🌍 Earth Magnetism

Air navigation relies heavily on magnetic references because most aircraft use magnetic compasses. However, the Earth’s magnetic field is not perfectly aligned with geographic (true) directions. This chapter explains how True, Magnetic, and Compass directions differ, and how pilots apply corrections like variation and deviation.

---

🧭 1. True Direction

True direction is based on the geographical poles of the Earth.

• Measured relative to True North
• Derived from maps and charts
• Independent of magnetism

✈️ Key Idea:

When you draw a line between two points on a chart and measure its direction, you are measuring True Direction

---

🧲 2. Magnetic Direction

The Earth behaves like a giant magnet due to movement of molten material in its core.

• Magnetic field is irregular
• Magnetic poles are not aligned with True poles
• Magnetic field resembles a distorted bar magnet

✔ Magnetic Direction:

• Measured from Magnetic North
• Expressed in degrees (e.g., 120°M)

---

🧭 3. Magnetic North

• Direction indicated by a freely suspended magnetic needle
• Varies depending on location
• Defines magnetic meridian at that point

---

📐 4. Variation (Very Important)

✔ Definition:

Variation is the angle between True North and Magnetic North

✔ Key Points:

• Measured East or West from True North
• Changes with location

---

✈️ Example (Modified)

If:

• True direction = 090°
• Variation = 12°E

👉 Magnetic direction =
= 090° – 12° = 078°M

---

✔ Rule:

• East Variation → Magnetic Least
• West Variation → Magnetic Best

---

🌍 5. Isogonals and Agonic Line

• Isogonals → Lines joining points of equal variation
• Agonic Line → Line of zero variation

✈️ Key Concept:

Along the agonic line:
👉 True North = Magnetic North

---

🌎 6. Variation Changes with Position

Variation depends on:

• Observer’s location
• Relative position of magnetic and true poles

✔ Observations:

• Some regions → Easterly variation
• Some regions → Westerly variation
• Near certain lines → Zero variation

---

🌐 7. Variation at the Poles

• Maximum possible variation = 180°
• Occurs near poles
• Direction of magnetic north may reverse relative to true north

---

🗺️ 8. Real Variation Map

Actual variation distribution is irregular because:

• Magnetic poles are not symmetrical
• Earth’s magnetic field is uneven

👉 Multiple agonic lines exist across Earth

---

⏳ 9. Change of Variation with Time

Variation is not constant.

✔ Types of Changes:

✈️ Additional Factors:

• Solar activity (magnetic storms)
• Movement of molten core
• Local magnetic anomalies

---

🧭 10. Magnetic Dip

✔ Definition:

Angle between the horizontal plane and Earth’s magnetic field

---

✔ Components:

---

✔ Key Points:

• Dip = 0° at magnetic equator
• Dip = 90° at magnetic poles
• Horizontal component decreases near poles

---

⚠️ 11. Effect of Dip

• Causes compass needle to tilt
• Leads to:
  • Turning errors
  • Acceleration errors

---

🧭 12. Deviation

✔ Definition:

Deviation is the difference between Magnetic North and Compass North

✔ Cause:

• Aircraft structure
• Electrical systems
• Metal components

---

✔ Rule:

• East Deviation → Compass Least
• West Deviation → Compass Best

---

🔄 13. Conversion Between Directions

✔ Order:

👉 True → Magnetic → Compass

---

✈️ Example (Modified)

Given:

• True heading = 120°
• Variation = 10°W
• Deviation = 4°E

---

Step 1: True → Magnetic

120° + 10° = 130°M

Step 2: Magnetic → Compass

130° – 4° = 126°C

---

📊 14. Summary Table

---

🧠 FINAL SUMMARY

• True direction → based on geographic poles
• Magnetic direction → based on Earth’s magnetic field
• Variation = True vs Magnetic
• Deviation = Magnetic vs Compass
• Dip affects compass accuracy
• Variation changes with location & time
• Agonic line = zero variation
• Isogonals = equal variation lines

---

⚠️ COMMON MISTAKES

• Confusing variation and deviation
• Applying wrong sign (East/West)
• Forgetting conversion sequence
• Ignoring dip effects
• Assuming variation is constant