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
	45	### 1.2 Spin and gravity
	46
	47	\| Quantity \| Value \|
	48	\|---\|---\|
	49	\| Angular velocity ω \| 0.014005 rad/s \|
	50	\| Spin period \| 448.6 s = 7 min 29 s \|
	51	\| Rotation rate \| 0.134 rpm \|
	52	\| Rim speed \| 700.2 m/s \|
	53	\| Floor gravity \| 1.000 g by construction \|
	54
	55	Consequences that the app should show or at least respect:
	56
	57	- Gravity falls off linearly with height. g(h) = ω²(R−h). A 1 km peak sits at
	58	0.98 g; the axis is weightless. Mirrors at the axis float.
	59	- Coriolis is mild but real. 2ω = 0.0280 s⁻¹. Walking is unaffected (0.4 % g).
	60	A ball thrown at 30 m/s deflects at 8.6 % g — enough that Tower's sports differ
	61	from Earth's, and enough that thrown objects visibly curve.
`769726ea` 2026-09-08 Pavel Safronov	62	- Nothing in the sky moves, except through the far cap. The wall is sealed
	63	(§1.4), so along 539 of the 540 km there is nothing to wheel past — the far side
	64	of the world overhead co-rotates with you and hangs perfectly still. The one
	65	exception is the far cap window, where the sky is outside and does not
	66	co-rotate: it turns once every 7 min 29 s about a single motionless star on the
	67	axis. The spin is invisible upward and obvious deepward, and only at night.
`866c7f96` 2026-09-07 Pavel Safronov	68	- But everything that falls leans. Rain hangs at a permanent 1.47° slant, smoke
	69	drifts the other way, and a 1 km waterfall lands 133 m off its own cliff (§1.9.2).
	70	This inversion is the alien fact the interior view must sell: **the sky is still
	71	and the water is not.**
	72
	73	### 1.3 The sun system
	74
	75	Sunlight enters through the sun-facing end cap and runs the full 540 km axis.
	76
	77	- Collector. A mirror array flies ahead of Tower, gathering sunlight and
	78	concentrating it into the beam. **46,900 km² of aperture — a disc ~244 km
	79	across**, two and a half times Tower's own diameter. It is the dominant visual
	80	element of the exterior view.
	81	- Beam. 10 km in circumference — 1,592 m radius, 7.96 km² cross-section —
	82	carrying 54.3 TW at a concentration of 5,000×, or 6.8 MW/m². It runs the
	83	axis from sun cap to far cap.
	84	- Diffusers. Not lamps — a swarm. See §1.3.1.
	85	- The beam is visible, and it glows by itself. There is real air at the axis
	86	(§1.5), and a 6.8 MW/m² beam Rayleigh-scattering through 3.18 km of it radiates
	87	779 W/m²/sr — **26× brighter than clear blue sky, 25,700× dimmer than the
	88	sun's disc.** It is a brilliantly sunlit cloud extruded into a rod, safe to look
	89	at directly. From the floor 50 km below it is **3.65° wide, about seven
	90	sun-widths**, running the whole length of the sky. This glow is a property of the
	91	beam and the air, not of the diffusers — it would be there if the beam were empty.
	92
`9c891d2e` 2026-09-08 Pavel Safronov	93	#### 1.3.0 Tower never sees the sun
	94
	95	The collector is 244 km across and flies 320 km ahead of the sun cap, which puts
	96	it 41.8° wide in Tower's sky against the sun's 0.53° — **79 times the sun's
	97	diameter**. It sits squarely between Tower and the sun, so it occults it completely,
	98	and the umbra it casts is 120 km across where Tower's 51 km sits inside it. The
	99	eclipse is total, permanent, and self-inflicted.
	100
	101	So nobody aboard can see the sun, for the same reason nobody can see Earth
	102	(§1.4). What they can see, through the sun cap window, is the back of the
	103	collector: 46,938 km² of structure radiating its own waste heat at about 245 K —
	104	infrared, and therefore black. It is the only object in Tower's sky that is neither
	105	Tower nor a star, and it never moves, because the collector flies in formation
	106	rather than spinning. D-050.
	107
`49b0d64d` 2026-09-08 Pavel Safronov	108	Its edge is not black. The face is a mirror pointed away, but the collector is
	109	backlit, and the structure holding 46,938 km² flat stands proud of the mirror
	110	plane at the rim, where the sun strikes it side-on. With the scatter any real mirror
	111	throws at grazing incidence, the edge reads as a thin sun-coloured ring — so the
	112	collector is a hole in the stars with a lit rim rather than an absence, which is how
	113	anything backlit looks. It is not sunlight leaking past the edge: Tower is wholly
	114	inside the umbra, so the sun's disc is hidden from every point in the world. The ring
	115	is lit structure. D-054.
	116
`17ccee99` 2026-09-08 Pavel Safronov	117	And Tower keeps its clock on it. Two lit lines from the collector's hub: one
	118	running the full radius, fixed normal to the plane of the solar system, and one
	119	stopping short of the rim at an angle θ from it, where θ runs 0 to 2π over a day.
	120	The collector is the only surface in the sky worth writing on — it does not spin,
	121	and it is always dead ahead through the sun cap window.
	122
	123	The time is the angle between the two lines, and it has to be. From inside, the
	124	whole collector face appears to wheel once every 7 min 29 s along with the stars,
	125	because the observer co-rotates and it does not. A single hand against a fixed dial
	126	would be unreadable, since the dial is not fixed; the angle between two marks is
	127	invariant under that rotation. It is a clock built for people who cannot stop the
	128	dial turning. D-055.
	129
`866c7f96` 2026-09-07 Pavel Safronov	130	#### 1.3.1 The diffuser swarm
	131
	132	The diffusers must be individually invisible, or the sky becomes a row of separate
	133	suns. Two thresholds govern this, and only the second one binds:
	134
	135	- Resolvability. At 50 km, one arcminute is 14.5 m. Anything larger is a
	136	resolved disc with a visible edge.
	137	- Visibility. Being unresolved is not enough — stars are unresolved and we see
	138	them. A mirror preserves radiance, so a mirrorlet aimed at an observer shows
	139	the sun's own surface brightness over a tiny angle. A 1 m mirrorlet reads at
	140	magnitude −13.6, brighter than the full moon. Blending into daylight requires
	141	~1.2 cm, at magnitude −4.0, just under Venus.
