A partial lunar eclipse is observable when only a part of the Moon passes through the umbra. The penumbral variety occurs when the Moon moves solely through the outer part of the shadow. Lunar eclipses generally occur twice a year. In some years, however, there may be none, whereas in others, one or possibly three may take place. The Moon revolving in its orbit around the Earth passes through the shadow of the Earth.
Umbra is the total shadow,penumbra the partial shadow.
Lunar eclipses are of limited value for astronomical research. They have enabled scientists to study the reactions of materials on the Moon's surface to the sudden removal of solar radiation. The results of this research have led to a better understanding of the structure and heat conductivity of lunar soil. From the Earth, the Moon appears against a background of distant stars. As the Moon moves eastward across the constellations, it occasionally passes in front of one of the brighter stars or a planet, causing an occultation.