diff --git a/.changeset/allow-astrometry-dependency.md b/.changeset/allow-astrometry-dependency.md
new file mode 100644
index 0000000..14a08c8
--- /dev/null
+++ b/.changeset/allow-astrometry-dependency.md
@@ -0,0 +1,5 @@
+---
+'@observerly/orderly': minor
+---
+
+Add @observerly/astrometry 0.69.0 as orderly's one runtime dependency, observerly's own library with no runtime dependencies of its own, ahead of the sun schedules built on it; the dependency guard now admits exactly that name and fails the package checks on any other.
diff --git a/README.md b/README.md
index e5c517a..2530e64 100644
--- a/README.md
+++ b/README.md
@@ -4,8 +4,8 @@ Single file, no pushing. Type-safe middleware that brings a little bit of order
 
 orderly removes the two failure modes every raw queue consumer carries: messages that are never
 decided, and one bad message crashing a whole batch. Every message resolves to exactly one explicit
-outcome, every outcome is recorded as an event, and the consumer never rejects. Zero runtime
-dependencies.
+outcome, every outcome is recorded as an event, and the consumer never rejects. Zero third-party
+runtime dependencies.
 
 ## Installation
 
@@ -358,8 +358,9 @@ subtract(deadline, { days: 1 });
 ## Requirements
 
 orderly runs on [workerd](https://github.com/cloudflare/workerd) and is tested inside it. It ships
-as ESM with TypeScript declarations, requires nothing at runtime, and follows Standard Schema
-rather than depending on any validation library.
+as ESM with TypeScript declarations and follows Standard Schema rather than depending on any
+validation library. Its one runtime dependency is observerly's own `@observerly/astrometry`, which
+itself depends on nothing at runtime; a guard fails the package checks should another ever appear.
 
 ## Contributing
 
diff --git a/package.json b/package.json
index 63a5d8a..fb29aff 100644
--- a/package.json
+++ b/package.json
@@ -52,6 +52,9 @@
     "test:watch": "vitest",
     "typecheck": "tsc --noEmit && tsc --noEmit -p tests/tsconfig.json && tsc --noEmit -p scripts/tsconfig.json"
   },
+  "dependencies": {
+    "@observerly/astrometry": "0.69.0"
+  },
   "devDependencies": {
     "@arethetypeswrong/cli": "0.18.5",
     "@changesets/changelog-github": "1.0.0",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 05e0acc..4269b6b 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -7,6 +7,10 @@ settings:
 importers:
 
   .:
+    dependencies:
+      '@observerly/astrometry':
+        specifier: 0.69.0
+        version: 0.69.0
     devDependencies:
       '@arethetypeswrong/cli':
         specifier: 0.18.5
@@ -555,6 +559,9 @@ packages:
     resolution: {integrity: sha512-6QEf6yqFbETdwGITKq57aYoPfX/3K8XFNwsAlx0C1M7o8cb79sv1M3w+tWuWvIcSbNqrLF7OD7YpZMVVz335hQ==}
     engines: {node: '>=20.0.0'}
 
+  '@observerly/astrometry@0.69.0':
+    resolution: {integrity: sha512-bgPl2sKbq+T1ixdDTThsYPFx7TgVLRnR2Xaab0CC+vxIEc+tKNolbEGQx+o1mmo2nMn74gMTZVR21Is5co/rKA==}
+
   '@oxc-project/types@0.144.0':
     resolution: {integrity: sha512-nuhZIOLuI6TFQ32I/WnUx+SCPY7SdSKwgnFHydAuoS1+Z4BRcaP+RRJmGzl9lw+0OFF7UmaESf7KQRXaNLHypg==}
 
@@ -965,6 +972,9 @@ packages:
   '@types/estree@1.0.9':
     resolution: {integrity: sha512-GhdPgy1el4/ImP05X05Uw4cw2/M93BCUmnEvWZNStlCzEKME4Fkk+YpoA5OiHNQmoS7Cafb8Xa3Pya8m1Qrzeg==}
 
