diff --git a/app/api/routes-f/moon-phase/__tests__/route.test.ts b/app/api/routes-f/moon-phase/__tests__/route.test.ts
new file mode 100644
index 00000000..3278cb56
--- /dev/null
+++ b/app/api/routes-f/moon-phase/__tests__/route.test.ts
@@ -0,0 +1,102 @@
+/**
+ * @jest-environment node
+ */
+import { NextRequest } from "next/server";
+import { GET, moonPhaseAt, SYNODIC_MONTH, PHASE_NAMES } from "../route";
+
+function callGet(query: string) {
+  return GET(
+    new NextRequest(`http://localhost/api/routes-f/moon-phase${query}`)
+  );
+}
+
+describe("moonPhaseAt", () => {
+  it("reports a near-new moon on known new-moon dates", () => {
+    // NASA: new moon on 2024-01-11 and 2025-01-29.
+    for (const day of ["2024-01-11", "2025-01-29"]) {
+      const result = moonPhaseAt(new Date(`${day}T00:00:00Z`));
+      expect(result.phase_name).toBe("new");
+      expect(result.illumination_percent).toBeLessThan(2);
+    }
+  });
+
+  it("reports a near-full moon on known full-moon dates", () => {
+    // NASA: full moon on 2024-01-25 and 2025-01-13.
+    for (const day of ["2024-01-25", "2025-01-13"]) {
+      const result = moonPhaseAt(new Date(`${day}T00:00:00Z`));
+      expect(result.phase_name).toBe("full");
+      expect(result.illumination_percent).toBeGreaterThan(95);
+    }
+  });
+
+  it("reports ~0% illumination at the reference new moon epoch", () => {
+    // The epoch itself: 2000-01-06 18:14 UTC. age_days wraps to ~0.
+    const result = moonPhaseAt(new Date(Date.UTC(2000, 0, 6, 18, 14, 0)));
+    expect(result.age_days).toBeCloseTo(0, 1);
+    expect(result.illumination_percent).toBeCloseTo(0, 1);
+    expect(result.phase_name).toBe("new");
+  });
+
+  it("reports ~100% illumination half a synodic month after new moon", () => {
+    const epoch = Date.UTC(2000, 0, 6, 18, 14, 0);
+    const halfCycle = new Date(epoch + (SYNODIC_MONTH / 2) * 86_400_000);
+    const result = moonPhaseAt(halfCycle);
+    expect(result.phase_name).toBe("full");
+    expect(result.illumination_percent).toBeCloseTo(100, 1);
+    expect(result.age_days).toBeCloseTo(SYNODIC_MONTH / 2, 1);
+  });
+
+  it("walks through the eight phases across one synodic month", () => {
+    const epoch = Date.UTC(2000, 0, 6, 18, 14, 0);
+    const seen = new Set<string>();
+    for (let i = 0; i < 8; i++) {
+      const t = new Date(epoch + ((i * SYNODIC_MONTH) / 8) * 86_400_000);
+      seen.add(moonPhaseAt(t).phase_name);
+    }
+    expect(seen.size).toBe(8);
+    for (const name of PHASE_NAMES) {
+      expect(seen.has(name)).toBe(true);
+    }
+  });
+
+  it("keeps age_days within [0, synodic month)", () => {
+    for (const day of ["1999-06-01", "2000-01-06", "2030-12-31"]) {
+      const { age_days } = moonPhaseAt(new Date(`${day}T00:00:00Z`));
+      expect(age_days).toBeGreaterThanOrEqual(0);
+      expect(age_days).toBeLessThan(SYNODIC_MONTH);
+    }
+  });
+});
+
+describe("GET /api/routes-f/moon-phase", () => {
+  it("returns the phase for a valid date", async () => {
+    const res = await callGet("?date=2024-01-25");
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body).toEqual({
+      phase_name: "full",
+      illumination_percent: expect.any(Number),
+      age_days: expect.any(Number),
+    });
+    expect(body.illumination_percent).toBeGreaterThan(95);
+  });
+
+  it("rejects a missing date param", async () => {
+    const res = await callGet("");
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toBe("Invalid query parameters");
+  });
+
+  it("rejects a malformed date param", async () => {
+    const res = await callGet("?date=Jan-25-2024");
+    expect(res.status).toBe(400);
+  });
+
+  it("rejects a well-formed but impossible calendar date", async () => {
+    const res = await callGet("?date=2024-02-30");
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toBe("Invalid query parameters");
+  });
+});
diff --git a/app/api/routes-f/moon-phase/route.ts b/app/api/routes-f/moon-phase/route.ts
new file mode 100644
index 00000000..a220d60f
--- /dev/null
+++ b/app/api/routes-f/moon-phase/route.ts
@@ -0,0 +1,122 @@
+import { NextResponse } from "next/server";
+import { z } from "zod";
+import { validateQuery } from "@/app/api/routes-f/_lib/validate";
+
+/**
+ * Mean length of a synodic month (one new-moon-to-new-moon cycle), in days.
