TITLE The Astrolabe PROMPT A museum exhibit of a planispheric astrolabe, the analogue computer of the medieval sky, drawn in SVG as a richly engraved brass instrument whose parts the visitor can turn and take apart. The geometry is real stereographic projection computed in the page (tropics and equator as concentric circles; almucantars: circles of equal altitude: and azimuth arcs for a chosen latitude on the plate; the ecliptic as an off-centre circle on the rete). The star pointers on the rete are presented as an illustrative, stylised set (about a dozen flame-shaped pointers labelled with decorative invented names in the exhibit's own cartouche style, and the label says plainly that the star positions on this demonstration model are schematic), so no inaccurate star data is claimed. Concept: let people handle an instrument they would never be allowed to touch. Mood: scholarly, warm, golden-age-of-instruments; a vitrine in a history-of-science museum. Palette: vellum/parchment gallery ground #EADFC6, brass in two tones: shadow brass #8A6A2F and bright brass #C8A951 (with a pale highlight #E9D79A), ink blue-black #1F2A30 for type and engraving shadows, cinnabar red #7B2D26 for the rule's index line, numerals of note and the exhibit number. Light page with soft drop shadows as if the instrument lies on a parchment-covered table under a lamp. The instrument (layered SVG, each layer independently transformable, with engraved line work: hairlines, double rings, degree scales with ticks at 1°, 5°, 10°, numbers every 10° or 15° rotated along the circle): (1) the mater: the body with a raised limb engraved with a 360° scale and the 24 hours (two sets of I–XII), and the throne at the top with a decorative openwork design and a suspension ring (shackle) that swings slightly; (2) the plate (tympan) for latitude: stereographic almucantars every 5° or 10° from horizon to zenith, azimuth arcs, the horizon line, twilight line, and unequal-hour lines below the horizon; three plates available for latitudes 30°, 40° and 52°: switching them lifts the rete, swaps the plate with a slide, and sets everything back; (3) the rete: the pierced star map: a network of curved brass straps with flame/leaf-shaped star pointers, the ecliptic ring divided into twelve 30° signs marked by engraved zodiac names in small capitals and degree ticks; ornate but legible; (4) the rule: a straight double-ended arm with a declination scale, pivoting at the centre; (5) the pin and the "horse" wedge that holds it all together. Interaction: drag the rete to rotate it (the daily turning of the heavens): it turns with weight and a little inertia, and a faint ratchet tick; drag the rule separately. A live panel reads the instrument as the visitor works it: the visitor chooses a date (the page finds the Sun's place on the ecliptic from the date with the standard approximate longitude formula) and the instrument shows the Sun's point on the ecliptic with a small red bead; rotating the rete until that point reaches the eastern horizon line shows the time of sunrise read off the limb by the rule: the panel explains in plain steps ("1. Find today on the ecliptic: 17° Scorpio. 2. Turn the rete until that point touches the horizon in the east. 3. Lay the rule through it. 4. Read the limb: about 7:10."). A "Set to now" control turns the rete to the current real local time for the chosen plate, so the instrument shows which part of the ecliptic is rising at this moment. "Take it apart" (signature moment): the instrument explodes in a slow, precise axonometric separation: pin and horse withdraw, the rule lifts, the rete rises and tilts, the plate slides out, each part casting its own shadow and gaining a label with a leader line and a one-sentence explanation, then reassembles on request in reverse order with soft brass-on-brass settles. Exhibit apparatus: a left column of museum text: object label ("Planispheric astrolabe (demonstration model). Brass, engraved. After instruments made between the 10th and 16th centuries across the Islamic world and Europe."), a three-paragraph wall text, plain and accurate: what it did (tell time by Sun or stars, find the direction and times for prayer, survey heights, cast horoscopes, teach astronomy), why the latitude plate must match the observer, why the map looks strange (the sky projected from the south pole onto the equator's plane). "Try it" tasks as three numbered cards: find sunrise; find how long today is; find what is culminating now. Exhibit number in red "IV.12". Typography: wall text in a dignified book serif (Baskerville/Palatino/Georgia), title in spaced capitals; engraved lettering on the instrument in small roman capitals following the curves (SVG textPath) with varying sizes; numerals engraved in an old-style manner. Entrance: the instrument is lowered onto the table (scale and shadow settle), the shackle swings, and the rete makes one slow full turn before coming to rest at "now": the first three seconds. Hover states: parts glow slightly brighter brass with a thin