	142
	143	Diverting 54.3 TW needs 11.3 km² of mirror, so:
	144
	145	\| \| \|
	146	\|---\|---\|
	147	\| Mirrorlet size \| 1.2 cm, ~1.4 cm², 2 mg \|
	148	\| Count \| 78 billion \|
	149	\| Total mass \| 158 tonnes — the entire sun system \|
	150	\| Mean spacing in beam \| 3.8 m — sparse enough to fly through \|
	151	\| Optical depth over 540 km \| 1.41, so 24 % of light reaches the far cap \|
	152
	153	The swarm cannot be passive — it must fly, always. The axis is not weightless.
	154	At the beam's edge gravity is 0.312 m/s², enough that a foil sinks outward at
	155	0.26 m/s and leaves the beam in 1.7 hours. A drifting swarm dissolves the same
	156	afternoon. Every foil is therefore under continuous individual control. This is not
	157	a flourish on a working system; it is the only configuration that works.
	158
	159	They have absurd power to do it with. Each foil intercepts 979 W of beam.
	160	Harvesting 0.1 % gives it ~1 W on a 2 mg body — 489,600 W/kg, some fifty times
	161	a jet engine's specific power.
	162
	163	\| \| \|
	164	\|---\|---\|
	165	\| Down-beam, riding radiation pressure \| 0.85 m/s, 7.4 days \|
	166	\| Back up-beam under its own power at 1.5 m/s \| 15 µW, 4.2 days \|
	167	\| Harvestable from ambient world-light \| 14,400 µW — a 1000× margin \|
	168
	169	So there is no recirculation machinery, at either cap. The foils fly home on
	170	scavenged daylight. Nothing needs to be built for them, and the 1000× power margin
	171	means Tower can afford to move them wastefully — in spirals, shoals and patterns
	172	that cost energy for no reason but preference.
	173
	174	Lighting is a render target, not a schedule. 78 billion independently aimed
	175	mirrors put any intensity on any patch of ground at any time. Dawn, dusk, moving
	176	cloud shadows and seasons are compositing problems, not optical ones.
	177
	178	Aiming resolution is human-sized. A 1.2 cm mirror is diffraction-limited to
	179	4.6×10⁻⁵ rad, so its reflection lands as a 2.3 m spot on the floor 50 km below.
	180	Tower can put a glint on one person that someone five metres away cannot see. The
	181	privacy is set by the wavelength of light, not by any protocol.
	182
	183	Mirrorfall. A failed foil sinks out of the beam and falls. Terminal velocity at
	184	the floor is 0.48 m/s; the descent takes about two days and it lands like a
	185	snowflake. There is a permanent slow glitter coming down out of Tower's sky.
	186
	187	#### Illumination is a dial, not a constant
	188
	189	54.3 TW is Tower's capability, not its setting. Spread over the inner surface it
	190	is 320 W/m² mean, which — with roughly half the surface lit at a time and a
	191	sinusoidal day profile — puts **local noon at ~1,000 W/m², Earth clear-sky
	192	daylight**. But the diffusers are software-controlled, so brightness is genuinely
	193	adjustable in-world across the full range:
	194
	195	\| Setting \| Mean W/m² \| Feels like \|
	196	\|---\|---\|---\|
	197	\| Twilight \| 30 \| permanent overcast dusk \|
	198	\| Temperate \| 200 \| Earth's global mean \|
	199	\| Full \| 320 \| Earth clear-sky noon \|
	200
	201	This should be a control in the app, not a fixed number. It is the most
	202	characterful thing about a world whose sun is a machine.
	203
	204	#### Headroom
	205
	206	5,000× is 11 % of the étendue limit (46,223× at 1 AU). Tower could in principle
	207	push the beam 9.2× further and cook itself; the diffuser network is as much a safety
	208	system as a lighting system. 5,000× was chosen because it is demonstrated
	209	technology — the Odeillo solar furnace reaches ~10 MW/m² on Earth today. Nothing in
	210	Tower's sun system requires physics that does not exist. See `decisions.md` D-003.
	211
`392bb8a7` 2026-09-08 Pavel Safronov	212	#### 1.3.2 Night, and the difference between banked and off
	213
	214	Night cannot be made by aiming the swarm away. The glow above belongs to the
	215	beam and the air, not to the diffusers, so a fully lit beam lights the world
	216	whatever the mirrorlets are doing: 779 W/m²/sr over a strip 3.65° wide running
	217	horizon to horizon is about 0.2 sr, which lands ~100 W/m² on the floor — half
	218	of temperate, and no kind of night. Tower's night is therefore made by **banking
	219	the beam itself, to a thousandth of full: 0.32 W/m²**, a rod you can still find
	220	overhead and a world you can just make out under it (D-046).
	221
	222	Shutting the beam down entirely is a different state and a much worse one. No
	223	airlight, no lit ground, no rod — the only thing left in the world is the sky
	224	through the far cap. §1.9.4 gives the thermal mass about a week of coasting, so
	225	this is a state Tower can survive and not one it can sit in.
	226
	227	\| Setting \| Floor \| What it is \|
	228	\|---\|---\|---\|
	229	\| Dark \| 0 \| the beam shut down. Stars, and nothing else. \|
	230	\| Night \| 0.32 W/m² \| the beam banked to a thousandth \|
	231	\| Twilight \| 30 W/m² \| \|
	232	\| Temperate \| 200 W/m² \| \|
	233	\| Full \| 320 W/m² \| Earth clear-sky noon \|
	234
`da6bcb0e` 2026-09-08 Pavel Safronov	235	#### 1.3.3 The far cap mirror, and where the last quarter of the beam goes
	236
	237	The swarm has optical depth 1.41, so 24% of the beam survives all 540 km — and
	238	it survives collimated, in the beam's own 7.96 km² cross-section. That is the
	239	whole shape of the problem: not 13 TW spread over a 7,854 km² cap, but **1.7 MW/m²
	240	in a 3.2 km spot, about 1,200 suns. An absorber there sits at 2,330 K**.
	241
	242	So the spot is a mirror (D-048), and only the spot: it is a thousandth of the
	243	cap's area, which is what leaves the other 99.9% free to be window (§1.4). A mirror
	244	is the cheap answer rather than the expensive one — at 99.9% it absorbs **1,665
	245	W/m²**, about 1.2 Earth solar constants, so the thing that would have glowed white
	246	is merely warm.
	247
	248	It also recovers the energy. The residual goes back up the column, which makes
	249	the optical path 1,080 km instead of 540 and utilisation 1 - e^(-2τ) = 94%
	250	rather than 76%. What survives even the return trip leaves through the beam
	251	aperture toward the collector, which sends it back again; in the limit the whole
	252	54.3 TW lands on the inner surface. **This is why full brightness is 320 W/m² and
	253	not 243** — the figure in §1.3 assumes the entire beam is eventually used, and with
	254	the cavity closed at both ends it is.
	255
	256	Aiming light at a region to warm it is a separate lever and stays one (§1.9.4). The
	257	mirror is about not wasting the beam; the desert is about where the weather goes.
	258
`2bb3e0de` 2026-09-08 Pavel Safronov	259	#### 1.3.4 The day, and why dawn has a direction
	260
	261	Tower's day is 24 hours, chosen for the people living under it — nothing
	262	physical offers one, since the spin is 7.5 minutes and has nothing to do with light.
