In the present times the world is facing challenges and adversities from multiple directions. Disturbed ecosystem, depleting natural resources, alarming rise in environmental pollution, pesticide-treated natural products, adulterated food, toxic waste, risk of nuclear radiations, global warming, ever-new harmful viruses/microbes and consequent health hazards, dreaded diseases, growing cases of tension, anxiety, depression, stress, and related psychosomatic ailments, mental complexities, psychological weaknesses and disorders, expanding terrorism, fanaticism, social disharmony, rising trends of broken families, population-explosion, economic uncertainties, increasing percentage of unemployment, etc., have put a Big Question Mark on sustainability of life and future of the earth. Everyone, who can think, is keen to find an answer.
In the views of many experts, great visionaries and authentic foretellers, the world is passing through a decisive phase of transition since past few decades. Is this a phase of epochal change? Some ancient texts of different schools of thoughts and deeper knowledge across the world provide clear indications. Here we highlight some scholarly research findings with some pre-historical scriptural calculations on epochal phases of Nature.
In common language of conversations we often use the term “Era”, “Age” (called “yuga” in Sanskrit and Hindi) to indicate a characteristic period or span of time associated with a predominant phase of history or social/political/ economic change, etc. For example, the “Stone Age”, “Vedic Era”, “Medieval Era”, “Electronics Age”, “Computer Age”, or “Bhakti Yuga”, “Vaigyanik Yuga”, etc. However, the term era (yuga or equivalent) in the ancient texts refers to period of epochal characteristics of Nature.
In astrological calculations, for practical purposes, in terms of periodic movements of the positions of the r3ïis (signs of the Zodiac), the period of 12 years is also referred as ‘yuga’. Interestingly, this period is important in terms of biological clock and change in the patterns of circadian rhythms of person. During a period of 12 years, almost all the cells of our body are renewed; in normal course, age-related natural changes in human body also manifest most significantly in successive periods of 12 years (e.g. onset of puberty/teenage at the age of 12 years; bloom of youth at the age of 24 years; start of middle age at 36 years and so on...). Astronomical studies and related scientific research also show importance of the cycle of 12 years, as it is found that periodic changes in the internal condition of the Sun occur every 12 years.
In terms of cosmological sciences, the calculations of cosmic cycles as presented in Vedic texts are supposed to be most comprehensive. However, it is extremely complex to decipher them on the common scale of time we use at present. We shall discuss here only what is generally accepted and referred/documented by most of the scholars in the modern times as authentic.
The ancient scriptures of Vedic cosmology describe different kinds of cosmic cycles as yugas, and mah3-yugas (deva-yugas, great aeons) in relation to epochal changes in the sublime and manifested domains of Nature  . Noted French-Egyptian philosopher René Guénon, who also had deep scholarly knowledge of the ancient oriental scriptures, explains the concept of cosmic cycles more explicitly  as follows: “In the most general sense of the term, a cosmic cycle must be considered as - representing the process of development of some state of (cosmic) manifestation, or, in the case of minor cycles, of one of the more or less restricted and specialized modalities of that state”. Moreover, in virtue of the law of correspondence which links all things in Universal Existence, there is necessarily and always a certain analogy, either among the different cycles of the same order or among the principal cycles (aeons) and their secondary divisions”. This allows using one and the same mode of expression when speaking about the (cosmic) cycles, although this must often be understood only symbolically, and this alludes here especially to the ‘chronological’ form under which the doctrines of cosmic cycles are interpreted.
In the terminology of Indian scriptures, a Kalpa represents the total duration of a state or degree of Universal Existence (Srasti - manifested Nature, the entire cosmic creation); a Kalpa is divided into 14 Manvantaras. This is further divided into mah3-yugas. The duration of a deva-yuga (maha-Yuga) is interpreted as about 432000 years. (The term ‘year’ is used in this article in usual sense of the modern scientific calendar). Within a Manvantar, there are several cycles of chatur-yugas (four principal eras) each signifying distinct characteristic tendencies of Nature and its manifestation. Notably, as described in the next Section, the total duration (24000 years) of these yugas is a multiple of 12, indicating the consonance with the periodicity of the solar and Zodiac systems as well.
