Difference between the true cross-sectional geometry of a source (solid ellipse) and the maximum-area (dashed) ellipse derived from observations taken by two observers separated by an angle θ. The source has semi-major and semi-minor axes a and b, and an orientation angle, ϕ, between its semi-major axis and the lines of sight of observer 1. The derived ellipse has conjugate diameters, d₁ and d₂, with the same lengths as the parallel sides of the bounding parallelogram (solid lines) formed by the tangential lines of sight (dotted lines). The solid blue line represents the x-axis, and the dotted orange line represents the distance, 2c₁, between the lines of sight from observer 1.