–200 km into the
	302	haze, so from anywhere but the deepward end the airlight in front of it swamps
	303	whatever is behind, and the cap reads as a disc of sky. The stars are there at
	304	noon and unseeable, which is a fact about eyes and not about the glass.
`da6bcb0e` 2026-09-08 Pavel Safronov	305	- The beam does not go through it. §1.3.3 puts the surviving quarter of the
	306	beam on a 3.2 km bullseye at the centre of the cap, and that bullseye is a mirror
	307	rather than glass: it returns the residual up the column for a second pass. The
	308	window is everything else, which is 99.9% of the disc.
	309	- The mirror is not the pinwheel's centre, and it is worth being clear why. The
	310	sky turns about the direction parallel to the spin axis, and from anywhere on
	311	the floor that direction is horizontal — so **the pinwheel's centre is at eye
	312	level, dead ahead**, down where the window meets the ground, not overhead and not
	313	at the middle of the cap. The mirror is a physical object on the axis, which
	314	from 270 km sits about 10° above that. They coincide only for an observer
	315	standing on the axis itself, and nobody can. By day the mirror blazes — it is
	316	returning 1.7 MW/m² — and is the brightest thing in Tower's sky; at night it
	317	banks with the beam and goes dark.
`866c7f96` 2026-09-07 Pavel Safronov	318
	319	Nobody aboard can ever see Earth. Tower holds a terminator orbit (D-004), which
	320	puts Earth roughly 90° off the axis — out the side of the cylinder, which is now
	321	solid hull. A habitat in Earth orbit whose inhabitants have never seen the planet
	322	they orbit.
	323
	324	The caps are therefore the only places in the world with a horizon that is not
	325	Tower. Whatever grows up around them, it will not be ordinary.
	326
	327	### 1.5 Atmosphere and the look of the sky
	328
	329	Air co-rotates with the shell. Because g falls linearly to zero at the axis, the
	330	atmosphere is much deeper than Earth's:
	331
	332	- Pressure at the axis, 50 km up: 5.2 kPa — thin, but genuinely air.
	333	- A vertical column floor-to-axis is ~10,600 kg/m², about one Earth atmosphere.
	334
	335	So the interior sky behaves like this, and the shader must reproduce it:
	336
	337	- Straight up you look through ~1 atmosphere and see the far side of the world
	338	100 km away — visible, but washed to pale blue like a very distant mountain range.
	339	- Along the axis you look through many atmospheres. The far end cap is not
	340	visible as a wall; it dissolves into blue haze around 100–200 km out. The world
	341	reads as endless in the axial direction and closed in the circumferential one.
	342	- The ground curves up on both sides into that same blue, meeting overhead.
	343	- The beam is a self-luminous scattering shaft 3.65° wide at 779 W/m²/sr —
	344	render it as a bright sunlit cloud, not as a light source with a halo. With the
	345	hull sealed, it is the only thing in the sky that is not land.
	346
	347	### 1.6 Orbit and attitude
	348
	349	- Tower holds a high circular orbit with its **orbital plane kept normal to the
	350	Earth–Sun line** — a terminator orbit. This is why one cap can face the sun
	351	permanently and Tower is never eclipsed.
	352	- The plane precesses 360° per year to track the sun. Tower's angular momentum is
	353	enormous, so this 1°/day reorientation is actively driven, continuously, forever.
	354	Canon treats this as Tower's defining maintenance burden.
	355	- Consequence for the interior view: the star field slowly drifts across the year
	356	while spinning fast every 7.5 minutes. Two nested motions.
	357
	358	### 1.7 Coordinates
	359
	360	The canonical surface coordinate is (θ, z):
	361
	362	- `θ` — angle around the cylinder, 0 to 2π. θ = 0 is the centre of land strip 0.
	363	- `z` — distance from the sun cap, 0 to 540 km.
	364
	365	The 2D map is the direct unroll: `x = z`, `y = (θ − θ₀) · R`. No projection
	366	distortion — a cylinder unrolls exactly. This is a genuine advantage over mapping a
	367	globe and the map view should lean on it.
	368
	369	The map shows the surface once, bounded, so its extent is readable. That means
	370	it needs an edge, and the edge is placed at θ₀ = the centre of the last slat.
	371	Land strip 0 is centred on θ = 0, so cutting there would slice a whole strip in
	372	half across the top and bottom margins; cutting at a slat centre leaves every land
	373	strip whole and splits only a band of stars, which carries no geography. Same
	374	reasoning as putting Earth's map edge at the antimeridian. A sightline whose short
	375	route crosses the edge is drawn as two segments, so the drawn path always matches
	376	the measured distance.
	377
	378	---
	379
	380	### 1.8 Sightlines: Tower has two horizons
	381
	382	Along the axis the floor is perfectly flat and never falls away, so nothing is ever
	383	hidden by curvature. Tower's horizon is haze. Circumferentially the floor curves
	384	up, so distant land rises into the sky rather than sinking below a line. The two
	385	directions behave completely differently, and this is one of the most alien facts
	386	about living there.
