diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9033c92..eebefd9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -20,8 +20,8 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y shellcheck bats
 
-      - name: Shellcheck install.sh
-        run: shellcheck install.sh
+      - name: Shellcheck shell scripts
+        run: shellcheck ./*.sh
 
-      - name: Run install.sh test suite
-        run: bats tests/install.bats
+      - name: Run test suite
+        run: bats tests/*.bats
diff --git a/moon.sh b/moon.sh
new file mode 100755
index 0000000..0612bc0
--- /dev/null
+++ b/moon.sh
@@ -0,0 +1,58 @@
+#!/usr/bin/env sh
+# moon.sh — print the current phase of the Moon. A tiny, dependency-free
+# flourish for agent-ready; deliberately NOT wired into install.sh.
+#
+#   ./moon.sh [EPOCH_SECONDS]
+#
+# With no argument it uses the current time. Pass a Unix timestamp (whole
+# seconds since 1970-01-01 UTC) to ask about any other moment — which is what
+# makes it testable and lets you check a date that isn't today.
+
+set -eu
+
+# Reference new moon: 2000-01-06 18:14 UTC, as Unix epoch seconds.
+KNOWN_NEW_MOON=947182440
+# One synodic month (new moon to new moon): 29.53058867 days, in seconds.
+SYNODIC=2551443
+
+case "${1:-}" in
+  -h|--help) echo "usage: $0 [EPOCH_SECONDS]"; exit 0 ;;
+  -*) echo "unknown option: $1" >&2; exit 2 ;;
+esac
+
+# `date +%s` is a near-universal extension (GNU and BSD alike); there is no
+# pure-POSIX way to read the current epoch. An explicit argument sidesteps it.
+now="${1:-$(date +%s)}"
+
+# Reject anything non-numeric up front so the arithmetic below can't misbehave.
+case "$now" in
+  '' | *[!0-9]*)
+    echo "error: EPOCH_SECONDS must be a non-negative integer: '$now'" >&2
+    exit 2
+    ;;
+esac
+
+# Seconds elapsed into the current lunar cycle (0 .. SYNODIC-1). The extra
+# +SYNODIC guards against a reference-preceding timestamp yielding a negative
+# remainder before we normalise it.
+elapsed=$(( ( (now - KNOWN_NEW_MOON) % SYNODIC + SYNODIC ) % SYNODIC ))
+
+# Age of the Moon in whole days since the last new moon.
+age=$(( elapsed / 86400 ))
+
+# Snap to one of eight named phases. The +SYNODIC/2 rounds to the nearest
+# segment centre, so a full moon lands on "Full Moon" rather than just past it.
+phase=$(( (elapsed * 8 + SYNODIC / 2) / SYNODIC % 8 ))
+
+case "$phase" in
+  0) emoji="🌑"; name="New Moon" ;;
+  1) emoji="🌒"; name="Waxing Crescent" ;;
+  2) emoji="🌓"; name="First Quarter" ;;
+  3) emoji="🌔"; name="Waxing Gibbous" ;;
+  4) emoji="🌕"; name="Full Moon" ;;
+  5) emoji="🌖"; name="Waning Gibbous" ;;
+  6) emoji="🌗"; name="Last Quarter" ;;
+  7) emoji="🌘"; name="Waning Crescent" ;;
+esac
+
+printf '%s  %s (day %d of ~29.5)\n' "$emoji" "$name" "$age"
diff --git a/tests/moon.bats b/tests/moon.bats
new file mode 100644
index 0000000..62fa422
--- /dev/null
+++ b/tests/moon.bats
@@ -0,0 +1,89 @@
+#!/usr/bin/env bats
+# Exercises moon.sh against fixed timestamps so every phase is deterministic.
+# Run with: bats tests/moon.bats
+#
+# The offsets below are measured from the script's reference new moon
+# (2000-01-06 18:14 UTC = epoch 947182440), one per eighth of the lunar cycle.
+
+setup() {
+  KIT="$(cd "$BATS_TEST_DIRNAME/.." && pwd)"
+  MOON="$KIT/moon.sh"
+}
+
+@test "reference instant reads as a new moon" {
+  run "$MOON" 947182440
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"🌑"* ]]
+  [[ "$output" == *"New Moon"* ]]
+}
+
+@test "quarter into the cycle reads as first quarter" {
+  run "$MOON" 947820300
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"🌓"* ]]
+  [[ "$output" == *"First Quarter"* ]]
+}
+
+@test "half into the cycle reads as a full moon" {
+  run "$MOON" 948458161
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"🌕"* ]]
+  [[ "$output" == *"Full Moon"* ]]
+}
+
+@test "past full but before last quarter reads as waning gibbous" {
+  run "$MOON" 948777083
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"🌖"* ]]
+  [[ "$output" == *"Waning Gibbous"* ]]
+}
+
+@test "three quarters into the cycle reads as last quarter" {
+  run "$MOON" 949096022
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"🌗"* ]]
+  [[ "$output" == *"Last Quarter"* ]]
+}
+
+@test "reports the moon's age in whole days" {
+  run "$MOON" 948458161
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"day 14 of ~29.5"* ]]
+}
+
+@test "no argument uses the current time and still succeeds" {
+  run "$MOON"
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"of ~29.5"* ]]
+}
+
+@test "a timestamp before the reference new moon does not crash" {
+  run "$MOON" 0
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"of ~29.5"* ]]
+}
+
+@test "rejects a non-numeric timestamp" {
+  run "$MOON" abc
+  [ "$status" -eq 2 ]
+  [[ "$output" == *"non-negative integer"* ]]
+}
+
+@test "rejects an unknown option" {
+  run "$MOON" --nope
+  [ "$status" -eq 2 ]
+  [[ "$output" == *"unknown option"* ]]
+}
+
+@test "--help prints usage and exits 0" {
+  run "$MOON" --help
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"usage:"* ]]
+}
+
+@test "works under a strict POSIX sh (dash), not just bash" {
+  command -v dash >/dev/null 2>&1 || skip "dash not installed"
+  run dash "$MOON" 948458161
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"Full Moon"* ]]
+}