+ * Source: Jean Meeus, "Astronomical Algorithms" (2nd ed.), the mean synodic
+ * month is 29.530588853 days. We use the commonly cited 29.53058867 value.
+ */
+export const SYNODIC_MONTH = 29.53058867;
+
+/**
+ * A well-documented reference new moon: 2000-01-06 18:14 UTC. Lunar age for any
+ * other instant is the elapsed time since this epoch reduced modulo the synodic
+ * month. This is the standard "synodic-month approximation" — it ignores the
+ * small irregularities in the Moon's orbit and is accurate to within ~1 day.
+ */
+export const REFERENCE_NEW_MOON_UTC = Date.UTC(2000, 0, 6, 18, 14, 0);
+
+/**
+ * The eight conventional moon phases, ordered from new moon through a full
+ * cycle. The four "principal" phases (new, first quarter, full, last quarter)
+ * sit at exact 1/4 points; the four intermediate phases fill the gaps.
+ */
+export const PHASE_NAMES = [
+  "new",
+  "waxing crescent",
+  "first quarter",
+  "waxing gibbous",
+  "full",
+  "waning gibbous",
+  "last quarter",
+  "waning crescent",
+] as const;
+
+export type PhaseName = (typeof PHASE_NAMES)[number];
+
+export interface MoonPhaseResult {
+  phase_name: PhaseName;
+  illumination_percent: number;
+  age_days: number;
+}
+
+/**
+ * Compute the moon phase for the given UTC instant.
+ *
+ * `age_days` is the time elapsed since the most recent new moon (0 .. synodic
+ * month). `illumination_percent` is the fraction of the lunar disc that appears
+ * lit, derived from the age via (1 - cos(2π·age/synodic)) / 2. `phase_name` is
+ * the nearest of the eight conventional phases, so each principal phase is
+ * centred on its exact age.
+ */
+export function moonPhaseAt(date: Date): MoonPhaseResult {
+  const elapsedDays = (date.getTime() - REFERENCE_NEW_MOON_UTC) / 86_400_000;
+
+  // Reduce into [0, SYNODIC_MONTH). `%` keeps the sign of the dividend, so add
+  // a synodic month before the final modulo to handle dates before the epoch.
+  const ageDays =
+    ((elapsedDays % SYNODIC_MONTH) + SYNODIC_MONTH) % SYNODIC_MONTH;
+
+  const cyclePosition = ageDays / SYNODIC_MONTH; // 0 .. 1
+  const illumination = (1 - Math.cos(2 * Math.PI * cyclePosition)) / 2;
+
+  // Round to the nearest eighth so principal phases occupy a narrow window
+  // centred on their exact age; `% 8` folds the wrap-around back onto "new".
+  const phaseIndex = Math.round(cyclePosition * 8) % 8;
+
+  return {
+    phase_name: PHASE_NAMES[phaseIndex],
+    illumination_percent: Math.round(illumination * 1000) / 10,
+    age_days: Math.round(ageDays * 100) / 100,
+  };
+}
+
+/**
+ * Parse a strict `YYYY-MM-DD` string as a UTC midnight instant, returning
+ * `null` for impossible calendar dates. `new Date(...)` silently rolls
+ * overflowing days into the next month (e.g. "2024-02-30" → Mar 1), so we
+ * verify the parsed components round-trip back to the input.
+ */
+export function parseCalendarDate(value: string): Date | null {
+  const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
+  if (!match) {
+    return null;
+  }
+
+  const [, year, month, day] = match.map(Number);
+  const date = new Date(Date.UTC(year, month - 1, day));
+  if (
+    date.getUTCFullYear() !== year ||
+    date.getUTCMonth() !== month - 1 ||
+    date.getUTCDate() !== day
+  ) {
+    return null;
+  }
+  return date;
+}
+
+const schema = z.object({
+  date: z.string().regex(/^\d{4}-\d{2}-\d{2}$/, "date must be YYYY-MM-DD"),
+});
+
+export async function GET(request: Request): Promise<NextResponse> {
+  const { searchParams } = new URL(request.url);
+  const result = validateQuery(searchParams, schema);
+  if (result instanceof NextResponse) {
+    return result;
+  }
+
+  const date = parseCalendarDate(result.data.date);
+  if (!date) {
+    return NextResponse.json(
+      {
+        error: "Invalid query parameters",
+        details: "date is not a real calendar date",
+      },
+      { status: 400 }
+    );
+  }
+
+  return NextResponse.json(moonPhaseAt(date));
+}