	387
	388	Rayleigh transmission from a floor-level observer:
	389
	390	\| Distance \| Along the axis \| Across the interior (overhead) \|
	391	\|---\|---\|---\|
	392	\| 26 km \| 78 % \| 77 % \|
	393	\| 50 km \| 56 % \| 70 % \|
	394	\| 100 km \| 31 % \| 78 % \|
	395	\| 270 km (far cap from midpoint) \| 4 % — a ghost \| — \|
	396
	397	Two consequences:
	398
	399	- The axial horizon closes at 100–150 km. From the midpoint you cannot see
	400	either end cap. Everyone can see 100 km, knows there is more beyond, and cannot
	401	resolve it — a different psychology of distance than Earth's 5 km cliff-edge.
	402	- The far side of the world is clearer than the ground at the same distance,
	403	because a sightline across the interior rises through thin axis air while one
	404	along the floor grinds through full density. It gets better with distance: the
	405	land directly overhead, 100 km away, is the crispest far object in Tower at 78 %.
	406	Distant geography is best seen by looking up. People would orient by what is
	407	overhead; the ground is the vague part. This inverts the human default.
	408
	409	### 1.9 Water
	410
	411	Tower's hydrology is stranger than its geometry, and most of it follows from one
	412	fact and one absence.
	413
	414	#### 1.9.1 Coriolis never pushes surface water sideways
	415
	416	The floor's tangent plane is spanned by θ̂ and ẑ, and the spin axis is ẑ. For any
	417	flow along the surface:
	418
	419	- along the axis — `ẑ × ẑ = 0`, no Coriolis force at all;
	420	- around the circumference — `ẑ × θ̂ = −ŝ`, the force is purely radial.
	421
	422	The Coriolis term never has a component inside the surface. So **Tower's rivers
	423	do not curve.** No hemispheres, no Baer's law, no preferred bank erosion, no gyres,
	424	no cyclones. Every large-scale rotational habit of Earth's water and weather is
	425	absent, and a meander in Tower is pure flow dynamics with no thumb on the scale.
	426
	427	#### 1.9.2 What it does instead: everything vertical leans
	428
	429	The force goes into the floor rather than across it, so it appears whenever water
	430	moves up or down. Deflection of a free fall goes as h^1.5:
	431
`28f9a6bc` 2026-09-07 Pavel Safronov	432	\| Fall \| Lands (retrograde) \| Lean of the path \| Lean at impact \|
	433	\|---\|---\|---\|---\|
	434	\| 100 m \| 4.2 m \| 2.4° \| 3.6° \|
	435	\| 500 m \| 47 m \| 5.4° \| 8.1° \|
	436	\| 1,000 m \| 133 m \| 7.6° \| 11.3° \|
	437	\| 2,000 m \| 377 m \| 10.7° \| 15.8° \|
	438
	439	Two different angles, and the interior view needs both. The path leans by where
	440	the water lands against how far it fell — the tilt of a waterfall's plume seen
	441	whole. The impact lean is the direction the water is actually moving when it
	442	arrives, which is what a streak of it is drawn at, and it is half again the other in
	443	tangent. `src/canon/motion.ts` computes both.
`866c7f96` 2026-09-07 Pavel Safronov	444
	445	Rain falls permanently slanted — a raindrop at terminal velocity sits at a fixed
	446	1.47° against the spin, ~51 m of drift over a 2 km fall, never varying, never
`28f9a6bc` 2026-09-07 Pavel Safronov	447	reversing. It is fixed because drag holds the fall speed constant, so unlike free
	448	fall the angle does not grow with the drop: rain leans the same 1.5° whatever the
	449	cloud. Set against a waterfall's 11°, **the waterfall is the compass and the rain is
	450	not**, which is the opposite of what §2.4 originally assumed (D-034). Rising things lean the opposite way: smoke drifts spinward, rain drifts
`866c7f96` 2026-09-07 Pavel Safronov	451	anti-spinward. Every plume and every shower in Tower tilts, and they tilt against
	452	each other.
	453
	454	The same applies to anything that descends, not just water — see §1.11 on what it
	455	does to skiing.
	456
	457	Horizontal flow changes weight rather than direction (Eötvös): +0.57 % for a
	458	2 m/s prograde river, negligible hydrologically. It only matters at speed — an
	459	express train at 100 m/s gains 29 %, and anything moving retrograde at the 700 m/s
	460	rim speed is stationary in space and therefore weightless.
	461
	462	#### 1.9.3 A perfect cylinder has no slope anywhere
	463
	464	Potential depends only on radius, so **the whole floor is a single equipotential
	465	surface.** Water on it has no preferred direction: a perfect Tower has no rivers at
	466	all, forever.
	467
	468	Every river in Tower exists because the floor was shaped. There are no
	469	tectonics and no uplift, so erosion is strictly one-way — a 3 km range wears flat in
	470	~30 Myr under a wet engineered climate, ~150 Myr under a dry one. Tower's rivers
	471	are steadily destroying its landscape and the highlands must be rebuilt to keep them
	472	running. This is D-004's theme again: Tower's rivers are plumbing it chose to
	473	install and has to keep re-installing.
