(This article originally appeared in CURRENT SCIENCE, VOL. 97, NO. 3, 10 AUGUST 2009 and is reproduced here with the permission of the Author and Publisher)

Year to year variation of Indian monsoon rainfall is described qualitatively in some ancient Sanskrit texts. Interestingly, these are cyclic with periods of 3, 5, 7, 18 and 60 years. Time series analysis of actual sea-sonal rainfall data shows that at very near the above periods the spectrum has significant peaks. The Venus visibility portent stated in the Arthaśāstra appears to be a proxy for the near three-year fluctuation cycle in the rainfall.

The season that brings rainfall to large parts of the country, now called the mon-soon, has left its imprint in all types of literature starting from the Ṛgveda. Vedic literature describes figuratively the evaporation–rainfall cycle (Ṛgveda: 1.164.51), and knows the sun as the cause of rain-fall. It even has a hymn dedicated to the frogs who sound in chorus at the start of the rains, much like the students learning the Veda. While a bounteous monsoon kept the people happy, its vagaries were dreaded. Stories of droughts and failure of rainfall were quite common in the legends, particularly when people shifted their location. One such 12-year long drought was the reason for Bhadrabāhu and his followers to emigrate from Bihar to distant Karnataka in the 3rd century BCE¹. Failure of rainfall for 12-years might be an exaggeration, but people should have been as eager as now are to have a reliable forecast on what they could expect from the clouds. The most significant feature of the monsoon is its annual recurrence and the year-to-year variation in the amount of rainfall, called inter annual variability (IAV).

At present, IAV is characterized by fixing a normal (time average) and expressing the yearly rainfall in terms of percentage variation about this long-term normal. It is now well known that the variation comprises of oscillatory patterns around 2–3, 5–7, 11–12, 18–20 and 60 years². Narasimha and Kailas³ through wavelet analysis found almost the same periods for the first four modes. These are not perfect repeating cycles, but narrow-band oscillations with slowly varying amplitude and phase around the indicated central periods. The above quantitative understanding is based on the analysis of long-time series data of Indian rainfall for the last 100 years. However, all persons dependent on the monsoon for their profession recognize IAV of the seasonal rainfall in their own way. It is common among farmers and villagers to talk of a season being similar to the one a few years back. They intuitively recognize IAV in terms of similarity or lack of it, compared with some past year fresh in their memory. This would be clearly due to the influence that rainfall has on their personal lives. A question that arises in this connection is; considering the deep influence that monsoon has had on Indian culture and life, how was IAV described and characterized? Was there any effort in ancient India to quantify IAV? The present note attempts to address these questions by tracing IAV-related descriptions in ancient Sanskrit literature.

Venus appearance cycle

Kautilya’s Arthaśāstra is a book on Statecraft, with considerable information on administrative procedures of the governments of his days (c. 4th century BCE)⁴. He describes under the chapter on agriculture (II.24) how to measure rainfall and also gives the amount of rainfall (presumably some kind of average) in the important provinces of his kingdom. Since rainfall figures were collected by empowered officials and used by the decision makers, Kautilya’s methods are expected to be rational. Quite intriguingly, he mentions that rainfall for the season depends on the visibility of Venus (तस्योपलब्धिः ... शुक्रोदयास्तमयचारेभ्यः ... शुक्राद्वृष्टिरिति ॥ tasyopalabdhiḥ ……śukrodayāstamayacārebhyaḥ……śukrādvrstiriti||).         At first reading this appears to be an astrological prescription, based more on belief rather than empirical observations. However, on closer scrutiny this statement is seen to reflect the near three-year oscillation in monsoon rainfall. Kautilya expects good rainfall if Venus were to be sighted in the eastern sky during the monsoon season. The season being of four months according to the text, this precursor for making a forecast should refer to the first month of the season. Now, Venus as a morning object is visible for about eight months and becomes invisible for about 50 days, before rising in the evening in the western sky.

The synodic period of Venus is nearly 584 days. Hence, once seen in the early part of the monsoon season, Venus will not be seen in the subsequent season, which is only one year away. Also, when seen next after its cycle of 584 days, the season will not be rainy. But, interestingly, after one more round, that is, after nearly three years, Venus would be visible in the beginning of the monsoon season. Thus, the corre-lation prescribed by Kautilya is based on an uncanny observation of occurrence of two contemporaneous natural events with nearly the same period. Since any such cyclic trend is not perfectly periodic, the correlation will drift over time with no unique practical implication. This could be the reason for the development of longer cycles for predictive purposes.

Five-year cycle

In the Vedic period the country followed luni-solar calendar. While the original form and evolution of this is not known in all its details, what has come down to us from the Vedānga Jyotiṣa⁵ is a calendar comprising five years as a unit, called the Yuga. It appears that this calendar was used till about 4–5 century CE, when the yearly calendar of the siddhāntic astronomy became popular. Varāha-mihira, in his Brhat-samhitā⁶, presents the older five-year division for rainfall prognosis. This appears in Chapter 8, verses 24 and 25.