The Four Principal Eras of Nature:
In the context of deeper science of Nature, especially in the scriptural parlance, ‘change of an era’ (yuga parivartan) mostly implies change in the subtle environment of Nature. This transformation is also reflected in epochal changes on multiple fronts of life including culture, social trends, goals and types of development, etc. Change in the subtle environment of Nature is characterized in the Vedic texts by change in the levels of the triguñas 1 .
In Krat-Yuga, which is referred as Sat-Yuga (Age of Truth and Light), there is predominance of sat-guña. The levels of raj-guña begin to rise with the onset of Treta (Treta) -Yuga and this guña begins to dominate by the completion of this era. The presence of tam-guña begins to manifest with gradually higher intensity with passage of Dwapar (Dwapara)-Yuga. Tam becomes predominant tendency in Kali-Yuga which also nurtures negative influence of raj. [An equivalence of these four yugas in terms of the Greco-Latin antiquity is the four ages of gold, silver, bronze and iron.]
It is obvious that epochal changes cannot occur instantaneously or overnight. There is substantial transitional period at the juncture (sandhi) of two successive yugas. This transition period is also counted in the total duration of a yuga. This period is significant for prior conditioning and preparations for adaptation to the drastic changes. Its importance would be clear if we recall our routine experiences of change of seasons. As we know, the period between change of two successive seasons of weather is quite important to get accustomed to the change in atmospheric conditions; change in the weather affects the metabolism, sleep pattern, body temperature, etc, and hence overall health.
Spans of the Four Principal Yugas:
At a glance there appears discrepancy between scholarly interpretations of scriptural excerpts, and the implications of astronomical calculations (e.g. [1-6]) of the durations of the four principal eras. However, a comprehensive interpretation and analysis in the light of ancient astronomical texts clarifies the matter and leads to a convincing conclusion.
According to some translations of Indian scriptures, the Bhavishya Parv of Harivanha Puran, Manusmiriti (67-68), Ling Puran (9-12), and Vanparva (22-26) of the Epic “Mahabharata” cite the total duration of the four principal yugas as 12000 years. Some scriptures [e.g. Devi Bh3gvat Puran (3|1|39)] describe that the successive spans of four principal yugas (from Krata-yuga to Kali- yuga) and their transition periods are in the ratio 4:3:2:1. That is, the span of Kali-yuga is shortest, that of Dwapar is double of it, and so on.
However, it should be noted that the Puranas are written in a narrative style, couplets in other scriptures too should be viewed in continuity with context-dependent meanings of text. Thus the meaning of a single or set of few couplets (hymns) should be interpreted as interlinked with other hymns/texts of the same or related (w.r.t. the context) ancient treatises. Such details indicate that the durations stated in the above- referred couplets correspond only to one phase of a complete cycle of the four eras. As in one full day there are two phases - day and night, similarly in a complete cycle of the chatur-yugas, there are two phases - descending and ascending - of equal duration of 12000 years. Thus the total span (including sandhi periods) of the four principal yugas is 2X12000 = 24000 years.
The ratio 4:3:2:1 of the durations of the successive (in descending phase) yugas also appears to correspond mainly to the smaller periods within a yuga in which predominant quality of the yuga is fully developed and manifested. Chronologically, the duration of each of the four principal yugas is equal. Thus the span of each is about 3000 years (2700 years + sandhi period of 300 years) in each phase. The total duration of Kaliyuga is thus about 5400 years with sandhi period of about 300 years of dawn and 300 years of twilight. This measure is supported by authentic documents like - the Indian calendar used in the times Chandra Gupta Maurya, the first ever documented astronomical treatise Surya Siddhant  , and the thesis “Aryabhattia” written in 499 CE by globally renowned Indian mathematician and astronomer Aryabhatta  , and calculation of several other famous astronomers of early centuries including Paulisa, Srishena, and Vishnucandra. Great leader of Indian freedom movement Lokmanya Tilak, and revered yogi Paramhans Yoganand  , and scholar Yutkeshwar  have also cited similar views.