	474
	475	Two corollaries:
	476
	477	- No river can circumnavigate. A closed loop has no net descent.
	478	- Lakes curve upward. Level means constant radius, so across a 30 km lake the
	479	far shore rides 2.3 km above your tangent plane — Earth would hide it 18 m
	480	below the horizon. You look up at the opposite beach, and water appears to
	481	climb away from you.
	482
	483	#### 1.9.4 There is no passive climate at all
	484
	485	Tower has no latitude — every point is equidistant from the light — and with the
	486	hull sealed it has no windows either. Nothing breaks the thermal uniformity. A
	487	uniform hull radiates at 274 K everywhere, so left alone the world settles into a
	488	flat calm: no gradient, no wind, no evaporation, no rain, no rivers.
	489
	490	So every weather system in Tower is deliberate. Tower makes climate the only way
	491	left to it — by aiming light. It warms where it wants convection and shades where it
	492	wants condensation; the hull's uniform loss is the sink. Every cloud, every
	493	rainfall, every jet stream is something Tower is doing, continuously, on purpose.
	494
	495	This is D-009 arriving at its conclusion. The mirrorlet swarm was already Tower's
	496	body language; now it is also its lungs and its weather. There is no autonomic
	497	layer underneath. If Tower's attention ever lapses, the rain stops.
	498
	499	Costs, for scale against a 54.3 TW budget:
	500
	501	\| \| \| \|
	502	\|---\|---\|---\|
	503	\| Earth-like rainfall, 1,000 mm/yr \| 12.2 TW \| 22.4 % — Earth spends the same fraction \|
	504	\| Recirculating all runoff (1,792 m³/s) against 500 m \| 8.8 GW \| 0.016 % \|
	505
	506	#### 1.9.5 Rivers are a closed, pumped circulation
	507
	508	With no natural cycle and no natural slope, a Tower river is a loop: a headwater
	509	reservoir, a shaped descent through the landscape, a lake at the bottom, and a lift
	510	back to the top. Source and sink are the same system.
	511
	512	Two ways to close the loop, and they give completely different rivers:
	513
	514	- The pump outfall. A river begins at a machine. Walk to the source of any
	515	river and find a wall of water coming out of a structure — not a spring, an
	516	outlet. Costs a rounding error (above).
	517	- The made cloud. Tower evaporates the lake with aimed light and condenses it
	518	over the highlands, so rain falls exactly where the river should start. No
	519	machinery: the swarm is the pump. Costs the 22.4 % above, and buys real weather
	520	with it.
	521
	522	Canon uses both — made weather for the world's general hydrology, pumps for
	523	watercourses Tower wants under precise control.
	524
	525	Two consequences of the sealed hull:
	526
	527	- A river can circle the world. The surface is one unbroken sheet, so a channel
	528	can run the full 314 km circumference and close on itself — a lazy river returning
	529	to its own source. Gravity cannot drive a closed loop, so it is pumped.
	530	- The water inventory is conserved and exactly known. Nothing evaporates to
	531	space. Tower knows how much water it has, to the litre, forever.
	532
	533	Lifting is also slightly cheaper than on Earth, since gravity falls off with height:
	534	1 % cheaper at 1 km, 5 % at 5 km, 25 % at 25 km.
	535
	536	### 1.10 Slat orientation — RESOLVED: none
	537
	538	Axial strips versus rings was weighed at length (D-016) and answered by removing
	539	slats entirely (D-017). The comparison is kept in D-016 because it established what
	540	the choice cost, and because the reasoning — that slat layout sets the shape of
	541	Tower's countries, its climate map and its astronomy all at once — is what made
	542	"none" a real option rather than an absence.
	543
	544	### 1.11 The world as built
	545
	546	Authored geography lives in `content/geography.json` — the source of truth — and
	547	`npm run terrain` renders it. Names are provisional. Tower is laid out in **belts
	548	along the axis**, which is the natural grain of an engineered climate (§1.9.4):
	549
	550	\| z \| \| \|
	551	\|---\|---\|---\|
	552	\| 0–42 km \| The Sunward Sea \| ring ocean at the foot of the sun cap \|
`03607270` 2026-09-08 Pavel Safronov	553	\| 55–140 km \| the range \| Westwall, The Sisters, Snowcrown (4,300 m), Eastwall — the named high points of The Girdle, which rings the world at z 90 km (D-051) \|
`866c7f96` 2026-09-07 Pavel Safronov	554	\| 148–218 km \| The Dry Reach \| desert ringing the world, 22,000 km², 13% of the surface \|
	555	\| 176/190/203 km \| three ridges \| ringing, concentric — and The Narrows is the only breach in all three \|
	556	\| 214–430 km \| Greatwood \| forest belt \|
	557	\| 248–402 km \| The Deep \| 154 × 32 km sea, long down the axis and narrow around \|
	558	\| 270 km \| The Capital / The Spindle \| axial midpoint, on The Deep's antispinward shore \|
	559	\| 496–540 km \| The Deepward Sea \| ring ocean at the foot of the far cap \|
	560
	561	#### Why the desert rings
	562
	563	On a cylinder you cannot go around a band. Every desert on Earth can be
	564	circumnavigated; a ringed one cannot. It is an absolute barrier across the world's
	565	waist, and with the three ridges ringing too it becomes three concentric walls.
