Here is detailed explanation of time of yugas and more from http://www.stephen-knapp.com/
Timings of the Four Yugas
By Stephen Knapp
When describing the length of the yugas or ages, and which yuga we are in and how far along we are in it, there is sometimes confusion about how to calculate them. A number of theories have cropped over the years regarding how the yugas transpire. The problem is when they are figured only according to years of earth’s time, they will never be accurate. They are described in the Vedic literature according to the celestial years, or years of the Devas. Nonetheless, there are specific references in the Vedic texts which make it clear how to calculate them. For starters, the Mahabharata (Shanti Parva, 231.12-20) explains it in detail:
"The rishis, measuring time, have given particular names to particular portions [of time]. Five and ten winks of the eye make what is called a Kastha. Thirty Kasthas make what is called a Kala. Thirty Kalas, with the tenth part of a Kala, make a Muhurta. Thirty Muhurtas make one day and night. Thirty days and nights form a month, and twelve months form a year. Persons well-read in mathematical science say that a year is made up of two solar motions, meaning the northern and southern. The sun makes the day and night for men. The night is for the sleep of all living creatures, and the day is for work. A month of human beings is equal to a day and night of the departed manes [ancestors who have gone on to the subtle worlds]. That division consists in this: the light half of the month is their day which is for work; and the dark fortnight is their night for sleep. A year (of men) is equal to a day and night to the gods [Devas or celestials]. This division consists in this: the half year for which the sun travels from the vernal to the autumnal equinox is the day of the gods, and the half year for which the sun moves from the latter to the former is their night. [Thus, an earth year is but a day for the Devas.] Calculating by the days and nights of human beings about which I have told you, I shall speak of the day and night of Brahma and his years also. I shall, in their order, tell you the number of years, that are for different purposes calculated differently, in the Krita, the Treta, the Dvapara, and the Kali yugas. Four thousand celestial years is the duration of the first or Krita age. The morning of that cycle consists of four hundred years and its evening is of four hundred years. [Note: This says celestial years, or years of the demigods on the higher planets. Such years are much longer than those of planet earth. So 4000 celestial years, with the morning or Sandhya of 400 celestial years and the evening or Sandhyansa, or intermediate period, of another 400 years equals 4800 celestial years or 1,728,000 human years.]
"Regarding the other cycles [or yugas], the duration of each gradually decreases by a quarter in respect of both the principal period with the minor portion and the conjoining portion itself. These periods always keep up the never-ending and eternal worlds. They who know Brahma, O child, regard this as Immutable Brahma." (Mb, Shanti Parva, 231.21-22)
This means that as each age or yuga appears, from the Krita, Treta, Dvapara, to Kali, each yuga decreases by a quarter of the previous yuga, in addition to the conjoining Sandhya and Sandhyansa periods with each yuga. In this way, it is roughly calculated that a whole cycle of the four yugas, namely Krita, Treta, Dvapara, and Kali-yuga together, total about 12,000 celestial years in length.
The Mahabharata (Shanti Parva, 231.29-32) continues: "The learned say that these 12,000 celestial years form what is called a cycle. A thousand such cycles form a single day of Brahma. The same is the duration of Brahma’s night. With the beginning of Brahma’s day the universal entities come into being. During the period of universal dissolution the Creator [Brahma] sleeps in Yoga-meditation. When the period of sleep expires, He awakes. What is Brahma’s day covers a thousand such cycles. His night also covers a thousand similar cycles. They who know this are said to know the day and the night. On the expiry of His night, Brahma, waking up, modifies the indestructible intelligence by causing it to be overlaid with ignorance. He then causes Consciousness to spring up, whence it originates Mind which is at one with the Manifest." [This creative process is described in detail in my book How the Universe was Created and Our Purpose In It.]
In calculating the duration of the different yugas, there are a few differences between the Puranas, otherwise their descriptions are all the same. The Brahmanda Purana (1.2.29.31-34) specifically states that Krita or Satya-yuga is 1,440,000 human years in length, Treta-yuga is 1,080,000 years, Dvapara-yuga is 720,000 years, and Kali-yuga is 360,000 years in length. The Linga Purana (4.24-35) also agrees with this except for Treta-yuga, which it says is 1,800,000 years in length. These figures are correct when we leave out the conjoining Sandhya [at the beginning of each yuga] and the Sandhyansa [ending of each yuga] periods.