	263
	264	Dawn cannot be a dimmer, and it cannot be the swarm either. §1.3 gives the glow
	265	to the beam and the air, so the sky, the airlight and the rod overhead all brighten
	266	together the moment beam power comes up; there is no way to have one end of the axis
	267	light before the other. But the ground is lit by the diffusers, and they can put
	268	the light wherever they like. So Tower brings the day up as a **front running
	269	deepward from the sun cap, 540 km in 40 minutes — 225 m/s**, near Tower's own rim
	270	speed, and slow enough to stand and watch arrive.
	271
	272	The result is a morning nothing on Earth has: a fully lit sky over dark ground,
	273	with a line of daylight coming toward you along the world. §1.9.4 says Tower has no
	274	latitude. The cycle gives it longitude — 40 minutes of it, sun cap to far cap.
	275
	276	Dusk runs the same way, so the light withdraws in the direction it arrived and the
	277	sun-cap end always has the shorter day by a hair. Nothing forces that; the
	278	alternative was a dusk that retreats, and a consistent direction of time reads as
	279	designed rather than as an oscillation.
	280
`866c7f96` 2026-09-07 Pavel Safronov	281	### 1.4 Sealed hull, and the cap windows
	282
	283	The cylinder wall is opaque and unbroken. No slats, no windows, no view out
	284	along any of the 540 km. From anywhere on the inner surface, the sky is land, haze
	285	and the beam — nothing else. See D-017.
	286
	287	The only views out are at the end caps, and they are peculiar:
	288
	289	- A cap is a wall, not a floor. Gravity there points radially outward, which is
	290	tangent to the cap's surface, so nothing rests on it — you would slide to the
	291	rim. Each cap is a 50 km vertical cliff standing at the end of the world, and
	292	its windows sit at its base, at eye level for anyone standing on the floor.
	293	- The sky through them does not sweep — it turns. A cap window looks along the
	294	spin axis, which is the celestial pole. Stars rotate about a fixed centre, one
	295	full turn every 7.5 minutes, with one motionless star at the middle. **A pinwheel
`769726ea` 2026-09-08 Pavel Safronov	296	sky.** It is the only moving sky anyone in Tower ever sees — and because the axis
	297	is horizontal from the floor, it wheels about a point on the horizon rather
	298	than overhead.
	299	- The far cap is glazed whole — a 100 km window looking into deep anti-sun
	300	space, not a wall with windows at its foot (D-045). Its structure survives as the
	301	window's mullions, on the same polar tessellation the checker used and at the
	302	same cell pitch, so the one object in the world with a known scale still is one:
	303	the rule now reads against the sky instead of against a plate. The sun cap
`9c891d2e` 2026-09-08 Pavel Safronov	304	faces the collector rather than the sun, and is glazed too (D-050): §1.3.0
	305	retires the "heavy glare management" this section used to demand of it, because no
	306	direct sunlight has ever reached it. Through it, a 244 km black disc, stars round
	307	the edge of it, and the beam blazing out of the aperture at its centre.
`769726ea` 2026-09-08 Pavel Safronov	308	- By day the window is simply blue. §1.5 puts the far cap 100–200 km into the
	309	haze, so from anywhere but the deepward end the airlight in front of it swamps
	310	whatever is behind, and the cap reads as a disc of sky. The stars are there at
	311	noon and unseeable, which is a fact about eyes and not about the glass.
`da6bcb0e` 2026-09-08 Pavel Safronov	312	- The beam does not go through it. §1.3.3 puts the surviving quarter of the
	313	beam on a 3.2 km bullseye at the centre of the cap, and that bullseye is a mirror
	314	rather than glass: it returns the residual up the column for a second pass. The
	315	window is everything else, which is 99.9% of the disc.
	316	- The mirror is not the pinwheel's centre, and it is worth being clear why. The
	317	sky turns about the direction parallel to the spin axis, and from anywhere on
	318	the floor that direction is horizontal — so **the pinwheel's centre is at eye
	319	level, dead ahead**, down where the window meets the ground, not overhead and not
	320	at the middle of the cap. The mirror is a physical object on the axis, which
	321	from 270 km sits about 10° above that. They coincide only for an observer
	322	standing on the axis itself, and nobody can. By day the mirror blazes — it is
	323	returning 1.7 MW/m² — and is the brightest thing in Tower's sky; at night it
	324	banks with the beam and goes dark.
`866c7f96` 2026-09-07 Pavel Safronov	325
	326	Nobody aboard can ever see Earth. Tower holds a terminator orbit (D-004), which
	327	puts Earth roughly 90° off the axis — out the side of the cylinder, which is now
	328	solid hull. A habitat in Earth orbit whose inhabitants have never seen the planet
	329	they orbit.
	330
	331	The caps are therefore the only places in the world with a horizon that is not
	332	Tower. Whatever grows up around them, it will not be ordinary.
	333
	334	### 1.5 Atmosphere and the look of the sky
	335
	336	Air co-rotates with the shell. Because g falls linearly to zero at the axis, the
	337	atmosphere is much deeper than Earth's:
	338
	339	- Pressure at the axis, 50 km up: 5.2 kPa — thin, but genuinely air.
	340	- A vertical column floor-to-axis is ~10,600 kg/m², about one Earth atmosphere.
	341
	342	So the interior sky behaves like this, and the shader must reproduce it:
	343
	344	- Straight up you look through ~1 atmosphere and see the far side of the world
	345	100 km away — visible, but washed to pale blue like a very distant mountain range.
	346	- Along the axis you look through many atmospheres. The far end cap is not
	347	visible as a wall; it dissolves into blue haze around 100–200 km out. The world
	348	reads as endless in the axial direction and closed in the circumferential one.
	349	- The ground curves up on both sides into that same blue, meeting overhead.
	350	- The beam is a self-luminous scattering shaft 3.65° wide at 779 W/m²/sr —
	351	render it as a bright sunlit cloud, not as a light source with a halo. With the
	352	hull sealed, it is the only thing in the sky that is not land.
	353
	354	### 1.6 Orbit and attitude
	355
	356	- Tower holds a high circular orbit with its **orbital plane kept normal to the
	357	Earth–Sun line** — a terminator orbit. This is why one cap can face the sun
	358	permanently and Tower is never eclipsed.
	359	- The plane precesses 360° per year to track the sun. Tower's angular momentum is
	360	enormous, so this 1°/day reorientation is actively driven, continuously, forever.
	361	Canon treats this as Tower's defining maintenance burden.
	362	- Consequence for the interior view: the star field slowly drifts across the year
	363	while spinning fast every 7.5 minutes. Two nested motions.
