From 195520d66f4a718a399ba53ffebbf7c7df882a8e Mon Sep 17 00:00:00 2001
From: Leo Vasanko <leo@git>
Date: Wed, 20 Aug 2025 19:49:24 -0600
Subject: [PATCH] Lunar phases

---
 calendar.js   | 11 +++++++++--
 cells.css     |  6 ++++++
 date-utils.js | 30 ++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/calendar.js b/calendar.js
index 4f7a18f..3089bfa 100644
--- a/calendar.js
+++ b/calendar.js
@@ -13,6 +13,7 @@ import {
   getLocalizedWeekdayNames,
   getLocalizedMonthName,
   formatDateRange
+  ,lunarPhaseSymbol
 } from './date-utils.js'
 
 class InfiniteCalendar {
@@ -330,8 +331,14 @@ class InfiniteCalendar {
         labelYear = cur.getFullYear()
       }
 
-      const day = document.createElement('h1')
-      day.textContent = String(cur.getDate())
+  const day = document.createElement('h1')
+  day.textContent = String(cur.getDate())
+
+  // lunar phase symbol (only for main phases)
+  const moon = document.createElement('span')
+  moon.className = 'lunar-phase'
+  moon.textContent = lunarPhaseSymbol(cur) || ''
+  if (moon.textContent) cell.appendChild(moon)
 
       const date = toLocalString(cur)
       cell.dataset.date = date
diff --git a/cells.css b/cells.css
index c40279a..d7b64ae 100644
--- a/cells.css
+++ b/cells.css
@@ -22,6 +22,12 @@
   transition: background-color .15s ease;
   font-size: 1em;
 }
+.cell .lunar-phase {
+  position: absolute;
+  z-index: 1;
+  top: .25em;
+  left: 50%;
+}
 .cell.today h1 {
   border-radius: 2em;
   background: var(--today);
diff --git a/date-utils.js b/date-utils.js
index 076306e..446a045 100644
--- a/date-utils.js
+++ b/date-utils.js
@@ -121,6 +121,35 @@ function formatDateRange(startDate, endDate) {
   return `${startISO}/${endISO}`
 }
 
+/**
+ * Compute lunar phase symbol for the four main phases on a given date.
+ * Returns one of: 🌑 (new), 🌓 (first quarter), 🌕 (full), 🌗 (last quarter), or '' otherwise.
+ * Uses an approximate algorithm with a fixed epoch.
+ */
+function lunarPhaseSymbol(date) {
+  // Reference new moon: 2000-01-06 18:14 UTC (J2000 era), often used in approximations
+  const ref = Date.UTC(2000, 0, 6, 18, 14, 0)
+  const synodic = 29.530588853 // days
+  // Use UTC noon of given date to reduce timezone edge effects
+  const dUTC = Date.UTC(date.getFullYear(), date.getMonth(), date.getDate(), 12, 0, 0)
+  const daysSince = (dUTC - ref) / DAY_MS
+  const phase = ((daysSince / synodic) % 1 + 1) % 1
+  const phases = [
+    { t: 0.0, s: '🌑' }, // New Moon
+    { t: 0.25, s: '🌓' }, // First Quarter
+    { t: 0.5, s: '🌕' }, // Full Moon
+    { t: 0.75, s: '🌗' }  // Last Quarter
+  ]
+  // threshold in days from exact phase to still count for this date
+  const thresholdDays = 0.5 // ±12 hours
+  for (const p of phases) {
+    let delta = Math.abs(phase - p.t)
+    if (delta > 0.5) delta = 1 - delta
+    if (delta * synodic <= thresholdDays) return p.s
+  }
+  return ''
+}
+
 // Export all functions and constants
 export {
   monthAbbr,
@@ -136,4 +165,5 @@ export {
   getLocalizedWeekdayNames,
   getLocalizedMonthName,
   formatDateRange
+  ,lunarPhaseSymbol
 }