When explaining the various measurements of time, the Vishnu Purana (Book One, Chapter Three) and the Srimad-Bhagavatam (3.11.19), along with the Bhagavad-gita (8.17), the Vayu Purana (Chapter 57), and Bhavishya Purana (Brahma Parva, 2.86-107), and others, such as the Mahabharata as quoted above, also all agree on the measurements of the durations of the yugas. This shows that these time measurements are corroborated throughout the Vedic texts, and if anyone has a different theory about the Vedic time frame, then they are going outside the Vedic descriptions. We will describe this in more simplified terms as follows:
In the explanations of the measurements of time found therein, one cycle of the four yugas together is 12,000 years of the demigods, called divine years. Each of these years is composed of 360 days, and each of their days is equal to one human year. So Krita-yuga is 4000 divine years in length, Treta-yuga is 3000 divine years in length, Dvapara-yuga is 2000 divine years in length, and Kali-yuga is 1000 divine years long, each one a quarter less than Krita or Satya-yuga, with the addition of the conjoining portions of the Sandhya and Sandhyansa.
In this way, each yuga is preceded by a period called a Sandhya, which is as many hundred years in length as there are thousands of years in that particular yuga. Each yuga is also followed by a period of time known as a Sandhyansa, which is also as many hundreds of years in length as there are thousands of years in the yuga. In between these periods of time is the actual yuga. Therefore, we have:
Krita-yuga = 4000 divine years, Sandhya = 400 divine years, Sandhyansa = 400 divine years. Total = 4800 divine years x 360 days = 1,728,000 human years.
Treta-yuga = 3000 divine years, Sandhya = 300 divine years, Sandhyansa = 300 divine years. Total = 3600 divine years x 360 days = 1,296,000 human years.
Dvapara-yuga = 2000 divine years, Sandhya = 200 divine years, Sandhyansa = 200 divine years. Total = 2400 divine years x 360 days = 864,000 human years.
Kali-yuga = 1000 divine years, Sandhya = 100 divine years, Sandhyansa = 100 divine years. Total = 1200 divine years x 360 days = 432,000 human years.
This equals 4,320,000 human years, or 12,000 divine years, in one cycle of the four yugas together, and 1000 cycles of these yugas equals a Chaturyuga at 12,000,000 divine years and 4,320,000,000 human years in one day of Brahma. It is also explained that Kali-yuga began with the disappearance of Lord Krishna from the planet. This has been calculated to be 3102 BCE. Since Kali-yuga is described as being 432,000 earth years in length, with 5,000 years and more already passed, then the age of Kali-yuga has approximately 326,000 more years to go.
To be more clear about this, these calculations are further confirmed in the Brahma-Vaivarta Purana which relates the same figures in the following way:
"Now I will tell you about the counting of time. As the days rise and end, similarly all the four yugas also come and end. A single divine day and night of Manu compares with the completed year of the humans. Those well-versed in the signs of time, ordain that after the passing of the three hundred and sixty yugas of the humans, a divine yuga of the gods is made. Seventy-one divine years [of the gods] comprise a manvantara. An age of Indra is equivalent to one manvantara. Thus, after the existence of twenty-eight Indras, there is one day and night of Brahma. Thus, after the lapse of one hundred and eight years, the life of Brahma comes to an end." (Brahma-Vaivarta Purana, Prakriti Khanda, 7.72-75)
The same time frame is calculated with a different description in the Kalki Purana (19.12-14) as follows:
"Twelve thousand years of the demigods is the duration of the four yugas on earth. The duration of Satya-yuga is four thousand celestial years, Treta-yuga is three thousand celestial years, Dvapara-yuga is two thousand celestial years, and Kali-yuga is one thousand celestial years. The transitional periods of the four yugas are four hundred, three hundred, two hundred, and one hundred celestial years respectively. In this way, the total comes to twelve thousand celestial years. The duration of the reign of each Manu is seventy-one cycles of the four yugas. Fourteen Manus reign during one day of Brahma. This is also the duration of Brahma’s night [which is as long as his day]."
