Current Ellipse

A current ellipse (tidal ellipse) is the path traced by the tip of the tidal-current velocity vector over one tidal cycle, the rotary form a tidal current takes in open water away from a channel that would force it back and forth along one line. Harmonic analysis resolves the current of each constituent (M2, S2, K1) into its ellipse, defined by semi-major and semi-minor axes, the orientation of the major axis, the sense of rotation, and the phase. The Coriolis force biases the rotation, clockwise in the northern hemisphere for many constituents, so the current never fully slacks. Pilots use the ellipse to predict set and drift at any hour, not just the rectilinear stream of a strait.