	566
	567	#### The drainage, and why it keeps the chokepoint
	568
	569	Four rivers come off the range — Westwall Water, Sisters Water, Eastwall Water and
	570	The Long Water. On Earth most drainages do not cut through a rising ridge; they
	571	are defeated and diverted, and only an antecedent river with enough power keeps
	572	its course. So three of the four are turned by First Ridge, run along its front
	573	gathering each other, and leave with the fourth through the single gap.
	574
	575	- The Narrows carries the entire world's runoff — ~392 m³/s off a 9,800 km²
	576	range, lifted 1,780 m for 6.9 GW (0.013% of budget).
	577	- The ridge-front reaches are the only circumferential water in Tower, and
	578	weigh fractionally more or less for it (§1.9.2). Everything else runs axially,
	579	where Coriolis is exactly zero.
	580	- Whoever holds The Narrows holds the axis of the world. The Capital sits behind it.
	581
	582	#### The Deep is oriented for the view
	583
	584	Long down the axis and narrow around it, so you cross it the short way — around
	585	θ, where the floor curves up. From the Capital the far shore is 32 km away and
	586	stands 2.6 km above eye level. Crossing it lengthwise instead would be dead
	587	flat. The orientation is the whole point.
	588
	589	It is also the world's flywheel: 19× the diurnal heat storage of bare ground, and
	590	5.9 days at full power to move its temperature one kelvin. Tower still supplies
	591	every joule (§1.9.4), but the world now coasts for about a week rather than
	592	stopping the moment attention lapses.
	593
	594	#### The cap seas are not designed
	595
	596	Gravity on an end cap points radially outward — tangent to the cap. Nothing rests
	597	there; water runs down to the rim and pools. **Both ring oceans exist whether anyone
	598	planned them or not**, each with a 314 km shoreline and a 50 km cliff standing
	599	behind it. The only windows in the world are at their feet (§1.4).
	600
	601	#### Skiing, because it is a different sport here
	602
	603	Level motion feels no Coriolis (§1.9.1) but a descending skier has radial velocity,
	604	and that does deflect:
	605
	606	- Down the axis: a steady sideways push, always antispinward — 1.5% of g at
	607	15 m/s on a 20° slope, 6.4% at 35 m/s on 40°.
	608	- Around the cylinder: you get heavier or lighter. 8.6% at 30 m/s, 11.4% at 40.
	609
	610	So a retrograde run gets lighter the faster you go, which is self-reinforcing,
	611	while a prograde run grips harder as it accelerates. Every slope in Tower has a
	612	handedness. No Earth mountain does. Settlements sit at the tarns — Sisters Station,
	613	Hollow Station — around 2,100 m, with the runs above them.
	614
	615	#### Open questions
	616
	617	- The Spindle's height. 3.2 km is a placeholder; an object dropped from its top
	618	lands ~760 m antispinward of its base.
`03607270` 2026-09-08 Pavel Safronov	619	- z 430–496 km is unauthored. The circumference outside the named peaks is no
	620	longer empty — The Girdle carries the range round the whole 314 km — but it is
	621	unwalked: peaks with no names, and no rivers drawn off them yet.
`866c7f96` 2026-09-07 Pavel Safronov	622
	623	## 2. Application architecture
	624
	625	### 2.1 Stack
	626
	627	- Vite + TypeScript (strict), no framework — the app is three canvases and
	628	a control panel, and a framework would not earn its weight.
`fc11b064` 2026-09-08 Pavel Safronov	629	- One layout breakpoint, at 720 px: above it the panel is a column beside the
	630	world, below it a drawer over it (D-043). The world is a full-bleed canvas in
	631	both, because all three views are drag-to-look and a scrolling page would be
	632	fighting them for every touch.
`866c7f96` 2026-09-07 Pavel Safronov	633	- Three.js for the exterior and interior 3D views.
	634	- Canvas 2D for the map view.
`62458672` 2026-09-07 Pavel Safronov	635	- GitLab CI → GitLab Pages, publishing `dist/` as `static/`.
