TITLE
A Year of Daylight

PROMPT
A data visualization of how long the sun is up, everywhere, every day of the year: computed in the page from astronomy (solar declination from the day of year via the sun's mean longitude and anomaly, then sunrise hour angle from latitude: cos ω = −tan φ · tan δ, with polar day and polar night where the formula has no solution). No external data. Concept: one elegant chart that makes the seasons physically obvious: the hourglass of light widening and pinching as you move from the equator to the poles.

Mood: luminous, clear, optimistic; a science-museum wall chart with a strong graphic identity. Palette: pale sunlight yellow #FFF4C2 (page ground and day), strong sun yellow #F7B801, orange #F18701 for twilight bands and highlights, deep twilight blue #1B2A5B, midnight navy #0B1026 for night. Light page: the ground is the pale yellow; night appears only inside the charts.

Main chart ("the daylight hourglass"): for the chosen latitude, a large chart with the 365 days of the year along the horizontal axis (month initials, solstices and equinoxes marked with thin rules and small labels) and the 24 hours of the day on the vertical axis (midnight at top and bottom, noon in the middle, in local solar time). The area between sunrise and sunset is filled sun-yellow; civil, nautical and astronomical twilight are drawn as three successively darker bands around it, orange, a plum mix #8A4660, then twilight blue (sun 6°, 12°, 18° below the horizon, each computed with the same formula using the altitude term); night is navy with a sprinkling of tiny stars. At the equator the band is a flat ribbon; at 52° it is a plump lens; at 69° it balloons into midnight sun in summer and closes entirely in winter. The selected date (today on load) is marked with a vertical hairline carrying sunrise and sunset times and a dot at the current time; when another date is scrubbed, today stays marked with a dotted line and its dot. Labels "MIDNIGHT SUN" and "POLAR NIGHT" appear inside the chart where they apply, and the chart's headline number is today's day length at that latitude in hours and minutes, set very large, with the daily change ("gaining 2 min 41 s a day").

Signature moment: dragging latitude. A vertical latitude control at the left, drawn as a meridian arc of a small globe with a sliding marker from 90°N to 90°S (drag with pointer/finger, arrow keys, or pick a city chip), reshapes the chart continuously and smoothly at 60fps as the marker moves: the lens swelling, pinching, flipping upside-down as you cross the equator into the southern hemisphere; the globe beside it shows the latitude ring and the day/night terminator for the selected date, tilting through the year. On load, the latitude sweeps by itself from the equator to the Arctic Circle and back to a default mid-latitude (London, 51.5°N) within the first three seconds so the viewer immediately sees the shape transform.

Secondary views: (1) "All latitudes at once": a heat map with day of year across and latitude down, colour = hours of daylight from navy (0) through blue, orange to yellow (24), with contour lines at 6, 9, 12, 15, 18 hours and labels, the 12-hour contours pinching together at the equinoxes (the −0.83° correction keeps them a few days apart); hovering reads off any cell ("68°N, 21 June: 24 h 00 m"). Clicking sets latitude and date; arrow keys move a probe and Enter selects. (2) A ring calendar for the chosen latitude: the year as a circle, each day a thin radial wedge whose yellow length is the day length: a sun-shaped figure that is round at the equator and a lopsided flower in Tromsø-like latitudes, with the twilight bands and stars beyond it; dragging around the ring sets the date. (3) A small table of superlatives computed live: longest day, shortest day, fastest change, dates of the first and last midnight sun and the start and end of polar night where they exist. (4) A short panel giving the general formula and the simplifications.

City chips (latitudes that are well-known and safe): Quito 0°, Singapore 1°N, Cairo 30°N, Tokyo 36°N, New York 41°N, London 51.5°N, Reykjavík 64°N, Tromsø 69.7°N, Longyearbyen 78°N, Sydney 34°S, Ushuaia 55°S; chips are simple outlined labels in navy that fill yellow when active. A date scrubber along the bottom of the main chart moves the hairline; a "today" control returns to the real date. A note in small type states the simplifications (no atmospheric refraction adjustment beyond the standard −0.83° for sunrise/sunset, local solar time, no equation of time unless implemented).

Typography: confident Swiss-style information design: a bold grotesk (Helvetica/Arial bold) for the headline number and title "A Year of Daylight" set large and tight in navy; small tabular numerals; axis labels in tracked caps at 10–11px; a three-sentence explanatory lede in a serif italic. Canvas for the charts with crisp DPR handling; smooth interpolation when switching cities (latitude eases to its target over 800ms, cubic-bezier(.3,0,.1,1)). Responsive: on phones the chart is full-width with the latitude control as a horizontal slider above it, secondary views stacked. Reduced motion: no intro sweep; latitude changes apply without easing.

DESCRIPTION
A pale sunlight-yellow museum wall chart in Swiss information-design dress: a tight navy grotesk title, a serif-italic lede, tracked-caps labels and thin navy rules. The centrepiece is the daylight hourglass, a navy block of starry night with a sun-yellow band of day running through it, edged by orange, plum and blue twilight. On load the latitude sweeps from the equator to the Arctic Circle and settles on London, so the flat ribbon visibly swells into a lens. Beside it a small desk globe with a graduated meridian arc carries the latitude bead, the selected parallel and a terminator that tilts with the date, coloured with the same twilight bands as the chart. Above the chart the day length for the chosen date is set very large with its daily gain or loss, sunrise and sunset. Below, a heat map shows every latitude and day at once with labelled contours, a ring calendar bends the year into a sun-shaped figure, and a table of superlatives recalculates live. Everything is computed in the page from latitude and date.

VISUAL TECHNIQUES
- Four canvases (hourglass chart, globe, heat map, ring calendar) drawn at device pixel ratio capped at 2 and redrawn only when state changes
- Sunrise and twilight hour angles computed per half-day sample from solar declination, with clamping for polar day and polar night
- Stacked band polygons for astronomical, nautical and civil twilight and daylight, with two-tone grid and solstice/equinox rules clipped to the day shape
- Deterministic star field that only shows where the sun is more than 18° below the horizon
- Edge-on orthographic globe with graticule, tilted terminator bands, subsolar sun glyph, two-tone latitude chord and a graduated meridian arc with tropic and polar-circle marks
- Heat map rendered to an offscreen image from a 365 by 361 grid, with marching-squares contour lines at 6, 9, 12, 15 and 18 hours and pill labels
- Ring calendar of 365 radial wedges with twilight bands, a dashed 12-hour reference circle and per-day rays
- Custom cubic-bezier easing solved in script for latitude and date tweens, plus a choreographed intro sweep
- Swiss typographic system: bold grotesk at extreme scale contrast, tabular numerals, tracked caps, serif italic annotations
- Paper grain from an inline turbulence filter; staggered title and rule entrance animations

INTERACTION MODEL
Drag the bead on the globe's meridian arc (or the horizontal slider on small screens) to change latitude from 90°N to 90°S; the chart, globe, ring, heat-map crosshair, headline number and superlatives all reshape continuously. Arrow keys step one degree (Shift for a tenth, Page keys for ten, Home and End for the poles). City chips ease the latitude to a known place over 800 ms and fill yellow when active; the bead also snaps lightly to those cities. Drag across the main chart, the date scrubber beneath it, or around the ring calendar to move the date hairline; arrow keys on the scrubber step a day, and the Today button eases back to the real date. Hovering the heat map reads off latitude, date and day length in a tooltip; clicking it (or moving the probe with arrow keys and pressing Enter) sets both latitude and date. With reduced motion requested there is no intro sweep or entrance animation and all changes apply instantly.