From 3443c1c65e9f1753e1e656b35d08416c1fa298f2 Mon Sep 17 00:00:00 2001
From: godosa <godosa@godosa.eu>
Date: Wed, 7 Oct 2026 00:14:38 +0200
Subject: worldmap-viewer: initial public history

---
 viewer/js/sky.js | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)
 create mode 100644 viewer/js/sky.js

(limited to 'viewer/js/sky.js')

diff --git a/viewer/js/sky.js b/viewer/js/sky.js
new file mode 100644
index 0000000..df4f000
--- /dev/null
+++ b/viewer/js/sky.js
@@ -0,0 +1,41 @@
+// sky.js — sun geometry (pure), values from planet.js. Viewer convention: day 0 = northern spring equinox,
+// moment 0 = midnight at longitude 0°. Circular orbit. A day has in-world moments (units.moment_s) or minutes;
+// a sun-locked world keeps its sun still.
+import { toVec } from './geo.js';
+import { PLANET } from './planet.js';
+
+export const TILT_DEG = PLANET.tilt_deg;
+export const YEAR_DAYS = Math.max(1, Math.round(PLANET.year_days));
+export const MOMENT_S = PLANET.moment_s || 60;
+export const DAY_MOMENTS = Math.round(PLANET.day_hours * 3600 / MOMENT_S);
+const q = YEAR_DAYS / 4;
+export const SEASONS = TILT_DEG > 0 ? [[0, 'northern spring equinox'], [q, 'northern midsummer'], [2 * q, 'northern autumn equinox'],
+  [3 * q, 'northern midwinter'], [YEAR_DAYS, 'northern spring equinox']] : [];
+const D = Math.PI / 180;
+
+export function subsolar(day, moment) {
+  if (PLANET.sun_lock) return { lat: PLANET.sun_lock[0], lon: PLANET.sun_lock[1] };
+  const t = day + moment / DAY_MOMENTS;
+  const lat = Math.asin(Math.sin(TILT_DEG * D) * Math.sin((2 * Math.PI * t) / YEAR_DAYS)) / D;
+  const lon = ((((180 - (360 * moment) / DAY_MOMENTS + 180) % 360) + 360) % 360) - 180;
+  return { lat, lon };
+}
+
+export function sunVec(day, moment) {
+  const s = subsolar(day, moment);
+  return toVec(s.lat, s.lon);
+}
+
+export function dayFraction(lat, day) {
+  const dec = subsolar(day, 0).lat * D;   // declination at the start of the day
+  const phi = Math.max(-89.9999, Math.min(89.9999, lat)) * D;
+  const x = -Math.tan(phi) * Math.tan(dec);
+  if (x <= -1) return 1;
+  if (x >= 1) return 0;
+  return Math.acos(x) / Math.PI;
+}
+
+export function seasonName(day) {
+  const hit = SEASONS.find(([d]) => Math.abs(day - d) <= Math.min(2, YEAR_DAYS / 20));
+  return hit ? hit[1] : '';
+}
-- 
cgit