	364
	365	### 1.7 Coordinates
	366
	367	The canonical surface coordinate is (θ, z):
	368
	369	- `θ` — angle around the cylinder, 0 to 2π. θ = 0 is the centre of land strip 0.
	370	- `z` — distance from the sun cap, 0 to 540 km.
	371
	372	The 2D map is the direct unroll: `x = z`, `y = (θ − θ₀) · R`. No projection
	373	distortion — a cylinder unrolls exactly. This is a genuine advantage over mapping a
	374	globe and the map view should lean on it.
	375
	376	The map shows the surface once, bounded, so its extent is readable. That means
	377	it needs an edge, and the edge is placed at θ₀ = the centre of the last slat.
	378	Land strip 0 is centred on θ = 0, so cutting there would slice a whole strip in
	379	half across the top and bottom margins; cutting at a slat centre leaves every land
	380	strip whole and splits only a band of stars, which carries no geography. Same
	381	reasoning as putting Earth's map edge at the antimeridian. A sightline whose short
	382	route crosses the edge is drawn as two segments, so the drawn path always matches
	383	the measured distance.
	384
	385	---
	386
	387	### 1.8 Sightlines: Tower has two horizons
	388
	389	Along the axis the floor is perfectly flat and never falls away, so nothing is ever
	390	hidden by curvature. Tower's horizon is haze. Circumferentially the floor curves
	391	up, so distant land rises into the sky rather than sinking below a line. The two
	392	directions behave completely differently, and this is one of the most alien facts
	393	about living there.
	394
	395	Rayleigh transmission from a floor-level observer:
	396
	397	\| Distance \| Along the axis \| Across the interior (overhead) \|
	398	\|---\|---\|---\|
	399	\| 26 km \| 78 % \| 77 % \|
	400	\| 50 km \| 56 % \| 70 % \|
	401	\| 100 km \| 31 % \| 78 % \|
	402	\| 270 km (far cap from midpoint) \| 4 % — a ghost \| — \|
	403
	404	Two consequences:
	405
	406	- The axial horizon closes at 100–150 km. From the midpoint you cannot see
	407	either end cap. Everyone can see 100 km, knows there is more beyond, and cannot
	408	resolve it — a different psychology of distance than Earth's 5 km cliff-edge.
	409	- The far side of the world is clearer than the ground at the same distance,
	410	because a sightline across the interior rises through thin axis air while one
	411	along the floor grinds through full density. It gets better with distance: the
	412	land directly overhead, 100 km away, is the crispest far object in Tower at 78 %.
	413	Distant geography is best seen by looking up. People would orient by what is
	414	overhead; the ground is the vague part. This inverts the human default.
	415
	416	### 1.9 Water
	417
	418	Tower's hydrology is stranger than its geometry, and most of it follows from one
	419	fact and one absence.
	420
	421	#### 1.9.1 Coriolis never pushes surface water sideways
	422
	423	The floor's tangent plane is spanned by θ̂ and ẑ, and the spin axis is ẑ. For any
	424	flow along the surface:
	425
	426	- along the axis — `ẑ × ẑ = 0`, no Coriolis force at all;
	427	- around the circumference — `ẑ × θ̂ = −ŝ`, the force is purely radial.
	428
	429	The Coriolis term never has a component inside the surface. So **Tower's rivers
	430	do not curve.** No hemispheres, no Baer's law, no preferred bank erosion, no gyres,
	431	no cyclones. Every large-scale rotational habit of Earth's water and weather is
	432	absent, and a meander in Tower is pure flow dynamics with no thumb on the scale.
	433
	434	#### 1.9.2 What it does instead: everything vertical leans
	435
	436	The force goes into the floor rather than across it, so it appears whenever water
	437	moves up or down. Deflection of a free fall goes as h^1.5:
	438
`28f9a6bc` 2026-09-07 Pavel Safronov	439	\| Fall \| Lands (retrograde) \| Lean of the path \| Lean at impact \|
	440	\|---\|---\|---\|---\|
	441	\| 100 m \| 4.2 m \| 2.4° \| 3.6° \|
	442	\| 500 m \| 47 m \| 5.4° \| 8.1° \|
	443	\| 1,000 m \| 133 m \| 7.6° \| 11.3° \|
	444	\| 2,000 m \| 377 m \| 10.7° \| 15.8° \|
	445
	446	Two different angles, and the interior view needs both. The path leans by where
	447	the water lands against how far it fell — the tilt of a waterfall's plume seen
	448	whole. The impact lean is the direction the water is actually moving when it
	449	arrives, which is what a streak of it is drawn at, and it is half again the other in
	450	tangent. `src/canon/motion.ts` computes both.
`866c7f96` 2026-09-07 Pavel Safronov	451
	452	Rain falls permanently slanted — a raindrop at terminal velocity sits at a fixed
	453	1.47° against the spin, ~51 m of drift over a 2 km fall, never varying, never
`28f9a6bc` 2026-09-07 Pavel Safronov	454	reversing. It is fixed because drag holds the fall speed constant, so unlike free
	455	fall the angle does not grow with the drop: rain leans the same 1.5° whatever the
	456	cloud. Set against a waterfall's 11°, **the waterfall is the compass and the rain is
	457	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	458	anti-spinward. Every plume and every shower in Tower tilts, and they tilt against
	459	each other.
	460
	461	The same applies to anything that descends, not just water — see §1.11 on what it
	462	does to skiing.
	463
	464	Horizontal flow changes weight rather than direction (Eötvös): +0.57 % for a
	465	2 m/s prograde river, negligible hydrologically. It only matters at speed — an
	466	express train at 100 m/s gains 29 %, and anything moving retrograde at the 700 m/s
	467	rim speed is stationary in space and therefore weightless.
	468
	469	#### 1.9.3 A perfect cylinder has no slope anywhere
	470
	471	Potential depends only on radius, so **the whole floor is a single equipotential
	472	surface.** Water on it has no preferred direction: a perfect Tower has no rivers at
	473	all, forever.
	474
	475	Every river in Tower exists because the floor was shaped. There are no
	476	tectonics and no uplift, so erosion is strictly one-way — a 3 km range wears flat in
	477	~30 Myr under a wet engineered climate, ~150 Myr under a dry one. Tower's rivers
	478	are steadily destroying its landscape and the highlands must be rebuilt to keep them
	479	running. This is D-004's theme again: Tower's rivers are plumbing it chose to
	480	install and has to keep re-installing.
	481
	482	Two corollaries:
	483
	484	- No river can circumnavigate. A closed loop has no net descent.
	485	- Lakes curve upward. Level means constant radius, so across a 30 km lake the
	486	far shore rides 2.3 km above your tangent plane — Earth would hide it 18 m
	487	below the horizon. You look up at the opposite beach, and water appears to
	488	climb away from you.
