A lunar eclipse occurs when the Moon moves into the Earth's shadow. This can occur only when the Sun, Earth, and Moon are exactly or very closely aligned (in syzygy) with Earth between the other two, which can happen only on the night of a full moon when the Moon is near either lunar node. The type and length of a lunar eclipse depend on the Moon's proximity to the lunar node.
The reddish color of a totally eclipsed Moon is caused by Earth completely blocking direct sunlight from reaching the Moon, with the only light reflected from the lunar surface has been refracted by Earth's atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of blue light.
Unlike a solar eclipse, which can only be viewed from a relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of Earth. A total lunar eclipse can last up to nearly 2 hours, while a total solar eclipse lasts only up to a few minutes at any given place, because the Moon's shadow is smaller. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than a normal full Moon.
The symbol for a lunar eclipse (or indeed any body in the shadow of another) is Lunar eclipse (U+1F776)
Earth's shadow can be divided into two distinctive parts: the umbra and penumbra. Earth totally occludes direct solar radiation within the umbra, the central region of the shadow. However, since the Sun's diameter appears about one-quarter of Earth's in the lunar sky, the planet only partially blocks direct sunlight within the penumbra, the outer portion of the shadow.
Penumbral lunar eclipse
This occurs when the Moon passes through Earth's penumbra. The penumbra causes a subtle dimming of the lunar surface, which is only visible to the naked eye when about 70% of the Moon's diameter has immersed into Earth's penumbra. A special type of penumbral eclipse is a total penumbral lunar eclipse, during which the Moon lies exclusively within Earth's penumbra. Total penumbral eclipses are rare, and when these occur, the portion of the Moon closest to the umbra may appear slightly darker than the rest of the lunar disk.
Partial lunar eclipse
This occurs when only a portion of the Moon enters Earth's umbra, while a total lunar eclipse occurs when the entire Moon enters the planet's umbra. The Moon's average orbital speed is about 1.03 km/s (2,300 mph), or a little more than its diameter per hour, so totality may last up to nearly 107 minutes. Nevertheless, the total time between the first and the last contacts of the Moon's limb with Earth's shadow is much longer and could last up to 236 minutes.
Total lunar eclipse
This occurs when the Moon falls entirely within the Earth's umbra. Just prior to complete entry, the brightness of the lunar limb-- the curved edge of the Moon still being hit by direct sunlight-- will cause the rest of the Moon to appear comparatively dim. The moment the Moon enters a complete eclipse, the entire surface will become more or less uniformly bright. Later, as the Moon's opposite limb is struck by sunlight, the overall disk will again become obscured. This is because as viewed from the Earth, the brightness of a lunar limb is generally greater than that of the rest of the surface due to reflections from the many surface irregularities within the limb: sunlight striking these irregularities is always reflected back in greater quantities than that striking more central parts, and is why the edges of full moons generally appear brighter than the rest of the lunar surface. This is similar to the effect of velvet fabric over a convex curved surface which to an observer will appear darkest at the center of the curve. It will be true of any planetary body with little or no atmosphere and an irregular cratered surface (e.g., Mercury) when viewed opposite the Sun.
Central lunar eclipse
This is a total lunar eclipse during which the Moon passes through the centre of Earth's shadow, contacting the antisolar point. This type of lunar eclipse is relatively rare.
The relative distance of the Moon from Earth at the time of an eclipse can affect the eclipse's duration. In particular, when the Moon is near apogee, the farthest point from Earth in its orbit, its orbital speed is the slowest. The diameter of Earth's umbra does not decrease appreciably within the changes in the Moon's orbital distance. Thus, the concurrence of a totally eclipsed Moon near apogee will lengthen the duration of totality.
A selenelion or selenehelion, also called a horizontal eclipse, occurs where and when both the Sun and an eclipsed Moon can be observed at the same time. The event can only be observed just before sunset or just after sunrise, when both bodies will appear just above opposite horizons at nearly opposite points in the sky. A selenelion occurs during every total lunar eclipse-- it is an experience of the observer, not a planetary event separate from the lunar eclipse itself. Typically, observers on Earth located on high mountain ridges undergoing false sunrise or false sunset at the same moment of a total lunar eclipse will be able to experience it. Although during selenelion the Moon is completely within the Earth's umbra, both it and the Sun can be observed in the sky because atmospheric refraction causes each body to appear higher (i.e., more central) in the sky than its true geometric planetary position.
The timing of total lunar eclipses is determined by what are known as its "contacts" (moments of contact with Earth's shadow):
P1 (First contact): Beginning of the penumbral eclipse. Earth's penumbra touches the Moon's outer limb.
U1 (Second contact): Beginning of the partial eclipse. Earth's umbra touches the Moon's outer limb.
U2 (Third contact): Beginning of the total eclipse. The Moon's surface is entirely within Earth's umbra.
Greatest eclipse: The peak stage of the total eclipse. The Moon is at its closest to the center of Earth's umbra.
U3 (Fourth contact): End of the total eclipse. The Moon's outer limb exits Earth's umbra.
U4 (Fifth contact): End of the partial eclipse. Earth's umbra leaves the Moon's surface.