red outline when grabbable; cursor changes to an open/closed hand drawn in SVG. Keyboard: Tab to rete or rule, arrows rotate by 1° (Shift 15°). Touch: one-finger drag rotates the nearest part. Reduced motion: no intro turn, no inertia, explode view cross-fades. Responsive: on phones the instrument takes the full width with the reading panel beneath; text labels on the instrument thin out below 400px. As built: almucantars are engraved every 5° (heavier every 10°), azimuth arcs every 15°, limb degree numerals every 10°; there are twelve star pointers; the rule is counter-changed so that its red index edge runs through the centre; the date is chosen with day and month steppers; the reading panel offers the three tasks as tabs with a "Show me" control that works the instrument through the chosen task, and the rete and rule settle gently onto the horizon, the Sun's point or the meridian when released close to them; the ratchet tick and the brass settles are heard only once the visitor switches sound on; the suspension ring can be flicked; on phones the exploded view labels carry names only and the one-sentence explanations move into the panel beneath. DESCRIPTION A history-of-science vitrine with the glass taken away. A large engraved brass astrolabe lies on a lamplit parchment table beside a column of museum text headed by the cinnabar exhibit number IV.12 and the invitation "Please touch." The instrument is lowered onto the table, its shackle swings, and the rete makes one slow turn before stopping at the present hour with the rule laid through the Sun's bead, so that at rest it is already telling the time. Every circle on it is real stereographic geometry for the chosen latitude, and a label-card panel reads the instrument back in plain language as the visitor turns it: the Sun's place in the zodiac, its height, the hour under the rule, the degree rising and the degree culminating. Three numbered tasks teach sunrise, day length and culmination step by step, ticking themselves off as the visitor succeeds. On request the whole instrument separates into a calm axonometric stack of pin, horse, rule, rete, plate and mater, each named and explained, and then settles back together. VISUAL TECHNIQUES - Six independently transformed inline-SVG layers (mater, plate, rete, rule, pin, horse) stacked in a CSS 3D stage with orthographic (axonometric) tilt and per-layer depth - Stereographic projection computed in the page: tropics and equator, almucantars, azimuth arcs, horizon, twilight line, unequal-hour arcs through three points, and the off-centre ecliptic with sign boundaries and degree ticks - Sun's ecliptic longitude from the date by the standard approximate formula, with rising and culminating degrees solved live - Brass rendered with layered gradients in the two brass tones and pale highlight, turbulence-filtered brushed texture, dark edge strokes drawn beneath bright fills so pierced straps read as one cut sheet, and hard-edge plus soft drop shadows for thickness - Engraved lettering on curves with SVG textPath: zodiac names sized to their unequal arcs, plate inscriptions, horizon and twilight names, invented star names along flame-shaped pointers; Roman hours and old-style numerals rotated around the limb - Cast shadows in the exploded view: a blurred silhouette of the actual pierced rete falls on the plate, clipped to the part beneath - Custom tween engine with hand-tuned cubic-bezier and settle curves, spring-and-friction rotation for the rete and rule, and a damped pendulum for the shackle - Leader lines and labels tracked to the moving parts in screen space - Parchment ground with fibre noise, lamp pool and vignette; hand cursors drawn as inline SVG - Web Audio ratchet ticks and brass settles, off until switched on INTERACTION MODEL Drag the rete to turn the heavens: it follows with weight, coasts with a little inertia and settles onto the horizon when the Sun's bead is released near it; drag the rule separately to lay it through a point and read the hour on the limb. Grabbable parts brighten with a thin red outline and the cursor becomes an open, then closed, hand; on touch a one-finger drag turns the nearest part. Tab reaches the rete and the rule, where the arrow keys turn one degree and Shift with an arrow turns fifteen. In the panel, the day and month steppers move the Sun's bead along the ecliptic, the three plate buttons lift the rete, slide the old plate out and a new one in, and the task tabs (or the three "Try it" cards in the wall text) choose a numbered procedure whose steps mark themselves done as the visitor performs them; "Show me" performs the task on the instrument. "Set to now" turns the rete to the present local hour. "Take it apart" separates the instrument into a labelled axonometric stack and "Put it back together" reassembles it in reverse order. The sound control enables the ratchet and settle sounds, and the suspension ring swings when flicked. With reduced motion the entrance turn and inertia are removed and the exploded view cross-fades.