In case of earth, the vertical plane containing the longitude circle and the axis of rotation of the earth is called the geographic meridian. In a similar way, one can define magnetic meridian of a place as the vertical plane which passes through the imaginary line joining the magnetic north and the south poles. This plane would intersect the surface of the earth in a longitude like circle. A magnetic needle, which is free to swing horizontally, would then lie in the magnetic meridian and the north pole of the needle would point towards the magnetic north pole. Since the line joining the magnetic poles is titled with respect to the geographic axis of the earth, the magnetic meridian at a point makes angle with the geographic meridian.
(i) The angle between the true geographic north and the north pole shown by a compass needle is called the magnetic declination (D) or simply declination. The declination is greater at higher latitudes and smaller near the equator. The declination in India is small, it being 0”41' E at Delhi and 0”58' W at Mumbai. Thus, at both these places a magnetic needle shows the true north quite accurately.
(ii) The angle that the total magnetic field (BE) of the earth makes with the surface of the earth and horizontal component of earth's magnetic field (HE) is called dip or inclination (I) at that place.
In most of the northern hemisphere, the north pole of the dip needle tilts downwards. Likewise in most of the southern hemisphere, the south pole of the dip needle tilts downwards.
(iii) To describe the magnetic field of the earth at a point on its surface, we need to specify three quantities, viz., the declination (D), the angle of dip or the inclination (I) and the horizontal component of the earth HE. These are known as the element of the earth magnetic field.
Representing the verticle component by ZE, we have