	489
	490	#### 1.9.4 There is no passive climate at all
	491
	492	Tower has no latitude — every point is equidistant from the light — and with the
	493	hull sealed it has no windows either. Nothing breaks the thermal uniformity. A
	494	uniform hull radiates at 274 K everywhere, so left alone the world settles into a
	495	flat calm: no gradient, no wind, no evaporation, no rain, no rivers.
	496
	497	So every weather system in Tower is deliberate. Tower makes climate the only way
	498	left to it — by aiming light. It warms where it wants convection and shades where it
	499	wants condensation; the hull's uniform loss is the sink. Every cloud, every
	500	rainfall, every jet stream is something Tower is doing, continuously, on purpose.
	501
	502	This is D-009 arriving at its conclusion. The mirrorlet swarm was already Tower's
	503	body language; now it is also its lungs and its weather. There is no autonomic
	504	layer underneath. If Tower's attention ever lapses, the rain stops.
	505
	506	Costs, for scale against a 54.3 TW budget:
	507
	508	\| \| \| \|
	509	\|---\|---\|---\|
	510	\| Earth-like rainfall, 1,000 mm/yr \| 12.2 TW \| 22.4 % — Earth spends the same fraction \|
	511	\| Recirculating all runoff (1,792 m³/s) against 500 m \| 8.8 GW \| 0.016 % \|
	512
	513	#### 1.9.5 Rivers are a closed, pumped circulation
	514
	515	With no natural cycle and no natural slope, a Tower river is a loop: a headwater
	516	reservoir, a shaped descent through the landscape, a lake at the bottom, and a lift
	517	back to the top. Source and sink are the same system.
	518
	519	Two ways to close the loop, and they give completely different rivers:
	520
	521	- The pump outfall. A river begins at a machine. Walk to the source of any
	522	river and find a wall of water coming out of a structure — not a spring, an
	523	outlet. Costs a rounding error (above).
	524	- The made cloud. Tower evaporates the lake with aimed light and condenses it
	525	over the highlands, so rain falls exactly where the river should start. No
	526	machinery: the swarm is the pump. Costs the 22.4 % above, and buys real weather
	527	with it.
	528
	529	Canon uses both — made weather for the world's general hydrology, pumps for
	530	watercourses Tower wants under precise control.
	531
	532	Two consequences of the sealed hull:
	533
	534	- A river can circle the world. The surface is one unbroken sheet, so a channel
	535	can run the full 314 km circumference and close on itself — a lazy river returning
	536	to its own source. Gravity cannot drive a closed loop, so it is pumped.
	537	- The water inventory is conserved and exactly known. Nothing evaporates to
	538	space. Tower knows how much water it has, to the litre, forever.
	539
	540	Lifting is also slightly cheaper than on Earth, since gravity falls off with height:
	541	1 % cheaper at 1 km, 5 % at 5 km, 25 % at 25 km.
	542
	543	### 1.10 Slat orientation — RESOLVED: none
	544
	545	Axial strips versus rings was weighed at length (D-016) and answered by removing
	546	slats entirely (D-017). The comparison is kept in D-016 because it established what
	547	the choice cost, and because the reasoning — that slat layout sets the shape of
	548	Tower's countries, its climate map and its astronomy all at once — is what made
	549	"none" a real option rather than an absence.
	550
	551	### 1.11 The world as built
	552
	553	Authored geography lives in `content/geography.json` — the source of truth — and
	554	`npm run terrain` renders it. Names are provisional. Tower is laid out in **belts
	555	along the axis**, which is the natural grain of an engineered climate (§1.9.4):
	556
	557	\| z \| \| \|
	558	\|---\|---\|---\|
	559	\| 0–42 km \| The Sunward Sea \| ring ocean at the foot of the sun cap \|
`03607270` 2026-09-08 Pavel Safronov	560	\| 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	561	\| 148–218 km \| The Dry Reach \| desert ringing the world, 22,000 km², 13% of the surface \|
	562	\| 176/190/203 km \| three ridges \| ringing, concentric — and The Narrows is the only breach in all three \|
	563	\| 214–430 km \| Greatwood \| forest belt \|
	564	\| 248–402 km \| The Deep \| 154 × 32 km sea, long down the axis and narrow around \|
	565	\| 270 km \| The Capital / The Spindle \| axial midpoint, on The Deep's antispinward shore \|
	566	\| 496–540 km \| The Deepward Sea \| ring ocean at the foot of the far cap \|
	567
	568	#### Why the desert rings
	569
	570	On a cylinder you cannot go around a band. Every desert on Earth can be
	571	circumnavigated; a ringed one cannot. It is an absolute barrier across the world's
	572	waist, and with the three ridges ringing too it becomes three concentric walls.
	573
	574	#### The drainage, and why it keeps the chokepoint
	575
	576	Four rivers come off the range — Westwall Water, Sisters Water, Eastwall Water and
	577	The Long Water. On Earth most drainages do not cut through a rising ridge; they
	578	are defeated and diverted, and only an antecedent river with enough power keeps
	579	its course. So three of the four are turned by First Ridge, run along its front
	580	gathering each other, and leave with the fourth through the single gap.
	581
	582	- The Narrows carries the entire world's runoff — ~392 m³/s off a 9,800 km²
	583	range, lifted 1,780 m for 6.9 GW (0.013% of budget).
	584	- The ridge-front reaches are the only circumferential water in Tower, and
	585	weigh fractionally more or less for it (§1.9.2). Everything else runs axially,
	586	where Coriolis is exactly zero.
	587	- Whoever holds The Narrows holds the axis of the world. The Capital sits behind it.
	588
	589	#### The Deep is oriented for the view
	590
	591	Long down the axis and narrow around it, so you cross it the short way — around
	592	θ, where the floor curves up. From the Capital the far shore is 32 km away and
	593	stands 2.6 km above eye level. Crossing it lengthwise instead would be dead
	594	flat. The orientation is the whole point.
	595
	596	It is also the world's flywheel: 19× the diurnal heat storage of bare ground, and
	597	5.9 days at full power to move its temperature one kelvin. Tower still supplies
	598	every joule (§1.9.4), but the world now coasts for about a week rather than
	599	stopping the moment attention lapses.
	600
	601	#### The cap seas are not designed
	602
	603	Gravity on an end cap points radially outward — tangent to the cap. Nothing rests
	604	there; water runs down to the rim and pools. **Both ring oceans exist whether anyone
	605	planned them or not**, each with a 314 km shoreline and a 50 km cliff standing
	606	behind it. The only windows in the world are at their feet (§1.4).
	607
	608	#### Skiing, because it is a different sport here
	609
	610	Level motion feels no Coriolis (§1.9.1) but a descending skier has radial velocity,
	611	and that does deflect:
	612
	613	- Down the axis: a steady sideways push, always antispinward — 1.5% of g at
	614	15 m/s on a 20° slope, 6.4% at 35 m/s on 40°.
