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A circle divided into a gibbous shape (red) and an astronomical crescent shape (blue) by a half-ellipse, where the major axis of the ellipse coincides with a diameter of the circle. According to geometry, an illuminated hemisphere of a sphere will appear to take on such gibbous or crescent shapes when viewed from an angle. In this case, the length of the major axis of the ellipse is twice the length of the minor axis (i.e. the eccentricity is half the square root of 3), so that the red gibbous area shows the theoretical shape of the visible lit area of the moon (for example) when the separation between the viewing angle and the angle of illumination (i.e. between Earth and Sun) is 60 degrees, while the blue crescent area shows the theoretical shape of the visible lit area of a sphere when the separation between the viewing angle and the angle of illumination is 120 degrees. See also Image:Gibbous-Crescent-half-ellipse-in-circle-outlined.png

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