An eclipse is the veiling or obscuring of one celestial body by another. It occurs when a planet or moon on its journey passes between another planet, moon or the Sun. During an eclipse two types of shadows are cast. The darkest portion of the shadow, which gets smaller as it goes away from the Sun, is called ‘umbra’, while the partial shadow surrounding the Umbra, which gets larger as it goes away from the Sun, is known as ‘Penumbra’.
The moon revolves round the Earth, while the Earth orbits the Sun. During the course of the journey, when the Earth lines up directly between the Sun and the Moon, the Earth blocks the light from the Sun to the Moon and the shadow of the Earth falls on the Moon. As the Moon does not have any light of its own, it cannot shine when the source of light is thus cut off. This is an eclipse of the moon, which is called a lunar eclipse. When the moon travels completely into the Earth's umbra, one observes a total lunar eclipse. On the other hand, a partial lunar eclipse occurs when only a portion of the moon enters the umbra.
However, during a total lunar eclipse, the Moon does not completely disappear, but it will be cast in an eerie darkness. This is because Earth's atmosphere refracts sunlight and indirectly lights up the Moon's surface. Earth's atmosphere removes or blocks parts of the sunlight's spectrum, leaving only the longer wavelengths. As a result, a totally eclipsed Moon usually looks red.
The lunar Eclipse can last as long as 3 hours and 40 minutes. If the moon is at its nearest to the Earth (at Perigee), the eclipse lasts longer, but if it is farthest from the Earth (at Apogee), the eclipse will be of a shorter duration.
Lunar eclipses occur at the time of a Full Moon, but not every Full Moon, because the Moon has to be near one of the nodes of intersection between its orbit and the ecliptic plane.
Sometimes the orbiting Moon comes between the Sun and the Earth. At that time, if their centres are in one line, the moon blocks the light of the sun from reaching Earth, and it results a Solar Eclipse. In other words, when the moon casts a shadow onto the Earth, a Solar Eclipse takes place. Only on New Moon days, the Moon comes in between the Sun and The Earth. But, since the Moon’s orbit is inclined to the ecliptic plane, an eclipse does not occur at every New Moon. There are four different types of solar eclipse, namely Partial eclipse, Annular eclipse, Total Eclipse and Hybrid Eclipse.
A partial solar eclipse occurs when only part of the Sun is covered by the Moon, which appears to take a “bite” out of the Sun. Partial eclipses occur when the umbra shadow of the Moon misses the Earth and only its penumbral shadow falls on the Earth’s surface. Since the penumbra is much larger than the umbra and it forms a very big circle around the umbra, it does not completely exclude the Sun and there is a partial eclipse wherever the penumbra touches the Earth. During a Partial Solar Eclipse the Moon covers only a part of the Suns disc, while the other part, which is not eclipsed by
the Moon, shines on the Earth. Observers within the area of the penumbra and close to the umbra will see the Sun almost covered while observers at the edge of the penumbra will see a crescent Sun.
Annular eclipses occur when the moon appears smaller than the Sun. Though the Sun is 400 times larger than the Moon, it is almost 400 times as far away from us as the Moon. Hence, both of them appear to be of the same size from the Earth. But, as the orbit of the Moon round the Earth is elliptical, the distance of the Moon from the Earth varies at different times. When it is at apogee, it appears to be very small and it cannot cover the solar disc completely. During that time, as the Moon is far away in its elliptical orbit, its umbra is too short to reach the Earth. But, since the moon is located exactly in front of the Sun, observers in the umbra area below will observe the centre of the Sun covered, leaving a dazzling ring (an annulus) of the Sun visible around the edges of the moon. However, observers in the penumbra area witness a normal partial eclipse instead.
A total solar eclipse occurs when the Moon completely covers the Sun. In a total solar eclipse, the moon, in its elliptical orbit, passes between the Sun and the Earth at a point where it is also quite close to the Earth. Totality during such an eclipse can only be seen from a limited area, shaped like a narrow belt, usually about 160 km (100 mi) wide and 16,000 km (10,000 mi) long. However, people outside this track will be able to see only a partial eclipse of the Sun. Though the total Solar Eclipse rarely lasts more than seven minutes in any place on the Earth, the partial eclipse may be as long as four hours. During the brief period of a total Solar Eclipse all direct sunlight is cut off and an eerie shadow is cast upon the Earth’s surface. Even the birds and the animals become scared. Stars and planets become visible in the midday sky and for a very few moments, just before the end of the process, the brilliant corona of the Sun becomes visible to the human eyes.