P4 (Sixth contact): End of the penumbral eclipse. Earth's penumbra no longer makes contact with the Moon.
Symbolism of Eclipse (Grahanam) in Hinduism
Eclipses were common celestial phenomena in ancient times, which people occasionally witnessed and were puzzled by their happening. Since they were associated with darkness rather than light, people ascribed to them several negative meanings. Find here the archetypal meaning, cultural significance and the symbolism of the eclipse (grahanam) in Hinduism
The gods of the Vedas possess great power. They are pure and immortal beings. Although not incorruptible, they remain on the side of Dharma and God, and follow his orders. As beings of light and delight, and as aspects of Brahman, they partake the nature of the Sun, who symbolizes him. Just as darkness cannot withstand light, the demons cannot withstand the power of gods. However just as darkness can temporarily overshadow light and prevail against it, gods may suffer a temporary defeat in the hands of demons due to fate and the overshadowing effect of the planetary deities.
The Vedas therefore attribute solar and lunar eclipses to the overshadowing effect of certain impure planetary deities. Like the demonic dark clouds that often hold rain in captivity and cover the sun and the moon, certain planets overshadow their light and positive influence and create temporary darkness.
In Sanskrit an eclipse (ecclipse) is called grahanam. The word has a negative connotation meaning planetary possession or adverse planetary influence. Eclipses torment not only gods but also humans. On specific occasions they overshadow their karma and destinies, creating chaos and suffering in their lives. Like the eclipses, sinful karma brings temporary darkness into the lives of people and eclipses their peace and happiness, until the light of good fortune shines upon them again.
Thus, in the Vedic symbolism, eclipses represented the dark and negative influence of planets, and a temporary aberration in the order and regularity of the worlds. Their influence would not last long, but they would leave their mark in the celestial map and upon the lives of individuals. Therefore, it is necessary for people to keep themselves abreast of their happening and safeguard themselves from their destructive influence.
As celestial phenomena, eclipses also represent the conflict between good and evil, light and darkness, or gods and demons. According to the Vedic beliefs, eclipses are caused especially by the actions of two shadowy planets, namely Rahu and Ketu. Since they have a score to settle with the Sun and the Moon, due to their betrayal at the time of the churning of the oceans for the elixir, they keep swallowing them on specific occasions to take revenge.
Due to their recurring and predictable nature, Vedic astrologers were able to calculate the movement of the planets and other celestial objects (including comets) and predict the occurrence of eclipses. It is not clear how they developed the ability, but we know that they took the eclipses seriously due to the fear associated with them.
The Vedas contain references to the eclipses. For example, the Taittiriya Brahmana mentions a solar eclipse, which was witnessed by sage Atri. We find similar references in the Tandya Brahmana and Sankhyayana also. These descriptions suggest that as early as the Rigvedic times, Vedic astronomers were familiar with the eclipses, and were able to predict their occurrence.
People feared as well as revered the dark planets. Both Rahu and Ketu are depicted in the images as having a serpent body and with and without a human head. Interestingly, in some other cultures also eclipses are believed to be caused by demonic beings, serpents, or dragons.
The Puranas trace their conflict to the churning of the oceans, during which the two demons tried to drink the elixir through deception. Fortunately, the Sun and the moon noticed it and informed Vishnu, who then cut off their heads to prevent them from becoming immortal. However, since a few drops of the elixir entered their bodies, they partially survived and became part of the planetary system.
Ever since, the two demons assumed serpent bodies and joined the Hindu pantheon as anti-gods. Since their plans were thwarted by the sun and moon, they take their revenge against them by swallowing them and causing the eclipses. For a short time on specific occasions Rahu swallows the Sun, and Ketu swallows the moon. When it happens, the two celestial beings said to remain in fear, overshadowed by the darkness of the demons, and feel relieved when they finally emerge out of their tails and become free.
Be it a solar eclipse or lunar eclipse, the overshadowing of one planet by another is considered an evil omen. In the Mahabharata, sage Vyasa explains to King Dhritarashtra the number of evil omens that appeared in the sky before the commencement of war, suggesting the impending doom and destruction arising from the war.
The scriptures leave no doubt about their inauspicious nature. Hence, when they happen, people are advised to stay indoors and not to look at them. They also advise people not to initiate any actions, engage in sexual intercourse, cook food or expose the food items to their negative influence. Instead they are advised to pray and give grants and charities to neutralize their ill effects and negative consequences.
Symbolically, eclipses are associated with demonic possession, suffering (pida) and adversity. When a person is going through difficulties or misfortune, people say that the person is possessed or overshadowed by an eclipse. The word is also used symbolically to convey dominance or conquest of one nation over another, or one person by another. A person's fame, wealth, or power may also be eclipsed by misfortune or adverse circumstances.
References to eclipses are found both in the Ramayana and Mahabharata also. The latter mentions a solar eclipse which happened during the great war at the end of the day. Lord Krishna was aware of its happening beforehand, but Arjuna had no idea. However, it miraculously saved him from a certain death and helped him slay Jayadratha, who was responsible for the death of his son Abhimanyu, and keep his pledge. Similarly, in the Ramayana it is mentioned that an eclipse appeared when Lord Rama was fighting a demon.