A reader in Westminster wondered why the dark portion of the moon is not always round: "The moon goes through its phases because the Earth blocks part of the light from the sun. So, one would expect the shadow always to reflect the shape of the Earth, no?" Oops. Back to school. Earth's round shadow touches the moon only during lunar eclipses. The phases change as the moon orbits Earth. The dark portion of the lunar sphere we see is simply the side facing away from the sun.