	615	- Around the cylinder: you get heavier or lighter. 8.6% at 30 m/s, 11.4% at 40.
	616
	617	So a retrograde run gets lighter the faster you go, which is self-reinforcing,
	618	while a prograde run grips harder as it accelerates. Every slope in Tower has a
	619	handedness. No Earth mountain does. Settlements sit at the tarns — Sisters Station,
	620	Hollow Station — around 2,100 m, with the runs above them.
	621
	622	#### Open questions
	623
	624	- The Spindle's height. 3.2 km is a placeholder; an object dropped from its top
	625	lands ~760 m antispinward of its base.
`03607270` 2026-09-08 Pavel Safronov	626	- z 430–496 km is unauthored. The circumference outside the named peaks is no
	627	longer empty — The Girdle carries the range round the whole 314 km — but it is
	628	unwalked: peaks with no names, and no rivers drawn off them yet.
`866c7f96` 2026-09-07 Pavel Safronov	629
	630	## 2. Application architecture
	631
	632	### 2.1 Stack
	633
	634	- Vite + TypeScript (strict), no framework — the app is three canvases and
	635	a control panel, and a framework would not earn its weight.
`fc11b064` 2026-09-08 Pavel Safronov	636	- One layout breakpoint, at 720 px: above it the panel is a column beside the
	637	world, below it a drawer over it (D-043). The world is a full-bleed canvas in
	638	both, because all three views are drag-to-look and a scrolling page would be
	639	fighting them for every touch.
`866c7f96` 2026-09-07 Pavel Safronov	640	- Three.js for the exterior and interior 3D views.
	641	- Canvas 2D for the map view.
`62458672` 2026-09-07 Pavel Safronov	642	- GitLab CI → GitLab Pages, publishing `dist/` as `static/`.
`866c7f96` 2026-09-07 Pavel Safronov	643
	644	### 2.2 Module layout
	645
	646	```
	647	src/
	648	canon/
	649	constants.ts single source of truth for every number in §1
	650	geometry.ts (θ,z) <-> 3D, slat layout, map <-> surface
`28f9a6bc` 2026-09-07 Pavel Safronov	651	atmosphere.ts the rotational density profile and its extinction (§1.5)
	652	motion.ts how things fall in the rotating frame (§1.9.2) — the only
`65484043` 2026-09-08 Pavel Safronov	653	evidence of the spin anywhere but the far cap window
`866c7f96` 2026-09-07 Pavel Safronov	654	views/
	655	palette.ts one colour per place kind, shared by map, list and editor
`28f9a6bc` 2026-09-07 Pavel Safronov	656	raster.ts what npm run terrain wrote: grid, and what white means
`866c7f96` 2026-09-07 Pavel Safronov	657	world/
	658	schema.ts world.json types + validation
	659	store.ts places CRUD, localStorage drafts, JSON import/export
	660	instruments.ts sightline, viewshed, lighting probe, travel (§3.3)
	661	state/
	662	store.ts selected point, time-of-day, spin phase, view mode, writer flag
	663	views/
`5f38efd0` 2026-09-07 Pavel Safronov	664	exterior.ts Three.js: hull, caps, collector, beam, spin, point picker
`866c7f96` 2026-09-07 Pavel Safronov	665	map.ts Canvas 2D: unrolled surface, slats, terminator, point picker
`28f9a6bc` 2026-09-07 Pavel Safronov	666	interior.ts Three.js: shell geometry, the haze shader of §1.5, beam, rain
`866c7f96` 2026-09-07 Pavel Safronov	667	ui/
`28f9a6bc` 2026-09-07 Pavel Safronov	668	app.ts the whole panel: tools, editor, readouts, view switching
	669	scales.ts slider mappings — kept out of views/ so the panel can label
	670	the eye-height slider without pulling in Three.js
`866c7f96` 2026-09-07 Pavel Safronov	671	main.ts
`6364748a` 2026-09-08 Pavel Safronov	672
	673	tools/ node-only, never bundled; plain .mjs with no build step
	674	terrain.mjs renders content/geography.json into static/ (§1.11)
	675	png.mjs minimal PNG encoder — zlib is its only dependency
	676	noise.mjs fbm and ridged noise for the terrain relief
	677	screenshots.mjs drives the running application in a headless browser and
	678	photographs it into docs/screenshots/ (D-042)
`866c7f96` 2026-09-07 Pavel Safronov	679	```
	680
	681	`canon/constants.ts` is the spine. Every derived figure — period, rim speed, land
	682	area, slat width — is computed from radius, length, slat count and target land
	683	area, never hard-coded. Changing the radius changes the whole app coherently.
	684
	685	### 2.3 State
	686
	687	One small observable store. Views subscribe; nothing owns anything else.
	688
	689	```ts
	690	type TowerState = {
	691	point: { theta: number; z: number } \| null // the picked location
	692	tod: number // 0..1, position in the diffuser day cycle
	693	brightness: number // W/m^2 mean, 30..320 -- the diffuser output setting
	694	spinPhase: number // 0..2pi, rotation for star motion
	695	spinning: boolean
	696	view: 'exterior' \| 'map' \| 'interior'
	697	writer: boolean // ?writer=1 -- a UI mode, not a security boundary
	698	probe: { from: SurfacePoint; to: SurfacePoint } \| null
	699	}
	700	```
	701
	702	The exterior and map pickers both write `point`; the interior view reads it. That
	703	is the entire interaction loop.
	704
`5f38efd0` 2026-09-07 Pavel Safronov	705	### 2.4 The exterior view
	706
	707	Scene units are kilometres, so the code reads in canon numbers. Tower's axis lies
	708	along world Z with the sun cap at z = 0, which puts a canon surface point at
	709	`(R cos θ, R sin θ, z)` in the hull's own frame. The hull spins about Z and carries
	710	that frame with it, so **picking resolves the ray hit back through
	711	`worldToLocal`** — otherwise every point picked off a turning hull would be wrong by
	712	the spin phase.
	713
	714	Two deliberate departures from physics, both reading aids of the same kind as the
	715	map's hillshade:
	716
	717	- Fill lighting. Real sunlight here runs parallel to the axis, so Tower's flank
	718	is edge-on to it and renders very nearly black — accurate and useless. Two fill
	719	lights lift the hull enough to read its shape.
	720	- A visible beam. In vacuum there is nothing between collector and cap to
`8b6287ba` 2026-09-07 Pavel Safronov	721	scatter light, so the converging beam would not be visible at all. It is drawn
	722	as rays because it is the thing the view is about.
	723
	724	Everything in the scene is captioned — collector, beam, both caps — with screen-space
	725	labels. A pale disc and a cone of light are not self-explanatory, and the collector
	726	is the least intuitive object in Tower: it is larger than the world it feeds.