To understand this in greater detail and to also show the consistency of these calculations throughout the Vedic literature, we can use the additional example. Here Suta Gosvami explains to the sages during the huge gathering at the forest of Naimisaranya 5000 years ago, all of the divisions of time and how the lengths of the yugas are formulated, as related in the Vayu Purana.
"In the context of the description of the earth, etc., I have already said about the four yugas. Now I shall recount them in detail. (Listen and) understand them. Calculating everything and mentioning in detail, I shall narrate in full the six aspects of yugas, namely yugabheda (difference in yugas), yugadharma (the particular characteristics of each yuga), yugasandhi (the junction of yugas), yugamsha (the parts of yugas), and yugasandhana (the joints of the yugas). The human year can be understood (calculated) by means of valid working knowledge. Calculating on the basis of that year [in the human time frame], I shall explain the four yugas. The time taken to utter a short syllable is equal to a Nimesha. Nimesha, Kashtha, Kala, and Muhurta are the units of time. Fifteen Nimeshas constitute a Kashtha, thirty Kashthas make one Kala, and thirty Kalas make one Muhurta. Thirty Muhurtas make one full day and night. The sun demarcates the human as well as the divine days and nights. The day is intended for activity and the night is meant for sleep. A (human) month constitutes the day and night of the Pitris [the departed ancestors, now included as part of the celestials]. Its division is thus: The dark half is the day for them and the bright half is their night for sleep [as calculated by the waning and waxing of the moon]." (Vayu Purana 57.2-9)
"Therefore, thirty human months make one month of the Pitris [the celestial ancestors]. On the basis of the human calculation, three hundred and sixty months constitute a year of the Pitris. A hundred human years constitute three years and four months of the Pitris. A human year in accordance with the human calculation is one day and one night of the Devas [the higher demigods]. This is the conclusion in this scripture." (Vayu Purana 57.10-12)
"So, the divine day and night together [of the Devas] make a human year. A further classification is that the Uttarayana period (northern transit of the sun) is the day and the Dakshinayana period (southern transit of the sun) is the night (of the Devas). Thirty of the days and nights of the gods, or thirty human years make one divine month (of the gods). One hundred human years make three divine months and three divine days. Thus is the divine reckoning for divisions of time." (Vayu Purana 57.13-15)
"Three hundred and sixty years according to human calculation constitute one divine year. Three thousand and thirty years according to human calculation constitute one year of the Seven Sages (Saptarishis) or the (astrological constellation of) the Great Bear. Nine thousand and ninety human years make one Kraunchan year. Thirty-six thousand human years should be known as one hundred divine years. Thus is the calculation of the divine unit of time. Persons well versed in calculation say that three hundred and sixty thousand (360,000) years reckoned on the basis of human time units constitute a thousand divine years [of the Devas]. It is thus that the sages sang about the divine calculation. They have formulated the reckoning of the yugas and their duration on the basis of this divine calculation." (Vayu Purana 57.16-21)
Now with all of this calculating we reach the point where it is used to establish the durations of the four yugas as followed:
"Wise people know that there are four yugas in Bharata Varsha [the area of India and sometimes considered planet earth in general]. The first one is Krita, followed by Treta, Dvapara, and Kali-yuga. One should reckon these yugas in this order. They say that the Krita-yuga consists of four thousand years. The Sandhya (transition period) consists of as many hundreds of years (400 divine years). The Sandhyamsha (part of the junction with the next yuga) is equal to the Sandhya period. In the Sandhyas and Sandhyamshas of the other yugas there is a gradual reduction of a thousand [years of the yuga] and hundred years [in the Sandhya and Sandhyamsha transitions]. [This means that] Treta-yuga contains three thousand years, and its Sandhya and Sandhyamsha [junctions with the preceding and following yugas] consist of three hundred years each. The wise say that Dvapara-yuga contains two thousand [celestial] years, and the Sandhya consists of two hundred years and the Sandhyamsha is equal to the Sandhya [two hundred years]. The learned say that Kali-yuga consists of a thousand years, and its Sandhya is of a hundred years and the Sandhyamsha is the same. This period of twelve thousand (divine) years is known as a Maha-yuga. The four yugas together are Krita, Treta, Dvapara, and Kali." (Vayu Purana 57.22-28)
"In this world the calculation of years is based on the human level. Now I shall mention the duration of Krita-yuga in accordance with that type of [human] calculation. The duration [of Krita-yuga] is one million, four hundred and forty thousand [human] years. As for Kali-yuga, it is one fourth of that. Thus the specific duration of time of the four yugas excluding the Sandhyas and Sandhyamshas is declared. Including the Sandhyas and Sandhyamshas the duration of the four yugas put together is four million three hundred and twenty thousand [4,320,000] human years." (Vayu Purana 57.29-32)
Thus, we have provided the different ways of explaining the units of time to calculate the length of the yugas according to the references in the Vedic texts. I hope this has provided enough information and evidence to clarify and conclude what is sometimes a confusing issue, and that the same figures and conclusions are arrived at in various texts.
