Unit of the cosmic cycle in Hindu cosmology. Each yuga is progressively shorter than the preceding one, corresponding to a decline in the moral and physical state of humanity. Four such yugas make up a mahayuga; 2,000 mahayugas make up the basic cosmic cycle, the kalpa. The first yuga was an age of perfection that lasted 1,728,000 years. The fourth and most degenerate yuga began in 3102 BC and will last 432,000 years. At the close of this yuga, the world will be destroyed, to be recreated after a period of quiescence as the cycle resumes.
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The descent from Satya to Kali is associated with a progressive deterioration of Dharma (righteousness) manifested as a decrease in both the length of human life and the quality of human moral standards. In the Vishnu Purana, for example, the Kali yuga is described thus:
"In the Kali Yuga, there will be numerous rulers vying with each other. They will have no character. Violence, falsehood and wickedness will be the order of the day. Piety and good nature will dwindle slowly... Passion and lust will be the only attraction between the sexes. Women will be the objects of sensual pleasure. Dishonest will be the bottom line of subsistence. Learned people will be ridiculed and put to shame; the word of the wealthy person will be the only law."
The traditional virtues accorded the highest value in the four epochs are:
Temples, wars, and writing are hallmarks of Dvapara and Kali yugas. In the higher ages (Treta and Satya), writing is unnecessary because people communicate directly by thought. Temples are unnecessary because people feel the omnipresence of God. Wars are rare but they do occur - one such war is described in the Ramayana.
Above numerical values are astrological/metrological and nothing to do with periods of time
Hence, Hindu scripture says the length of the yugas maintain a 4,3,2,1 ratio to each other. This four yugas consists of 10 parts of 432,000 years i.e 4,320,000 years . The Krita Yuga lasts for 4 parts; the name has the same consonants T and R of the word Chatur (four). The Treta Yuga consists of Tri (three) parts as it is apparent in the name 'Treta'. The Dvapara Yuga lasts for Dva (two) parts as it is apparent in the name 'Dvapara'. The Kali Yuga lasts for Eka (one) part; the consonant K appears in the name Kali.
Upon conclusion of seventy-one circuits of this cycle, there is a period equally long during which the world is inundated; then the cycle begins again
Another important thing to know about the four yugas, besides the fact the strength and virtue of man decreases and that the time of each yuga gets shorter, is that the life of man also decreases geometrically. In the beggining of the Satya yuga, the average life expectency is 100,000 years. In the beggining of the Treta yuga, the average life expectency is about 10,000 years. In the beggining of the Dvapara yuga, the life expectency is 1,000 years. Finally, at the beggining of the Kali yuga (the age that we are currently in) the life expectency is 100 years. While most people think that the life expectency of humans is increasing with the rise of science and technology, according to Hinduism, the average life expectency is actually decreasing. It is said that by the end of the yuga, an average human will live only to about 30 years and eat only meat.
In his book, The Holy Science, Sri Yukteswar explained that the descending phase of Satya Yuga lasts 4800 years, the Treta Yuga 3600 years, Dwapara Yuga 2400 years, and the Kali Yuga 1200 years. The ascending phase of the Kali Yuga then begins, also lasting 1200 years. The ascending phase of the Kali Yuga began in September of 499 AD. Since September 1699, we have been in the ascending phase of the Dwapara Yuga, according to Sri Yukteswar.
In The Holy Science, Sri Yukteswar wrote that the traditional or long count view is based on a misunderstanding. He says that at the end of the last descending Dvapara Yuga (about 700 BC), "Maharaja Yudhisthira, noticing the appearance of the dark Kali Yuga, made over his throne to his grandson [and]...together with all of his wise men...retired to the Himalaya Mountains... Thus there was none in the court...who could understand the principle of correctly accounting the ages of the several Yugas."
According to Sri Yukteswar, nobody wanted to announce the bad news of the beginning of the ascending Kali Yuga, so they kept adding years to the Dvapara date (at that time 2400 Dvapara) only retitling the epoch to Kali. As the Kali began to ascend again, scholars of the time recognized that there was a mistake in the date (then being called 3600+ Kali, even their texts said Kali had only 1200 years). "By way of reconciliation, they fancied that 1200 years, the real age of Kali, were not the ordinary years of our earth, but were so many daiva (or deva) years ("years of the gods"), consisting of 12 daiva months of 30 daiva days each, with each daiva day being equal to one ordinary solar year of our earth. Hence according to these men 1200 years of Kali Yuga must be equal to 432,000 years of our earth."
Sri Yukteswar explained that just as the cycle of day and night is caused by a celestial motion (the earth spinning on its axis in relation to the sun), and just as the cycle of the seasons are caused by a celestial motion (the earth with tilted axis orbiting the sun) so too is the yuga cycle (seen as the precession of the equinox), caused by a celestial motion. He explained this celestial motion as the movement of the whole solar system around another star. As our sun moves through this orbit, it takes the solar system (and earth) closer to and then further from a point in space known as the "grand centre" also called 'Vishnunabhi', which is the seat of the creative power, 'Brahma', [which]...regulates...the mental virtue of the internal world." He implied that it is the proximity of the earth and sun to this grand centre that determines which season of man or yuga it is.