+  '@types/geojson@7946.0.16':
+    resolution: {integrity: sha512-6C8nqWur3j98U6+lXDfTUWIfgvZU+EumvpHKcYjujKH7woYyLj2sUmff0tRhrqM7BohUw7Pz3ZB1jj2gW9Fvmg==}
+
   '@types/node@24.13.3':
     resolution: {integrity: sha512-Dh8vAsV36ig5wa9OX4pXvMc9D3Veibfw2wix0CUwYODLD8nkj9UsLjASr49nPg+2eKzxhBV+v7L8pXvT4e639Q==}
 
@@ -2398,6 +2408,10 @@ snapshots:
       tinyglobby: 0.2.17
       yaml: 2.9.0
 
+  '@observerly/astrometry@0.69.0':
+    dependencies:
+      '@types/geojson': 7946.0.16
+
   '@oxc-project/types@0.144.0': {}
 
   '@oxfmt/binding-android-arm-eabi@0.63.0':
@@ -2611,6 +2625,8 @@ snapshots:
 
   '@types/estree@1.0.9': {}
 
+  '@types/geojson@7946.0.16': {}
+
   '@types/node@24.13.3':
     dependencies:
       undici-types: 7.18.2
diff --git a/scripts/guard-dependencies.ts b/scripts/guard-dependencies.ts
index 746d39f..48d3a10 100644
--- a/scripts/guard-dependencies.ts
+++ b/scripts/guard-dependencies.ts
@@ -12,16 +12,51 @@ import process from 'node:process';
 
 /*****************************************************************************************************************/
 
-// orderly ships with zero runtime dependencies, and that is a guarantee rather than a habit. This guard makes the
-// guarantee structural: the moment a dependencies key appears in the manifest, the package checks fail, in CI and
-// locally alike. devDependencies remain allowed, as they are toolchain-only and never ship.
+// orderly ships with no third party runtime dependencies, and that is a guarantee rather than a habit. This guard
+// makes the guarantee structural: a dependency outside the one name allowed below fails the package checks, in
+// CI and locally alike. devDependencies remain allowed, as they are toolchain-only and never ship.
+
+/*****************************************************************************************************************/
+
+// The one runtime dependency orderly allows itself: observerly's own astrometry, which the sun schedules are
+// built on. It carries no runtime dependencies of its own; the types-only package its manifest names ships no
+// code. Nothing else may appear here without changing this line and the README sentence that promises it.
+const ALLOWED = new Set(['@observerly/astrometry']);
+
+/*****************************************************************************************************************/
+
 const manifest: unknown = JSON.parse(
   readFileSync(new URL('../package.json', import.meta.url), 'utf8'),
 );
 
-if (typeof manifest === 'object' && manifest !== null && 'dependencies' in manifest) {
+/*****************************************************************************************************************/
+
+// The names under dependencies, or none when the key is absent. Anything the key holds that is not a plain
+// object of names is refused too, rather than read as nothing.
+const dependenciesOf = (candidate: unknown): string[] => {
+  if (typeof candidate !== 'object' || candidate === null || !('dependencies' in candidate)) {
+    return [];
+  }
+
+  const { dependencies } = candidate;
+
+  if (typeof dependencies !== 'object' || dependencies === null || Array.isArray(dependencies)) {
+    console.error('package.json declares dependencies that are not a map of names to versions.');
+
+    process.exit(1);
+  }
+
+  return Object.keys(dependencies);
+};
+
+/*****************************************************************************************************************/
+
+const strangers = dependenciesOf(manifest).filter(name => !ALLOWED.has(name));
+
+if (strangers.length > 0) {
   console.error(
-    'package.json declares a dependencies key: @observerly/orderly ships with zero runtime dependencies.',
+    `package.json declares runtime dependencies @observerly/orderly does not allow: ${strangers.join(', ')}. ` +
+      `The only runtime dependency permitted is ${[...ALLOWED].join(', ')}.`,
   );
 
   process.exit(1);