Bird Migration: The Magnetic Compass (2D)

This simulator draws a 2D meridional cross-section of Earth's dipole magnetic field — the classic textbook slice through both poles — with field lines traced from the real McIlwain L-shell equation and a bird positioned at any latitude along the outline. Rather than reprojecting the 3D globe, it works entirely in this flattened plane: local field inclination and intensity are computed directly from latitude, and a small animated radical-pair diagram shows the quantum spin mechanism cryptochrome uses to sense that inclination. Drag to pan and scroll or pinch to zoom into the field-line geometry near the bird, click anywhere on Earth's outline to relocate it, flip the field's polarity to test whether the bird's course changes, and dial the light toward red or off to watch the radical-pair signal — and the bird's orientation confidence — fade out.