Yugas and Avatars

Yuga in Hindu philosophy is the name of an epoch or era within a four age cycle. According to Hindu cosmology, life in the universe is create and destroyed once every 4.1 to 8.2 billion years, which is one full day for Brahma. The cycles are said to repeat like the seasons, waxing and waning within a greater time-cycle of the creation and destruction of the universe. Each Yuga involves stages or gradual changes which the earth and the consciousness of mankind goes through as a whole. A complete Yuga cycle from a high Golden Age of Enlightenment to a Dark Age and back again is said to be caused by the solar system's motion around another star.

The four Yugas were Satya Yuga, Treta Yuga, Dvapara Yuga, and the last one which has not occurred yet is the Kali Yuga.

The Satya Yuga is the Yuga when humanity is governed by gods, and every manifestation or work is close to the purest ideal and humanity will allow intrinsic goodness to rule supreme. It is sometimes referred to as the "Golden Age". The Satya Yuga lasts 1,728,000 years. During this Yuga, we see four incarnations of Vishnu which appear to save humanity.

1. Matsya: the fish. Lord Vishnu takes the form of a fish to save Manu from a flood, after which he takes his boat to the new world along with one of every species of plant and animal, gathered in a massive cyclone.
2. Kurma: the tortoise. When the devas and asuras were churning the Ocean of Milk in order to get amrita, the nectar of immortality, the mount Mandara they were using as the churning staff started to sink and Lord Vishnu took the form of a tortoise to bear the weight of the mountain.
3. Varaha: the boar, He appeared to defeat Hiranyaksha, a demon who had taken the Earth, or Prithvi, and carried it to the bottom of what is described as the cosmic ocean in the story. The battle between Varaha and Hiranyaksha is believed to have lasted for a thousand years, which the former finally won. Varaha carried the Earth out of the ocean between his tusks and restored it to its place. 
4. Narasimha: he half-man/half-lion appeared in the Satya Yuga. The rakshasa(An evil person) Hiranyakashipu, the elder brother of Hiranyaksha, was granted a powerful boon from Brahma, not allowing him to be killed by man or animal, inside or out, day or night, on earth or the stars, with a weapon either living or inanimate. Vishnu descended as an anthropomorphic incarnation, with the body of a man and head and claws of a lion. He then disembowels the rakshasa at the courtyard threshold of his house, at dusk, with his claws, while he lay on his thighs.

Treta Yuga is the second out of four yugas. Treta means third in sanskrit. The famous events in this yuga were Lord Vishnu's fifth, sixth and seventh incarnations as Vamana, Parashurama and Rama respectively. 

1. Vamana: Vishnu incarnated as the dwarf son of Aditi to stop King Mahabali of the Asura race, the grandson of Prahlada, from completing a sacrifice which would allow him to overshadow Indra in power and splendor. The jealous and worried Devas persuaded Vishnu to send Bali, a bhakta of his, to Patala, the underworld. Vishnu did so by taking advantage of the King's kindness and refusal to go back on his word, but allowed him a luxurious dwelling and sovereignty in one of the lower worlds.
2. Parashurama, Rama with the axe. He is son of Jamadagni and Renuka. He received an axe after a penance to Shiva. King Kartavirya Arjuna and his army visited the father of Parashurama at his ashram, and the saint was able to feed them with the divine cow Kamadhenu. The king demanded the animal, Jamadagni refused, and the king took it by force and destroyed the ashram. Parashurama then killed the king at his palace and destroyed his army. In revenge, the sons of Kartavirya killed Jamadagni. Parashurama took a vow to kill every Kshatriya on earth twenty-one times over, and filled five lakes with their blood. Ultimately, his grandfather, the great rishi Rucheeka, appeared and made him halt. 
3. Rama: Ramachandra, the prince and king of Ayodhya.  Rama is a commonly worshiped avatar in Hinduism, and is thought of as the ideal heroic man. His story is recounted in one of the most widely read scriptures of Hinduism, the Ramayana. While in exile from his own kingdom with his brother Lakshman and the monkey king Hanuman, his wife Sita was abducted by the demon king of Lanka, Ravana. He travelled to Ashoka Vatika in Lanka, killed the demon king and saved Sita.

Dvapara Yuga is the third out of four Yugas. According to the Puranas, this Yuga ended at the moment when Krishna returned to his eternal abode of Vaikuntha. This Yuga last 864,000 years. The only two pillars in this age are Compassion and Truthfulness.

1. Krishna: was the eighth son of Devaki and Vasudev. Krishna is the most commonly worshiped deity in Hinduism and an avatar in Vaishnava belief. His name means 'dark' or 'attractive', and he appeared in the Dwapara Yuga alongside his brother Balarama, and was the central character of the Bhagavad Gita. He is mentor to Arjuna, delivering him the Gita at the Battle of Kurukshetra. He is often depicted playing the murali and having a mischievous spirit.
2. Buddha: In this age, the true devotion to Vedas was replaced by empty rituals. To enlighten the world, Lord Vishnu descended the earth as Buddha, the enlightened one. He was born as the crown prince of the Kapilavastu to King Suddhodana and Maya. He was named Siddhartha, meaning "All things fulfilled". But his mother died soon after his birth and was brought up by Prajapati, the sister of Maya. Buddha was saddened by death of living creatures, and vices like poverty. He wasn't happy with any answers that were provided to him and he decided to find out the meaning and the absolute truth and he left his wife and child to a hermit's life in the forest and became the enlightened one. Buddha advocated the Middle Path, in which he offered a balanced, harmonious way of life, steering between two extremes of self-indulgence and total abstinence.

Kali Yuga is the "age of Kali" and is the last of the four yugas. Kali Yuga is associated with the demon Kali. This is associated with strife, discord, quarrel, or contention. 

1. Kalki: is the final incarnation of Vishnu, foretold to appear at the end of Kali Yuga, our present epoch. He will be atop a white horse and his sword will be drawn, blazing like a comet. He is the harbinger of end time and will destroy all unrighteousness and evil at the end of Kali Yuga.

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“Whenever righteousness wanes and unrighteousness increases I send myself forth.For the protection of the good and for the destruction of evil,and for the establishment of righteousness,I come into being age after age.” (Gita:4.7–8)

DQ: Why do the Yugas get smaller every time? If Satya Yuga was so good, why did they need 4 avatars and we, in the Kali Yuga, only get one?