We can define two planes for a particular location on the surface of earth. One is called magnetic meridian and the other is called geographic meridian. Magnetic meridian for a point on the surface of earth is defined as a plane passing through this point and containing magnetic axis of earth. Similarly geographic meridian for a point on the surface of earth is defined as plane passing through this point and containing geographic axis of rotation. If we draw these two planes for a point on the surface of earth then in general these are found at some angle. Angle between these two planes is called declination for that place. Importance of magnetic meridian at a point is that net magnetic field vector of earth for that point lies on this plane. Usually net magnetic field of earth is inclined at some angle with the horizontal and this angle is called angle of dip for that place.
How many points are there on the surface of earth where declination is zero ?

Options

A. 2
B. 4
C. 8
D. infinite

Explanation

To determine how many points on the surface of the Earth have a declination of zero, we need to understand the concepts of magnetic meridian and geographic meridian.

### Step-by-Step Solution:

1. **Understanding the Definitions**:
- **Magnetic Meridian**: A plane that passes through a point on the Earth's surface and contains the magnetic axis of the Earth.
- **Geographic Meridian**: A plane that passes through a point on the Earth's surface and contains the geographic axis of rotation.

2. **Declination**:
- The angle between the magnetic meridian and the geographic meridian is known as declination. When this angle is zero, it means that the two planes coincide.

3. **Visualizing the Earth**:
- Imagine the Earth as a sphere with a geographic north pole and a geographic south pole. The magnetic poles are slightly tilted relative to the geographic poles.

4. **Identifying Points of Zero Declination**:
- The points where the magnetic meridian and geographic meridian align perfectly (i.e., where declination is zero) occur along a specific line or region on the Earth’s surface.

5. **Conclusion**:
- The points where declination is zero are located along the line where the magnetic meridian coincides with the geographic meridian. This line is often referred to as the "agonic line."
- Since this line wraps around the Earth, there are infinitely many points along this line where the declination is zero.

### Final Answer:
There are infinitely many points on the surface of the Earth where the declination is zero.

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