Penumbra. When light from a point falls upon an object a shadow of that object is cast behind it - that is, there is a region of darkness caused by the interception of the rays of light by the object. When, however, light from a surface of appreciable size falls on an object, two kinds of shadows are cast, one being a region of complete darkness and the other of partial darkness only. It is this partially dark region which is known as the penumbra. This phenomenon is of special interest when the sun is the source of light. If E and S are the earth and sun, it is seen that there are two pairs of tangents to the circles representing sections of these bodies. The one pair meet behind e, and form the darkly shaded region known as the umbra. The other two tangents cross in front of E, and into the region p - part of a cone - only some light can penetrate: this is the penumbra. If the moon M in her path enters these regions, we have an eclipse (q.v.), the surface of the moon becoming gradually less and less bright as she passes from penumbra to umbra. The presence of the earth's atmosphere causes the rays of light to be bent slightly, so that the two cones are somewhat distorted. Also, since some rays pass through more atmosphere than- others, the refraction is unequal. This, combined with the fact that. watery vapour transmits differently the different coloured. rays which go to make up white light, causes the moon to appear coloured during an eclipse, the colour varying from penumbra to umbra.