`866c7f96` 2026-09-07 Pavel Safronov	636
	637	### 2.2 Module layout
	638
	639	```
	640	src/
	641	canon/
	642	constants.ts single source of truth for every number in §1
	643	geometry.ts (θ,z) <-> 3D, slat layout, map <-> surface
`28f9a6bc` 2026-09-07 Pavel Safronov	644	atmosphere.ts the rotational density profile and its extinction (§1.5)
	645	motion.ts how things fall in the rotating frame (§1.9.2) — the only
`65484043` 2026-09-08 Pavel Safronov	646	evidence of the spin anywhere but the far cap window
`866c7f96` 2026-09-07 Pavel Safronov	647	views/
	648	palette.ts one colour per place kind, shared by map, list and editor
`28f9a6bc` 2026-09-07 Pavel Safronov	649	raster.ts what npm run terrain wrote: grid, and what white means
`866c7f96` 2026-09-07 Pavel Safronov	650	world/
	651	schema.ts world.json types + validation
	652	store.ts places CRUD, localStorage drafts, JSON import/export
	653	instruments.ts sightline, viewshed, lighting probe, travel (§3.3)
	654	state/
	655	store.ts selected point, time-of-day, spin phase, view mode, writer flag
	656	views/
`5f38efd0` 2026-09-07 Pavel Safronov	657	exterior.ts Three.js: hull, caps, collector, beam, spin, point picker
`866c7f96` 2026-09-07 Pavel Safronov	658	map.ts Canvas 2D: unrolled surface, slats, terminator, point picker
`28f9a6bc` 2026-09-07 Pavel Safronov	659	interior.ts Three.js: shell geometry, the haze shader of §1.5, beam, rain
`866c7f96` 2026-09-07 Pavel Safronov	660	ui/
`28f9a6bc` 2026-09-07 Pavel Safronov	661	app.ts the whole panel: tools, editor, readouts, view switching
	662	scales.ts slider mappings — kept out of views/ so the panel can label
	663	the eye-height slider without pulling in Three.js
`866c7f96` 2026-09-07 Pavel Safronov	664	main.ts
`6364748a` 2026-09-08 Pavel Safronov	665
	666	tools/ node-only, never bundled; plain .mjs with no build step
	667	terrain.mjs renders content/geography.json into static/ (§1.11)
	668	png.mjs minimal PNG encoder — zlib is its only dependency
	669	noise.mjs fbm and ridged noise for the terrain relief
	670	screenshots.mjs drives the running application in a headless browser and
	671	photographs it into docs/screenshots/ (D-042)
`866c7f96` 2026-09-07 Pavel Safronov	672	```
	673
	674	`canon/constants.ts` is the spine. Every derived figure — period, rim speed, land
	675	area, slat width — is computed from radius, length, slat count and target land
	676	area, never hard-coded. Changing the radius changes the whole app coherently.
	677
	678	### 2.3 State
	679
	680	One small observable store. Views subscribe; nothing owns anything else.
	681
	682	```ts
	683	type TowerState = {
	684	point: { theta: number; z: number } \| null // the picked location
	685	tod: number // 0..1, position in the diffuser day cycle
	686	brightness: number // W/m^2 mean, 30..320 -- the diffuser output setting
	687	spinPhase: number // 0..2pi, rotation for star motion
	688	spinning: boolean
	689	view: 'exterior' \| 'map' \| 'interior'
	690	writer: boolean // ?writer=1 -- a UI mode, not a security boundary
	691	probe: { from: SurfacePoint; to: SurfacePoint } \| null
	692	}
	693	```
	694
	695	The exterior and map pickers both write `point`; the interior view reads it. That
	696	is the entire interaction loop.
	697
`5f38efd0` 2026-09-07 Pavel Safronov	698	### 2.4 The exterior view
	699
	700	Scene units are kilometres, so the code reads in canon numbers. Tower's axis lies
	701	along world Z with the sun cap at z = 0, which puts a canon surface point at
	702	`(R cos θ, R sin θ, z)` in the hull's own frame. The hull spins about Z and carries
	703	that frame with it, so **picking resolves the ray hit back through
	704	`worldToLocal`** — otherwise every point picked off a turning hull would be wrong by
	705	the spin phase.
	706
	707	Two deliberate departures from physics, both reading aids of the same kind as the
	708	map's hillshade:
	709
	710	- Fill lighting. Real sunlight here runs parallel to the axis, so Tower's flank
	711	is edge-on to it and renders very nearly black — accurate and useless. Two fill
	712	lights lift the hull enough to read its shape.
	713	- A visible beam. In vacuum there is nothing between collector and cap to
`8b6287ba` 2026-09-07 Pavel Safronov	714	scatter light, so the converging beam would not be visible at all. It is drawn
	715	as rays because it is the thing the view is about.
	716
	717	Everything in the scene is captioned — collector, beam, both caps — with screen-space
	718	labels. A pale disc and a cone of light are not self-explanatory, and the collector
	719	is the least intuitive object in Tower: it is larger than the world it feeds.
`5f38efd0` 2026-09-07 Pavel Safronov	720
	721	The default camera frames the whole system rather than Tower alone: the collector
	722	is 244 km across and flies 320 km ahead, and it is the larger object.
	723
`65484043` 2026-09-08 Pavel Safronov	724	The hull has two readings, and the view switches between them (D-047). True
	725	is the outer skin: bare structure, the checkered sun cap, and the glazed far cap.
