The radius of the earth's shadow for ultraviolet sunlight is enlarged by an amount equal to the altitude of the upper boundary of the ozone layer. Among the effects of this ozone shadow are (1) a delay in ionization at sunrise in the ionosphere, from which the uppermost height of the ozone layer may be calculated, (2) variations in the intensity of ultraviolet light reflected from the moon during a lunar eclipse, furnishing information concerning the vertical distribution of ozone, (3) a premature decay in the intensity of the sodium D-line at sunset, leading to calulation of the height of emission of the D-line.