`5f38efd0` 2026-09-07 Pavel Safronov	727
	728	The default camera frames the whole system rather than Tower alone: the collector
	729	is 244 km across and flies 320 km ahead, and it is the larger object.
	730
`65484043` 2026-09-08 Pavel Safronov	731	The hull has two readings, and the view switches between them (D-047). True
	732	is the outer skin: bare structure, the checkered sun cap, and the glazed far cap.
	733	X-ray wraps the terrain raster onto the outside instead — from outside Tower is
	734	structure, so what that texture shows is what is inside, which makes it a diagram
	735	and not a view. Both are worth having and they are not the same claim, so the
	736	control is a two-way switch rather than a checkbox.
	737
	738	The wall is drawn as two meshes rather than one double-sided one, because its two
	739	faces show different things: the outer skin changes with the mode, and the inner
	740	surface is always the land. Since the far cap became a window (D-045) that inner
	741	surface is genuinely visible from outside — look in through the glass in true mode
	742	and the world is really there, lit and 540 km deep. It is the only line of sight
	743	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	744	way round, so the UVs are rewritten from the original attribute rather than derived
	745	from vertex positions — the seam vertices are duplicated, and deriving an angle from
	746	them smears one column of triangles across the whole image.
	747
`15d933c9` 2026-09-07 Pavel Safronov	748	The camera position persists across reloads, alongside the active view, so
	749	returning to the page returns to the vantage you left. Both end caps are places in
	750	their own right, so the view can frame them: a cap's marker sits on the axis,
	751	because a cap is a disc across the end and a point on its rim would ride round the
	752	edge as Tower turns.
	753
`11a13dbf` 2026-09-07 Pavel Safronov	754	Markers are scaled per frame to hold a roughly constant size on screen. A 4 km dot
	755	is legible from 200 km and sub-pixel from the 1,200 km of the default view, which is
	756	to say the places would simply not be there.
	757
`28f9a6bc` 2026-09-07 Pavel Safronov	758	### 2.4 The interior view — built
	759
	760	Scope held: real geometry, simple ground.
	761
	762	Real: the shell is the actual cylinder, curving up on both sides and closing
	763	overhead, displaced by `terrain-height.png` and painted with `terrain-color.png`.
`65484043` 2026-09-08 Pavel Safronov	764	Stars only through the far cap — the wall is sealed (§1.4), so for 539 of the
	765	540 km the sky is land and light. The far cap is glazed (D-045) and the sky beyond
	766	it is sampled by view direction in an azimuthal-equidistant projection about the
	767	spin axis, which makes the pinwheel fall out of the projection rather than being
	768	animated into it: azimuth plus spin phase, one turn every 7 min 29 s, and the
	769	point on the axis genuinely motionless.
`28f9a6bc` 2026-09-07 Pavel Safronov	770
	771	Simple: the ground is the raster plus a little procedural grain in the near field,
	772	not modelled scenery. No buildings, no vegetation meshes. Cells are 0.7 km and
	773	texels 264 m, so relief a valley away is real and the ground underfoot is not.
	774	Detailed procedural scenery stays deferred.
	775
	776	Haze is the shader, and it is integrated rather than faked. Every fragment
	777	samples the rotational density profile 24 times along the ray to the camera and
	778	extinguishes per channel — red survives twice as far as green, blue half as far,
	779	which is what makes distance blue rather than grey. From the floor it reproduces
	780	§1.8 from the inside: 78% straight up against 31% for the same 100 km along the
	781	ground, printed in the readout so the claim can be checked rather than believed.
	782	The airlight the haze adds is neutral; tinting it blue double-counts the
	783	wavelength dependence and turns even the desert mauve (D-034).
	784
`65484043` 2026-09-08 Pavel Safronov	785	Nothing inside the scene rotates, because the observer, the ground and the air
`e27aaf20` 2026-09-08 Pavel Safronov	786	all co-rotate. What falls carries the spin, and §1.9.2 is lopsided about how: rain
	787	leans a fixed 1.5° whatever the cloud, because drag pins its speed, while only free
	788	fall keeps accelerating and so keeps leaning. **The waterfall is the compass; the
	789	rain is barely a hint.** Rain is no longer drawn (D-049) — 1.5° is not a picture,
	790	and the pinwheel below does the job it was standing in for. The physics stays in
	791	`canon/motion.ts` and stays tested; it was only ever the rendering that could not
	792	carry it.
`65484043` 2026-09-08 Pavel Safronov	793
`9c891d2e` 2026-09-08 Pavel Safronov	794	The one thing that does turn is outside. The sky through the caps does not
`65484043` 2026-09-08 Pavel Safronov	795	co-rotate, so it wheels once every 7 min 29 s about the motionless star on the axis
`e27aaf20` 2026-09-08 Pavel Safronov	796	(D-045). It is by a wide margin the plainest evidence of the spin in the view, and
`9c891d2e` 2026-09-08 Pavel Safronov	797	it is the only evidence left now that the rain is gone. Both caps are windows
	798	(D-050), so it can be had looking either way along the axis — deepward into empty
	799	space, or sunward past the black disc of the collector.
`28f9a6bc` 2026-09-07 Pavel Safronov	800
	801	The observer stands on the terrain, which means the height raster is read back onto
	802	the CPU and sampled against the mesh's own cell corners — the GPU cannot be asked
	803	where the ground is (D-035).
	804
	805	Instruments in the view: eye height from 1.7 m to 25 km with the local gravity
`65484043` 2026-09-08 Pavel Safronov	806	beside it, the five canon illumination settings of §1.3, the field of view as a
`28f9a6bc` 2026-09-07 Pavel Safronov	807	pair of binoculars, and captions on every place still visible through the haze.
	808	Selecting a place walks the observer to it (D-038) — the haze horizon closes at
	809	100–150 km, so turning to face something usually just points at haze.
	810
`65484043` 2026-09-08 Pavel Safronov	811	The caps are no longer the same object. The sun cap is checkered (D-041) —
	812	polar, six rings, cells about 8 km. The far cap is glazed (D-045), and the
	813	checker survives on it as the window's mullions: the same tessellation at the same
	814	pitch, so it is still the only object in the world with a known size and still
	815	doubles as a scale for judging distance, now read against sky rather than plate.
	816	Both patterns are shared with the exterior view.
	817
	818	The far cap's glazing is set a kilometre into its own frame, so from inside there is
	819	a 1 km reveal around it. That is the only place the wall depth of D-044 can be
	820	seen from within the world, which is why it is the only place it is drawn.
`28f9a6bc` 2026-09-07 Pavel Safronov	821
	822	Not modelled: water has no surface. Lakes and seas are painted in the raster and cut
	823	into the shell as basins, so standing in one puts you on its floor.