As cited by a thorough researcher  , the original Yuga Cycle doctrine appears to have been very simple: A Yuga Cycle duration of 12,000 years, with each Yuga lasting for 3,000 years. This cycle is encoded in the “SaptarÌi Calendar” which has been used in India for thousands of years - authentic documents for this are available in the recorded chronological history of Indian Kings since 6676 BC  . It was used extensively during the Maurya period in the 4 th century BC, and is still in use in some parts of India. The term “SaptarÌi” refers to the “Seven Rishis” or the “Seven Vedic Sages” by whose names the seven stars of the Great Bear constellation (Ursa Major) are identified. They are regarded as the angelic sages who appear at the beginning of every cycle of the four yugas to cultivate human cultural values and disseminate the laws of civilization. The Saptarsi Calendar used in India had a cycle of 2,700 years; it is said that the Great Bear constellation stays for 100 years in each of the 27 Naksatra (lunar asterisms; 27 sectors along the ecliptic) which add up to a cycle of 2,700 years. 2,700 year cycle was also referred to as a “Saptarsi Era” or a “Saptarsi Yuga”.
This estimate also has a correspondence with the measurements of cosmic units in other ancient systems. For instance, the Zoroastrians believe that the world lasts for 12,000 years, which is divided into four equal ages of 3,000 years each. A Mexican source known as the Codex Rios (also referred to as Codex 3738 and Codex Vaticanus A) states that each age lasts for 4008, 4010, 4801 and 5042 years respectively, which amounts to a total of 17,861 years. We can see that in this case also the duration of each age is nearly the same.
Knowledge of the span of Kaliyuga - the age of vices and sufferings, in which we are living at present, does not matter to us unless it also conveys when is it going to get over? Several scholarly analyses of the ancient texts and predictions of the seer-sages of the modern times  indicate that the end of Kaliyuga and dawn of a golden age would occur in this very century - the 21 st Century AD. We shall discuss the details in successive articles.
1. Sharma Shriram Acharya (1995): Yug- Parivartana Kaise aur Kaba?” Pandit Shriram Sharma Acharya Samagra Vangmaya Vol. 27, Akhand Jyoti Sansthan, Mathura.
2. Guénon, R., Guinon R. (2003): Traditional Forms and Cosmic Cycles [Collected works of Guénon, Rene, edited by Fohr, S.D.] (ISBN: 978-0900588167). Publ. Sophia Perennis.
3. Burgess E. (1860): Translation of the S¿rya Siddh3nta – A Text Book of Hindu Astronomy. New Haven: For the American Oriental Soc. [Original article publ. In J. Amer. Oriental Soc. Vol VI; the book also contains detailed commentary by the author.]
4a. Bhau Daji (1865): Brief Notes on the Age and Authenticity of the Works of Aryabhata, Varahamihira, Brahmagupta, Bhattotpala, and Bhaskaracharya. J. Royal Asiatic Soc. Great Britain and Ireland, p. 392.
4b. Thurston Hugh (1996): Early Astronomy, Springer. (ISBN 0-387-948228).
5. Swami Sri Yukteswar Giri (1949): The Holy Science. Yogoda Sat-Sanga Soc. of India.
6. Yogananda, Paramhansa: Autobiography of a Yogi. Sterling Publ. Pvt. Ltd. (ISBN-13: 978- 8120725249).
7. Bibhu Dev Misra (2003): Unravelling the Yuga Cycle Timeline. Part-I of a Web Article. (https://grahamhancock.com/dmisrab6/)
8. Mitchiner John E. (2000): Tradition of the Seven Rsis. Motilal Banarsidass Publ. Pvt. Ltd. (ISBN-13: 978-8120817623).
1. Triguñas: The intrinsic tendencies of sat, raj and tama indwelling in the Nature are called triguña. Sat implies the embodiment of truth, the source of knowledge & cognitive faculties, and pure intelligence. Raj is the source of all activities; the basis of animate manifestation of Nature and expressions of consciousness (e.g. in generation of thoughts). Tamoguna implies ignorance, inertia; it is predominant in inanimate Nature.