Problem Statement
Find the value of magnetic declination if the magnetic bearing of sun at noon is 356°.
Step-by-Step Solution
Key Information & Setup
- Magnetic bearing of sun at noon = 356°
- Goal: Find the magnetic declination value and direction
Step 1: Identify the True Bearing of Sun at Noon
At local noon, the sun is directly south in the northern hemisphere (or directly north in the southern hemisphere). This is a fundamental astronomical fact used in surveying.
For the northern hemisphere:
True Bearing of Sun at Noon = 180°
For the southern hemisphere:
True Bearing of Sun at Noon = 0° or 360°
Based on the given magnetic bearing (356°), which is very close to north, and the solution hint, we’re working in the southern hemisphere where:
True Bearing of Sun at Noon = 360°
Step 2: Calculate Magnetic Declination
The magnetic declination can be calculated using the relationship:
Declination = True Bearing – Magnetic Bearing
Declination = 360° – 356°
Declination = 4°
Step 3: Determine the Direction of Declination
The sign of the declination determines its direction:
- Positive declination: Easterly (E)
- Negative declination: Westerly (W)
Since our calculated declination is positive (4°), the magnetic declination is easterly.
Magnetic Declination = 4° East
Final Result
Explanation of Sun Observations for Declination
Using Solar Observations to Determine Declination:
- Astronomical Reference: The sun’s position at local noon provides a reliable astronomical reference for true north or south.
- Hemisphere Considerations: In the northern hemisphere, the sun is due south (180°) at local noon. In the southern hemisphere, the sun is due north (0° or 360°) at local noon.
- Measuring Method: By measuring the magnetic bearing of the sun at local noon and comparing it to the known true bearing, surveyors can determine magnetic declination.
- Interpretation: When magnetic north is east of true north, magnetic bearings will be smaller than true bearings, resulting in easterly declination.
Declination Direction Rule:
If True Bearing > Magnetic Bearing → Easterly Declination
If True Bearing < Magnetic Bearing → Westerly Declination
This method of determining magnetic declination using solar observations is particularly useful in remote areas where declination maps might not be available or up to date. It’s also an important skill for verifying compass accuracy in field surveying.