	726	X-ray wraps the terrain raster onto the outside instead — from outside Tower is
	727	structure, so what that texture shows is what is inside, which makes it a diagram
	728	and not a view. Both are worth having and they are not the same claim, so the
	729	control is a two-way switch rather than a checkbox.
	730
	731	The wall is drawn as two meshes rather than one double-sided one, because its two
	732	faces show different things: the outer skin changes with the mode, and the inner
	733	surface is always the land. Since the far cap became a window (D-045) that inner
	734	surface is genuinely visible from outside — look in through the glass in true mode
	735	and the world is really there, lit and 540 km deep. It is the only line of sight
	736	into Tower that is not a projection. Three's cylinder runs u around and v along, while the raster is the other
`11a13dbf` 2026-09-07 Pavel Safronov	737	way round, so the UVs are rewritten from the original attribute rather than derived
	738	from vertex positions — the seam vertices are duplicated, and deriving an angle from
	739	them smears one column of triangles across the whole image.
	740
`15d933c9` 2026-09-07 Pavel Safronov	741	The camera position persists across reloads, alongside the active view, so
	742	returning to the page returns to the vantage you left. Both end caps are places in
	743	their own right, so the view can frame them: a cap's marker sits on the axis,
	744	because a cap is a disc across the end and a point on its rim would ride round the
	745	edge as Tower turns.
	746
`11a13dbf` 2026-09-07 Pavel Safronov	747	Markers are scaled per frame to hold a roughly constant size on screen. A 4 km dot
	748	is legible from 200 km and sub-pixel from the 1,200 km of the default view, which is
	749	to say the places would simply not be there.
	750
`28f9a6bc` 2026-09-07 Pavel Safronov	751	### 2.4 The interior view — built
	752
	753	Scope held: real geometry, simple ground.
	754
	755	Real: the shell is the actual cylinder, curving up on both sides and closing
	756	overhead, displaced by `terrain-height.png` and painted with `terrain-color.png`.
`65484043` 2026-09-08 Pavel Safronov	757	Stars only through the far cap — the wall is sealed (§1.4), so for 539 of the
	758	540 km the sky is land and light. The far cap is glazed (D-045) and the sky beyond
	759	it is sampled by view direction in an azimuthal-equidistant projection about the
	760	spin axis, which makes the pinwheel fall out of the projection rather than being
	761	animated into it: azimuth plus spin phase, one turn every 7 min 29 s, and the
	762	point on the axis genuinely motionless.
`28f9a6bc` 2026-09-07 Pavel Safronov	763
	764	Simple: the ground is the raster plus a little procedural grain in the near field,
	765	not modelled scenery. No buildings, no vegetation meshes. Cells are 0.7 km and
	766	texels 264 m, so relief a valley away is real and the ground underfoot is not.
	767	Detailed procedural scenery stays deferred.
	768
	769	Haze is the shader, and it is integrated rather than faked. Every fragment
	770	samples the rotational density profile 24 times along the ray to the camera and
	771	extinguishes per channel — red survives twice as far as green, blue half as far,
	772	which is what makes distance blue rather than grey. From the floor it reproduces
	773	§1.8 from the inside: 78% straight up against 31% for the same 100 km along the
	774	ground, printed in the readout so the claim can be checked rather than believed.
	775	The airlight the haze adds is neutral; tinting it blue double-counts the
	776	wavelength dependence and turns even the desert mauve (D-034).
	777
`65484043` 2026-09-08 Pavel Safronov	778	Nothing inside the scene rotates, because the observer, the ground and the air
`e27aaf20` 2026-09-08 Pavel Safronov	779	all co-rotate. What falls carries the spin, and §1.9.2 is lopsided about how: rain
	780	leans a fixed 1.5° whatever the cloud, because drag pins its speed, while only free
	781	fall keeps accelerating and so keeps leaning. **The waterfall is the compass; the
	782	rain is barely a hint.** Rain is no longer drawn (D-049) — 1.5° is not a picture,
	783	and the pinwheel below does the job it was standing in for. The physics stays in
	784	`canon/motion.ts` and stays tested; it was only ever the rendering that could not
	785	carry it.
`65484043` 2026-09-08 Pavel Safronov	786
`9c891d2e` 2026-09-08 Pavel Safronov	787	The one thing that does turn is outside. The sky through the caps does not
`65484043` 2026-09-08 Pavel Safronov	788	co-rotate, so it wheels once every 7 min 29 s about the motionless star on the axis
`e27aaf20` 2026-09-08 Pavel Safronov	789	(D-045). It is by a wide margin the plainest evidence of the spin in the view, and
`9c891d2e` 2026-09-08 Pavel Safronov	790	it is the only evidence left now that the rain is gone. Both caps are windows
	791	(D-050), so it can be had looking either way along the axis — deepward into empty
	792	space, or sunward past the black disc of the collector.
`28f9a6bc` 2026-09-07 Pavel Safronov	793
	794	The observer stands on the terrain, which means the height raster is read back onto
	795	the CPU and sampled against the mesh's own cell corners — the GPU cannot be asked
	796	where the ground is (D-035).
