The shape of the lit side of a spherical body (most notably the Moon) that appears to be less than half illuminated by the Sun as seen by the viewer appears in a different shape from what is generally termed a crescent in planar geometry: Assuming the terminator lies on a great circle, the crescent Moon will actually appear as the figure bounded by a half-ellipse and a half-circle, with the major axis of the ellipse coinciding with a diameter of the semicircle.
The shape of the lit side of a spherical body (most notably the Moon) that appears to be less than half illuminated by the Sun as seen by the viewer appears in a different shape from what is generally termed a crescent in planar geometry Assuming the terminator lies on a great circle the crescent Moon will actually appear as the figure bounded by a half-ellipse and a half-circle with the major axis of the ellipse coinciding with a diameter of the semicircle