Q.What is a geographic meridian? How does the declination vary with latitude? Where is it minimum?

[!TLDR] The geographic meridian is the vertical plane through the Earth's rotation (geographic) axis; declination is the small angle between it and the magnetic meridian, varying from place to place and vanishing along the agonic line where the two meridians coincide. [!ANSWER] Geographic …

The geographic meridian at a place is the vertical plane, perpendicular to the Earth's surface, that passes through the Earth's geographic (rotation) axis -- it is the plane that defines true geographic north at that place. The magnetic meridian, by contrast, is the vertical plane passing through the Earth's magnetic axis instead, and the Earth's own resultant field at a place always lies along (or parallel to) this magnetic meridian. The angle between these two meridian planes, at a given place, is the magnetic declination, $\alpha$. Because the Earth's magnetic axis is both tilted relative to, AND offset from, its geographic axis, declination is not the same everywhere -- it varies from place to place across the globe, and its value (and even whether it points east or west of true north) depends on the location. Places sharing the same declination are joined by isogonic lines on a magnetic map. In India specifically, declination is small: about $0^\circ58'$ west at Mumbai and about $0^\circ41'$ east at Delhi. Since the sign flips from west to east somewhere betw …

Define the geographic meridian directly from section 12.5, then explain declination's place-to-place variation using the offset/tilt between the Earth's geographic and mag …

Assuming declination is the same everywhere on Earth, or confusing it with the angle of dip (inclination), which is a completely different angle measured in …