	797
	798	Instruments in the view: eye height from 1.7 m to 25 km with the local gravity
`65484043` 2026-09-08 Pavel Safronov	799	beside it, the five canon illumination settings of §1.3, the field of view as a
`28f9a6bc` 2026-09-07 Pavel Safronov	800	pair of binoculars, and captions on every place still visible through the haze.
	801	Selecting a place walks the observer to it (D-038) — the haze horizon closes at
	802	100–150 km, so turning to face something usually just points at haze.
	803
`65484043` 2026-09-08 Pavel Safronov	804	The caps are no longer the same object. The sun cap is checkered (D-041) —
	805	polar, six rings, cells about 8 km. The far cap is glazed (D-045), and the
	806	checker survives on it as the window's mullions: the same tessellation at the same
	807	pitch, so it is still the only object in the world with a known size and still
	808	doubles as a scale for judging distance, now read against sky rather than plate.
	809	Both patterns are shared with the exterior view.
	810
	811	The far cap's glazing is set a kilometre into its own frame, so from inside there is
	812	a 1 km reveal around it. That is the only place the wall depth of D-044 can be
	813	seen from within the world, which is why it is the only place it is drawn.
`28f9a6bc` 2026-09-07 Pavel Safronov	814
	815	Not modelled: water has no surface. Lakes and seas are painted in the raster and cut
	816	into the shell as basins, so standing in one puts you on its floor.
`866c7f96` 2026-09-07 Pavel Safronov	817
	818	---
	819
`1affced2` 2026-09-08 Pavel Safronov	820	### 2.5 Surfaces — the Cascadia ruler
	821
	822	The app can paint the cylinder with a second, non-canon ground: **the Pacific
	823	Northwest at 1:1**, as a scale reference. In UTM 10N the ground from the Pacific
	824	surf at Kalaloch east to the Columbia at Entiat measures 312.8 km against Tower's
	825	314.2 km circumference, so western Washington closes the ring on real terrain with
	826	1.4 km to spare — beach, Olympics, Puget Sound, Cascades, east flank, Columbia,
	827	back to the beach. Seattle lands 150 km deepward of the sun cap, and the point
	828	100 km straight overhead is both Kalaloch and Entiat: one place, reached by walking
	829	either way round.
	830
	831	This is legitimate because Tower has no north (§1.7) — the compass is
	832	sunward/deepward/spinward/antispinward, so there is nothing for a real region's
	833	orientation to contradict.
	834
	835	It is not canon. It contradicts §1.9.3 (all relief in Tower is engineered) and
	836	its rivers do not lean antispinward the way §1.9.2 requires. It is labelled in the
	837	panel, defaults off, and draws no places. The one artefact is a seam: theta wraps
	838	and geography does not, leaving a scarp of 369–844 m through the settled half,
	839	treated as a landform rather than repaired. Rotating the frame and sliding the cut
	840	were both measured against it; see D-055 for why neither is used.
	841
	842	Every surface shares the raster grid and height window (§2.2), which is what keeps
	843	a surface to a swap of two PNG filenames rather than a second shader.
	844
	845	`npm run cascadia` builds it. See D-055.
	846
`866c7f96` 2026-09-07 Pavel Safronov	847	## 3. Writer mode and the world file
	848
	849	The app is an instrument for writing the novel as much as a thing to look at. It
	850	must answer questions like can the far face be seen from the midpoint?, *where
	851	does a central embassy go?, can a lake overhang?* — and let their answers be
	852	placed, named and kept.
	853
	854	### 3.1 Two modes over one data file
	855
	856	\| \| Reader (default) \| Writer (`?writer=1`) \|
	857	\|---\|---\|---\|
	858	\| World content \| committed `world.json`, read-only \| editable, drafts in `localStorage` \|
	859	\| Instruments (§3.3) \| sightline probe only \| all \|
	860	\| Output \| permalinks to a point or place \| Export JSON for the author to commit \|
	861	\| Panel \| read-only detail card \| full editor, canon controls, import/export \|
	862
	863	There is no backend — GitLab Pages is static. The loop is: the author edits in the
	864	browser, exports `world.json`, commits it, and the published site serves it. Simple,
	865	versioned in git alongside the prose, and diffable.
	866
	867	Writer mode is a UI mode, not a security boundary. Anyone may set `?writer=1`;
	868	they can only edit their own local copy, which persists nowhere and reaches nobody.
	869	This is fine, and arguably good — a reader who finds it gets a sandbox. It must not
	870	be mistaken for access control.
