From 8a4884a37784bfeb34e1ee497b5bb3a5439a979e Mon Sep 17 00:00:00 2001
From: Richard Zink <Richard Zink>
Date: Wed, 14 Oct 2026 07:48:52 +0200
Subject: [PATCH] fix: align sun altitude with sunrise sunset

---
 background.html | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/background.html b/background.html
index 353ac02..875f696 100644
--- a/background.html
+++ b/background.html
@@ -684,6 +684,11 @@ window.addEventListener('resize', scheduleFitInfoPanel);
 
         var SITE_LAT = (window.CONFIG && window.CONFIG.location && window.CONFIG.location.lat) || 51.5074;
         var SITE_LON = (window.CONFIG && window.CONFIG.location && window.CONFIG.location.lon) || -0.1278;
+        // SunCalc.getPosition returns the geometric centre altitude. The
+        // scene's sunrise/sunset times use the standard apparent upper-limb
+        // altitude (-0.833 deg: refraction + solar semidiameter), so the
+        // lighting altitude is shifted by the same amount.
+        var SUN_HORIZON_CORR_RAD = 0.833 * Math.PI / 180;
 
         var realDayProgress = 0;
 
@@ -923,14 +928,15 @@ window.sunPalettes = [
         function getColor(t) {
             var now = sceneNow();
             var sunPos = SunCalc.getPosition(now, SITE_LAT, SITE_LON);
+            var sunAlt = sunPos.altitude + SUN_HORIZON_CORR_RAD;
             var moonIllum = SunCalc.getMoonIllumination(now);
 
             // The day palette covers sunAlt down to sunPalettes[0].alt
             // (-pi/2, deep below horizon). Only below that do we hand off
             // to the moon-based night palette. This makes twilight a
             // smooth transition through the day palette entries.
-            if (sunPos.altitude > window.sunPalettes[0].alt) {
-                return getDayColor(sunPos.altitude);
+            if (sunAlt > window.sunPalettes[0].alt) {
+                return getDayColor(sunAlt);
             } else {
                 return getNightColor(moonIllum);
             }
@@ -1986,7 +1992,8 @@ window.weatherThemes = {
             // Sun altitude in radians. Negative = below horizon (night).
             var _sunNow = sceneNow();
             var _sunPos = SunCalc.getPosition(_sunNow, SITE_LAT, SITE_LON);
-            window.__currentSunAlt = _sunPos.altitude;
+            var _sunAlt = _sunPos.altitude + SUN_HORIZON_CORR_RAD;
+            window.__currentSunAlt = _sunAlt;
             window.__currentSceneDate = _sunNow;
             window.__flightDow = _sunNow.getDay();
             // Unit vector towards the sun in world space. SunCalc azimuth
@@ -1995,9 +2002,9 @@ window.weatherThemes = {
             // World frame: +X = west, +Z = north, +Y = up -> east is -X.
             var _sunBearingR = _sunPos.azimuth + Math.PI;
             window.__currentSunDir = [
-                -Math.cos(_sunPos.altitude) * Math.sin(_sunBearingR),
-                Math.sin(_sunPos.altitude),
-                Math.cos(_sunPos.altitude) * Math.cos(_sunBearingR)
+                -Math.cos(_sunAlt) * Math.sin(_sunBearingR),
+                Math.sin(_sunAlt),
+                Math.cos(_sunAlt) * Math.cos(_sunBearingR)
             ];
             // ---- Local sidereal time -------------------------------------
             // Same formula SunCalc uses internally (Meeus): GMST in degrees