THE MANU AVATARAS
To more clearly understand the division of time known as a manvantara, or the life span of one Manu, the full explanation is provided herein. First of all, the Manus appear for certain durations within a day of Brahma. Brahma’s day is calculated as 4,300,000 years (the time of one cycle of the four yugas) times 1,000, or about 4,320,000,000 human years. Within one day of Brahma there are 14 Manus. The list of the 14 Manus in this universe is as follows: Yajna is Svayambhuva Manu, Vibhu is Svarocisha Manu, Satyasena is Uttama Manu, Hari is Tamasa Manu, Vaikuntha is Raivata Manu, Ajita is Ckakshusha Manu, Vamana is Vaivasvata Manu (the Manu of the present age), Sarvabhauma is Savarni Manu, Rishabha is Daksha-savarni Manu, Vishvaksena is Brahma-savarni Manu, Dharmasetu is Dharma-savarni Manu, Sudhama is Rudra-savarni Manu, Yogesvara is Deva-savarni Manu, and Brihadbhanu is Indra-savarni Manu. These fourteen Manus cover the 4,320,000,000 solar years of one day of Brahma. (Srimad-Bhagavatam 1.3.5 purport & Chaitanya-caritamrita, Madhya-lila 20, 319-328)
To understand more completely how long these Manus reign we can consider the following information. For example, there are four ages, namely Satya-yuga, Treta-yuga, Dvapara-yuga, and Kali-yuga, which together comprise a divya-yuga, which is one set of the four yugas, also called a chatur-yuga. Let us remember that Satya-yuga lasts 1,728,000 years, Treta-yuga 1,296,000 years, Dvapara-yuga 864,000 years, and Kali-yuga 432,000 years. That is a total of 4,320,000 years. A day of Brahma, called a kalpa, lasts for 1,000 of these cycles, and is thus 4,320,000,000 solar years. There are 14 Manus in each day of Brahma. Each Manu is said to exist for one manvantara, which is a period of time lasting 71 divya-yugas. Therefore, each Manu exists for roughly 306,720,000 years. Additionally, Brahma lives for 100 years, composed of 365 of such days in a year. (Bhagavad-gita 8.17 & Srimad-Bhagavatam.3.11.20 & Vishnu Purana, Book One, Chapter Three, p.35)
From further analysis we can also discover the age of the earth from these Vedic calculations. The present Manu is the seventh in line, called Vaivasvata Manu, the son of Vivasvan. Twenty-seven divya-yugas, or cycles of the four yugas, of his age have now passed. So 27 divya-yugas means 116,640,000 years. It is scheduled that at the end of the Dvapara-yuga of the twenty-eighth divya-yuga of the seventh Manu, Lord Krishna appears on earth with the full paraphernalia of His eternal spiritual abode, named Vrajadhama or Goloka Vrindavana. Brahma’s day consists of 4,320,000,000 years. In this way, six of these Manus appear and disappear before Lord Krishna takes birth. This means that 1,975,320,000 years of the day of Brahma have gone by before the appearance of Lord Krishna. (Chaitanya-caritamrita, Adi-lila 3, 7-10 & Srimad-Bhagavatam 4.30.49 purport)
Therefore, this is also the age of the earth in this particular day of Brahma by these Vedic calculations. Science is sometimes surprised that such lengths of time were part of the ancient Vedic conception of the universe.
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