The slides, written out
AI-drawn illustration. Simplified to explain the idea, not to scale.
Slide 1 · GYRO COMPASS &
GYRO COMPASS &
MAGNETIC COMPASS
ESSENTIALS
Understanding Precession,
Apparent Wander,
Settling Time &
Compass Deviation
Gyro Compass
Fundamentals
Tilt, Drift &
Earth Rotation
Settling Time &
Practical Points
Why Deviation
Changes with Heading
N
S
E
W
Slide 2 · GYRO COMPASS FUNDAMENTALS
GYRO COMPASS FUNDAMENTALS
1. PRECESSION
OF THE AXIS
Precession is the gyroscope's most
counter-intuitive property — but once you
get the feel for it, it explains everything else.
APPLIED
TORQUE
(Force)
GYROSCOPIC
RESPONSE
(Axis does not move
in the direction of
the push)
90°
AHEAD
(Direction of
(wheel rotation)
REMEMBER:
Try to
TILT a gyro
→ it will
TURN.
Try to
TURN a gyro
→ it will
TILT.
Think of a bicycle:
To turn left,
you lean (tilt) left.
The front wheel
turns left
automatically.
Wheels are spinning
gyroscopes.
They precess.
Exactly the same
effect!
HOW PRECESSION MAKES A COMPASS
Controlled
Torque Applied
(By the manufacturer
or system)
Precession
Occurs
(Axis moves 90°
ahead)
Axis Turns
Toward Meridian
(North–South
Direction)
North-Seeking
Gyrocompass
Achieved
In a gyrocompass, this property is used deliberately to turn a "free" gyroscope
into an accurate, stable, north-seeking instrument.
Slide 3 · APPARENT WANDER EXPLAINED
APPARENT WANDER EXPLAINED
2. TILT, DRIFT & SETTLING TIME
A free gyroscope on Earth appears to move because the Earth rotates beneath it.
Space-stable Gyro Axis
DRIFT (AZIMUTH)
Earth rotates beneath a space-stable gyro, causing the axis to appear to move East or West.
TILT (ALTITUDE)
Depending on its orientation and the observer's latitude, a free gyro may appear to rise or fall relative to the local horizontal plane.
This apparent motion (earth-rate drift or apparent wander) depends on latitude and the orientation of the gyro axis.
This is also part of the apparent wander caused by Earth rotation.
APPARENT DRIFT (East / West)
APPARENT TILT (Up / Down)
EARTH ROTATION (West to East)
3. SETTLING TIME
TYPICAL RANGE 2–4 HOURS*
* Manufacturer-dependent. May vary from approx. 2 hours to several hours.
When a gyrocompass is started (or restarted), it requires time to:
Reach full operating speed
Develop controlled precession (north-seeking action)
Damp oscillations
Stabilise on the true meridian
WATCHKEEPER NOTE
Do not rely on gyro heading until the manufacturer's settling requirements are fully met. Follow the maker's instructions.
PRACTICAL POINTS
If the gyro has been switched off in port, allow the full settling period.
Some makers allow external precession to speed up settling. Check the manual.
Short port stays? Keep the gyro running continuously. Never shut down unless alongside for several days.
Slide 4 · MAGNETIC COMPASS DEVIATION
MAGNETIC COMPASS DEVIATION
3. WHY MAGNETIC
COMPASS DEVIATION
CHANGES WITH HEADING
A ship carries its own magnetism. As the heading changes,
the relationship with Earth's magnetic field changes —
so does the deviation.
PERMANENT MAGNETISM
• Acquired during construction, welding, hammering, etc.
• Remains with the ship for years.
+
INDUCED MAGNETISM
• Caused by Earth's magnetic field acting on the ship's soft iron.
• Varies with the ship's heading.
=
DEVIATION
Compass aligns with the resultant of Earth's field and the ship's magnetic field.
HOW HEADING AFFECTS DEVIATION
As the ship turns, the ship's magnetic field has a different angle with Earth's field — so the resultant (and deviation) changes.
HEADING 000°
Earth's Magnetic Field
Ship's Magnetic Field
Resultant Field
Example Deviation: +10°
HEADING 090°
Earth's Magnetic Field
Ship's Magnetic Field
Resultant Field
Example Deviation: -2°
HEADING 180°
Earth's Magnetic Field
Ship's Magnetic Field
Resultant Field
Example Deviation: -10°
WHAT THIS MEANS FOR WATCHKEEPERS
CHECK DEVIATION CARD
Always apply deviation correction for your current heading.
CONDUCT REGULAR SWINGS
Determine deviation across headings and keep the deviation card updated.
REASSESS AFTER CHANGES
Major structural changes, repairs, or equipment installation can alter deviation.
ENSURE ACCURACY
Correcting deviation ensures the magnetic compass remains a reliable backup to gyro heading.
Understanding compass behaviour improves navigational safety.
Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.
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