fix: resolve a full Moon that is still to come in moon module in @observerly/astrometry

src/moon.ts

@@ -996,8 +996,11 @@ export const isFullMoon = (datetime: Date): boolean => getLunarPhase(datetime) =
  * @returns The date of the next full Moon.
  */
 export const getNextFullMoon = (datetime: Date): Date => {
-  // Amend the date to midnight on the given date:
-  let date = new Date(new Date(datetime).setHours(0, 0, 0, 0))
+  // The search begins at the datetime given, and not at the midnight before it: the midnight of
+  // the host system is not the midnight of UTC, and so the search would begin at an instant that
+  // depends on where the host is, and it would resolve a full Moon that has already passed earlier
+  // on the same day as the one that follows it:
+  let date = new Date(datetime)
 
   // The maximum number of days in a synodic month is 29, so if we increment the
   // date by 1 hour until we reach a full Moon, we will eventually reach
@@ -1045,6 +1048,14 @@ export const getNextFullMoon = (datetime: Date): Date => {
     }
   }
 
+  // The Moon reads as full over a window of ~20 hours centred on the syzygy, and so the datetime
+  // given may lie within it but beyond the syzygy itself, e.g., the full Moon resolved from it has
+  // already passed. The search resumes two days on, which clears the remainder of that window, for
+  // the full Moon that follows it:
+  if (fullMoon <= datetime) {
+    return getNextFullMoon(new Date(fullMoon.getTime() + 2 * 24 * 60 * 60 * 1000))
+  }
+
   return fullMoon
 }
 

tests/moon.spec.ts

@@ -488,6 +488,18 @@ describe('getNextFullMoon', () => {
     const nextFullMoon = getNextFullMoon(datetime)
     expect(nextFullMoon.toISOString()).toBe('2021-05-26T11:31:30.000Z')
   })
+
+  it('should return a full Moon that is still to come for a datetime just after one', () => {
+    // The Moon reads as full for ~20 hours about the syzygy, and so a datetime within that window
+    // but beyond the syzygy resolved the full Moon that had already passed:
+    const passed = new Date('2024-08-19T18:26:00.000Z')
+
+    for (const hours of [1, 6, 12, 18]) {
+      const datetime = new Date(passed.getTime() + hours * 60 * 60 * 1000)
+
+      expect(getNextFullMoon(datetime).getTime()).toBeGreaterThan(datetime.getTime())
+    }
+  })
 })
 
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