`866c7f96` 2026-09-07 Pavel Safronov	824
	825	---
	826
	827	## 3. Writer mode and the world file
	828
	829	The app is an instrument for writing the novel as much as a thing to look at. It
	830	must answer questions like can the far face be seen from the midpoint?, *where
	831	does a central embassy go?, can a lake overhang?* — and let their answers be
	832	placed, named and kept.
	833
	834	### 3.1 Two modes over one data file
	835
	836	\| \| Reader (default) \| Writer (`?writer=1`) \|
	837	\|---\|---\|---\|
	838	\| World content \| committed `world.json`, read-only \| editable, drafts in `localStorage` \|
	839	\| Instruments (§3.3) \| sightline probe only \| all \|
	840	\| Output \| permalinks to a point or place \| Export JSON for the author to commit \|
	841	\| Panel \| read-only detail card \| full editor, canon controls, import/export \|
	842
	843	There is no backend — GitLab Pages is static. The loop is: the author edits in the
	844	browser, exports `world.json`, commits it, and the published site serves it. Simple,
	845	versioned in git alongside the prose, and diffable.
	846
	847	Writer mode is a UI mode, not a security boundary. Anyone may set `?writer=1`;
	848	they can only edit their own local copy, which persists nowhere and reaches nobody.
	849	This is fine, and arguably good — a reader who finds it gets a sandbox. It must not
	850	be mistaken for access control.
	851
	852	### 3.2 The world file
	853
	854	```jsonc
	855	{
	856	"version": 1,
	857	"places": [{
	858	"id": "hallow-crossing", // permalink target; see below
	859	"name": "Hallow Crossing",
	860	"kind": "city\|embassy\|lake\|region\|landmark\|note",
	861	"geometry": { "type": "point", "theta": 1.9635, "z": 270000 },
	862	// or { "type": "polygon", "points": [[theta,z], ...] } for lakes, regions
	863	"blurb": "public description, shown to readers",
	864	"notes": "the author's working notes — NEVER shipped",
	865	"public": true // false = not ready to exist publicly
	866	}]
	867	}
	868	```
	869
	870	Two text fields, deliberately separate. `blurb` is written for readers and
	871	ships; `notes` is the author's working thinking and never leaves the machine except
	872	in a backup. One field with a flag would have made every note a publication
	873	accident waiting to happen.
	874
	875	Ids follow the name until pinned. An id is a permalink target and must be
	876	stable once published, but during world-building names churn constantly and a file
	877	full of `new-city-7` is unreadable. So the id tracks the name while it still
	878	matches it, and editing the id field freezes it for good. The editor shows the id
	879	for exactly this reason.
	880
	881	Coordinates are canon (θ radians, z metres, §1.7) — never map pixels, so the file
	882	survives any change to how the map is drawn.
	883
	884	The `public` flag is a privacy trap worth naming. Working notes ("embassy here?
	885	check sightline to the cap") must not ship to readers. But a `world.json` committed
	886	to a public repository is public regardless of any flag in it. So export writes two
	887	files: `world.json` (public entries only, committed) and
	888	`world.private.json` (everything, gitignored by default). Whether private notes
	889	should nonetheless be version-controlled — in a private repo, or a second one — is
	890	the author's call and is deliberately unresolved.
	891
	892	### 3.3 Instruments
	893
	894	The measuring tools are the real substance of writer mode. Each answers a question
	895	the novel actually raised:
	896
	897	- Sightline probe. Pick two points; get surface distance, straight-line
	898	distance, optical depth, transmission, and a plain verdict (*crisp / hazy / ghost
	899	/ invisible*). This is the tool that answered §1.8. Available to readers too — it
	900	is the single most illustrative thing in the app.
	901	- Viewshed overlay. From a selected point, shade the whole map by transmission.
	902	It renders the axis-versus-overhead asymmetry as one picture, which is worth more
	903	than the table in §1.8.
	904	- Lighting probe. At a point, illumination across the day cycle at the current
	905	brightness setting (§1.3) — for writing a scene at a specific hour.
	906	- Travel. Surface distance and journey time between points at chosen speeds,
	907	including whether the route crosses a slat.
	908	- Sky-at-a-time. Star field, beam position and Earth transits for a given point
	909	and moment, so a described sky is a correct sky.
	910
	911	### 3.4 What readers get
	912
	913	The public link is the finished map: embassies, towns, lakes, named regions — the
	914	permanent things, behaving like any map application. Plus the exterior view, the
	915	interior view from any point, and permalinks so a reader can be handed a place and
	916	a moment. Reading the world, not editing it.
	917
	918	## 4. Phases
	919
	920	1. Design — this document and `decisions.md`. ← current
	921	2. Scaffold — Vite/TS/Three, CI to Pages, `canon/` implemented and unit-tested.
`5f38efd0` 2026-09-07 Pavel Safronov	922	3. Exterior — ✅ hull, caps, collector, beam, spin, and hull point picking.
`866c7f96` 2026-09-07 Pavel Safronov	923	4. Map — unrolled surface, slat rendering, picking, sync with exterior.
`28f9a6bc` 2026-09-07 Pavel Safronov	924	5. Interior — ✅ shell geometry, the haze shader, the beam, rain, eye height,
`65484043` 2026-09-08 Pavel Safronov	925	and the five illumination settings. Stars, but only through the far cap window.
`866c7f96` 2026-09-07 Pavel Safronov	926	6. Writer mode — ✅ *built ahead of 3–5, at the author's request, because the
	927	world needs developing before there is anything to render.* Map view, place
	928	editing for all thirteen kinds, points/polylines/polygons, localStorage
	929	autosave, backup and static exports, import, and the sightline probe.
	930	7. Content pipeline and reader mode — ✅ `content/geography.json` +
	931	`npm run terrain` (D-019); the rendered terrain drawn under the map; reader and
	932	writer modes (D-011); permalinks (D-010).
`6364748a` 2026-09-08 Pavel Safronov	933	8. Polish — mobile ✅ (the sidebar is a drawer below 720 px, D-043) and a
	934	screenshot tour that photographs all ten views through the UI, D-042.
`fc11b064` 2026-09-08 Pavel Safronov	935	Remaining: transitions, performance.
`866c7f96` 2026-09-07 Pavel Safronov	936
`28f9a6bc` 2026-09-07 Pavel Safronov	937	All five view phases are built. Phase 2's `canon/` landed with phase 6, since the
	938	map could not be drawn without it.
`866c7f96` 2026-09-07 Pavel Safronov	939
	940	Shareable permalinks are implemented (phase 7) — the app has
	941	readers, and "stand here and look at this" is the whole reason it exists as a
	942	companion piece. Named locations from the novel should be markable on the map for
	943	the same reason.
	944
	945	Later, explicitly out of scope for now: procedural scenery, settlements, weather
	946	systems.