Declination:
Declination is the horizontal angle between the true meridian and magnetic meridian. It is also known as magnetic declination or magnetic variation. The true meridian refers to the line of longitude passing through a specific location, while the magnetic meridian corresponds to the direction indicated by a magnetic compass.

Declination can vary depending on the geographic location and changes over time due to the movement of the Earth's magnetic field. It is an important consideration in navigation and surveying, as it helps determine the orientation of a magnetic compass relative to true north.

Causes of Declination:
Declination occurs due to the difference between the geographic North Pole and the magnetic North Pole. The magnetic North Pole is the point on Earth's surface where the geomagnetic field points vertically downwards. However, this point is not aligned with the geographic North Pole, which is the point of intersection between the Earth's rotational axis and its surface.

The magnetic North Pole is constantly on the move, and its position can change over time. Additionally, the Earth's magnetic field is not uniform and has irregularities caused by variations in the distribution of magnetic minerals in the Earth's crust. These factors contribute to the variation in declination across different locations.

Measurement and Use:
Declination can be measured using a magnetic compass or specialized instruments such as a theodolite or a total station. By comparing the magnetic bearing of a known reference point with its true bearing, the declination angle can be determined.

Surveyors and navigators often correct for declination to obtain accurate results. When using a magnetic compass, the compass reading must be adjusted by adding or subtracting the declination angle to obtain the true bearing. This correction ensures that the compass reading aligns with the true meridian.

Conclusion:
The horizontal angle between the true meridian and magnetic meridian is known as declination. It is caused by the difference between the geographic North Pole and the magnetic North Pole, as well as irregularities in the Earth's magnetic field. Declination is an important consideration in navigation and surveying, and it must be accounted for to obtain accurate results.