	871
	872	### 3.2 The world file
	873
	874	```jsonc
	875	{
	876	"version": 1,
	877	"places": [{
	878	"id": "hallow-crossing", // permalink target; see below
	879	"name": "Hallow Crossing",
	880	"kind": "city\|embassy\|lake\|region\|landmark\|note",
	881	"geometry": { "type": "point", "theta": 1.9635, "z": 270000 },
	882	// or { "type": "polygon", "points": [[theta,z], ...] } for lakes, regions
	883	"blurb": "public description, shown to readers",
	884	"notes": "the author's working notes — NEVER shipped",
	885	"public": true // false = not ready to exist publicly
	886	}]
	887	}
	888	```
	889
	890	Two text fields, deliberately separate. `blurb` is written for readers and
	891	ships; `notes` is the author's working thinking and never leaves the machine except
	892	in a backup. One field with a flag would have made every note a publication
	893	accident waiting to happen.
	894
	895	Ids follow the name until pinned. An id is a permalink target and must be
	896	stable once published, but during world-building names churn constantly and a file
	897	full of `new-city-7` is unreadable. So the id tracks the name while it still
	898	matches it, and editing the id field freezes it for good. The editor shows the id
	899	for exactly this reason.
	900
	901	Coordinates are canon (θ radians, z metres, §1.7) — never map pixels, so the file
	902	survives any change to how the map is drawn.
	903
	904	The `public` flag is a privacy trap worth naming. Working notes ("embassy here?
	905	check sightline to the cap") must not ship to readers. But a `world.json` committed
	906	to a public repository is public regardless of any flag in it. So export writes two
	907	files: `world.json` (public entries only, committed) and
	908	`world.private.json` (everything, gitignored by default). Whether private notes
	909	should nonetheless be version-controlled — in a private repo, or a second one — is
	910	the author's call and is deliberately unresolved.
	911
	912	### 3.3 Instruments
	913
	914	The measuring tools are the real substance of writer mode. Each answers a question
	915	the novel actually raised:
	916
	917	- Sightline probe. Pick two points; get surface distance, straight-line
	918	distance, optical depth, transmission, and a plain verdict (*crisp / hazy / ghost
	919	/ invisible*). This is the tool that answered §1.8. Available to readers too — it
	920	is the single most illustrative thing in the app.
	921	- Viewshed overlay. From a selected point, shade the whole map by transmission.
	922	It renders the axis-versus-overhead asymmetry as one picture, which is worth more
	923	than the table in §1.8.
	924	- Lighting probe. At a point, illumination across the day cycle at the current
	925	brightness setting (§1.3) — for writing a scene at a specific hour.
	926	- Travel. Surface distance and journey time between points at chosen speeds,
	927	including whether the route crosses a slat.
	928	- Sky-at-a-time. Star field, beam position and Earth transits for a given point
	929	and moment, so a described sky is a correct sky.
	930
	931	### 3.4 What readers get
	932
	933	The public link is the finished map: embassies, towns, lakes, named regions — the
	934	permanent things, behaving like any map application. Plus the exterior view, the
	935	interior view from any point, and permalinks so a reader can be handed a place and
	936	a moment. Reading the world, not editing it.
	937
	938	## 4. Phases
	939
	940	1. Design — this document and `decisions.md`. ← current
	941	2. Scaffold — Vite/TS/Three, CI to Pages, `canon/` implemented and unit-tested.
`5f38efd0` 2026-09-07 Pavel Safronov	942	3. Exterior — ✅ hull, caps, collector, beam, spin, and hull point picking.
`866c7f96` 2026-09-07 Pavel Safronov	943	4. Map — unrolled surface, slat rendering, picking, sync with exterior.
`28f9a6bc` 2026-09-07 Pavel Safronov	944	5. Interior — ✅ shell geometry, the haze shader, the beam, rain, eye height,
`65484043` 2026-09-08 Pavel Safronov	945	and the five illumination settings. Stars, but only through the far cap window.
`866c7f96` 2026-09-07 Pavel Safronov	946	6. Writer mode — ✅ *built ahead of 3–5, at the author's request, because the
	947	world needs developing before there is anything to render.* Map view, place
	948	editing for all thirteen kinds, points/polylines/polygons, localStorage
	949	autosave, backup and static exports, import, and the sightline probe.
	950	7. Content pipeline and reader mode — ✅ `content/geography.json` +
	951	`npm run terrain` (D-019); the rendered terrain drawn under the map; reader and
	952	writer modes (D-011); permalinks (D-010).
`6364748a` 2026-09-08 Pavel Safronov	953	8. Polish — mobile ✅ (the sidebar is a drawer below 720 px, D-043) and a
	954	screenshot tour that photographs all ten views through the UI, D-042.
`fc11b064` 2026-09-08 Pavel Safronov	955	Remaining: transitions, performance.
`866c7f96` 2026-09-07 Pavel Safronov	956
`28f9a6bc` 2026-09-07 Pavel Safronov	957	All five view phases are built. Phase 2's `canon/` landed with phase 6, since the
	958	map could not be drawn without it.
`866c7f96` 2026-09-07 Pavel Safronov	959
	960	Shareable permalinks are implemented (phase 7) — the app has
	961	readers, and "stand here and look at this" is the whole reason it exists as a
	962	companion piece. Named locations from the novel should be markable on the map for
	963	the same reason.
	964
	965	Later, explicitly out of scope for now: procedural scenery, settlements, weather
	966	systems.