An eclipse is an astronomical phenomenon during which an astronomical object, like the Sun or the Moon, is partially or completely obscured for a short duration of time, by passing into the shadow of another body or by having another body passing between it and the viewer.
Solar and lunar eclipses occur when the Earth, the Sun, and the Moon are exactly or very closely aligned. During a solar eclipse, the Moon passes between the Sun and the Earth, blocking totally or partially the Sun’s rays and casting a shadow on parts of the Earth. Such events can only take place during a new moon. A lunar eclipse occurs when the Moon is on the far side of the Earth relative to the Sun and passes through the Earth’s shadow. This can only happen during a full moon. Lunar eclipses can be seen from the entire nightside half of the Earth surface, while solar eclipses, especially total solar eclipses, are only visible from a relatively small area on the Earth’s surface.
A shadow, for example during an eclipse, is divided into three parts, the umbra, penumbra and antumbra, created by a light source after hitting an opaque object. The umbra is the central region of the shadow and is the darkest part, where the light is completely blocked. In the penumbra light is only partially blocked. The antumbra is the part where the occluding body appears entirely within the disc of the light source.