diff --git a/src/constants.ts b/src/constants.ts
index 813f949..af48928 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -106,3 +106,13 @@ export const J1970 = 2440587.5 as const
 export const J2000 = 2451545.0 as const
 
 /*****************************************************************************************************************/
+
+/**
+ *
+ * Two pi, e.g., the angle of a whole turn (in radians), or the circumference of a circle of a
+ * unit radius.
+ *
+ */
+export const TWO_PI = 2 * Math.PI
+
+/*****************************************************************************************************************/
diff --git a/src/index.ts b/src/index.ts
index 26bc608..56b8a22 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -90,7 +90,8 @@ export {
   SECONDS_IN_DAY,
   SECONDS_IN_HOUR,
   SECONDS_IN_YEAR,
-  SPEED_OF_LIGHT
+  SPEED_OF_LIGHT,
+  TWO_PI
 } from './constants'
 
 /*****************************************************************************************************************/
diff --git a/tests/constants.spec.ts b/tests/constants.spec.ts
new file mode 100644
index 0000000..f01ee8d
--- /dev/null
+++ b/tests/constants.spec.ts
@@ -0,0 +1,32 @@
+/*****************************************************************************************************************/
+
+// @author         Michael Roberts <michael@observerly.com>
+// @package        @observerly/astrometry/constants
+// @license        Copyright © 2021-2026 observerly
+
+/*****************************************************************************************************************/
+
+import { describe, expect, it } from 'vitest'
+
+/*****************************************************************************************************************/
+
+import { TWO_PI } from '../src/constants'
+
+/*****************************************************************************************************************/
+
+describe('TWO_PI', () => {
+  it('should be defined', () => {
+    expect(TWO_PI).toBeDefined()
+  })
+
+  it('should be two pi, e.g., the angle of a whole turn (in radians)', () => {
+    expect(TWO_PI).toBe(2 * Math.PI)
+    expect(TWO_PI).toBeCloseTo(6.283185307179586, 15)
+  })
+
+  it('should be the angle of a whole turn of 360°', () => {
+    expect((TWO_PI * 180) / Math.PI).toBe(360)
+  })
+})
+
+/*****************************************************************************************************************/