Illustration of the transit geometry and the chosen coordinate system. The small circles depict the planet at two different locations. The planet has radius R_● = 1/10 R_⋆ and is transiting the stellar disk (large circle segment) at impact parameter b = 0.7 R_⋆. The green line shows the assumed straight transit chord (parallel to the x-axis) and the dashed red line the exact transit chord for a (unrealistically) close-in planet orbiting the host star at a distance of 2 R_⋆. ∆y is the extension of the planet perpendicular to the transit chord. For further details, see the text.