Tools

Hour Angle Calculator

What This Calculator Tells You

Hour Angle (HA) measures how far an object is from your local meridian, expressed in hours. An hour angle of 0h means the object is on the meridian (due south for northern observers), at its highest point. Negative HA means the object is east of the meridian (rising), positive HA means west (setting).

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Understanding Hour Angle

  • HA = 0h: Object is on the meridian, at its highest altitude
  • HA = -6h: Object is rising in the east
  • HA = +6h: Object is setting in the west
  • 1h of HA = 15°: Hour angle can be converted to degrees

Common Objects and Their RA

ObjectRASeason
Orion Nebula (M42)5.59hWinter
Whirlpool Galaxy (M51)13.50hSpring
Ring Nebula (M57)18.89hSummer
Andromeda Galaxy (M31)0.71hAutumn
Pleiades (M45)3.78hWinter

Click a row to load its RA into the calculator.

Rules of Thumb

  • Observe when |HA| < 3h: Object is within 3 hours of the meridian, at a good altitude
  • Best at HA = 0: Always try to observe near the meridian when possible
  • HA changes ~1h per hour: At the sidereal rate (≈59m 50s per hour of HA)
  • Equatorial mount tracking: The mount rotates to counteract Earth's rotation and maintain constant HA

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