संवत्सरोऽग्निः परिवत्सरोऽर्क इदादिकः शीतमयूखमाली ।

प्रजापतिश्चाप्यनुवत्सरः स्यादिद्वत्सरः शैलसुतापतिश्च ॥

Samvatsaro’gniḥ parivatsaro’rkaidādikaḥ śītamayūkhamālī|

Prajāpatiścapyanuvatsaraḥ syādidvatsaraḥ śailasutāpatiśca|| 

 

वृष्टिः समाद्ये प्रमुखे द्वितीये प्रभूततोया कथिता तृतीये ।

पश्चाज्जलं मुंचति यगतुर्थं स्वल्पोदकं पंचममब्दमुक्तम् ॥ 

Vṛṣṭiḥ samādye pramukhe dvitīye prbhūtatoyā kathita tṛtīye| 

Paścājjalam muñcati yaccaturtham svalpodakam pañcamamabdamuktam||

Whether or not the forecasts came true, the variability here was taken to be cyclic with a five-year period. The concept of a normal year which gets rainfall evenly throughout the season is basic to this model. The five years of the Vedic calendar with their names, regent deities and rainfall character are said to be: Samvatsara–Fire; Parivatsara–Sun; Idāvatsara–Moon; Anuvatsara–Prajāpati and Idvatsara–Rudra. Rainfall would be evenly distributed in the first year. The second year gets good rainfall in the be-ginning of the season only. Rainfall will be excessive in the third year. In the fourth year of the cycle rainfall will be delayed. Rainfall will be deficient in the fifth year. This type of variability attribution should have been in vogue since an-cient times, the roots of which are lost. This also gives raise to a question about how to decide the position of a year in the cycle. The ancient Vedānga Jyotiṣa definition is quite unambiguous that the Samvatsara started with the winter sol-stice at the star Dhaniṣṭhā (~Beta-Delphini). This would have been the case around 1400 BCE⁵. However, Varāha-mihira prescribes another way of fixing the years of this cycle, which is not important for the present discussion.

Seven-year cycle

Among the available Sanskrit books on agriculture, the Kṛṣiparāśara occupies an important place as it records ancient practices quite accurately. Published and studied widely in the recent years⁷, the date of the work is unknown. The contents, however, hint at a period later than the Vedic five-year cycle discussed above. It appears to belong to the early centuries of the Common Era, since it uses the Śaka Era (78 CE) in its algorithm for prognosis of rainfall. It postulates a seven-year cycle, based on the presumed influence of the seven planets on the rainfall. Historically this has a parallel with the development of the names of the seven days of the week, attributed the sun, moon and the five planets which most probably happened at the beginning of the Common Era. Even though this basis appears hypothetical, the way the cycle was used leads to an interesting pattern. Perhaps the agriculturists observed more variation than they were able to verbally describe in terms of the previous three-and five-year cycles. The years in the seven-year cycle had no names and hence a peculiar way of prognosis was proposed. Let the year under considera-tion counted from the beginning of the Śaka Era be N. The forecast of the sea-sonal rainfall for the year was based on the remainder of the fraction [(3N + 2)/7]. The remainder would be one of the seven integers (1, 2, …7) associated with the seven planets. Depending on this result the rainfall was forecast, as shown in Table 1.

 

In the long run, the above thumb rule results in a seven-year cycle of remainders not in the above order, but as [… 2,5, 1, 4, 7, 3, 6, 2, 5, 1, 4, 7, 3, …]. It is interesting to note that the results do not follow the order of the planets, which is in the order of the names of the week days. There are two years described as Uttamā or best in the above prescription. These are associated with Mercury and Venus, but separated by either two or three years, which is reminiscent of the near three-year Venus visibility cycle. The formula mentioned above for fixing the year within the cycle is not available in the Bṛhat-samhitā⁶. But in chapter 19, similar rainfall descriptions are given for years ruled by the respective planets.

Eighteen-year cycle

Relation of this cycle with rainfall is through prescribing conditions for sowing of seeds, depending on the position of the imaginary dark planet Rāhu among the 27 nakṣatras or stars along the ecliptic. This principle, which is included in many of the currently available Hindu almanacs (pañcānga), goes under the name of Rāhu- or Phani-chakra, literally the Rāhu or the serpent-cycle. The relatively ancient nature of this practice will be clear when it is noted that modern Indian astrologers talk of Rāhu and Ketu as the ascending and descending lunar nodes. It may be noted here that identification of Rāhu with the ascending lunar node is a late development after mathematical astronomy got established firmly in India c. 5th century CE. Prior to this, as evidenced in the epics Mahābhārata and Rāmāyana, Rāhu was considered to be the intervening dark planet responsible for causing both solar and lunar eclipses. Clearly, the use of the Phanichakra should have been more ancient than the development of siddhāntic astro-nomy and knowledge of moon’s path cutting the ecliptic at two points. Tradi-tional almanacs contain a section on agri-culture and rainfall, where one can find a sinuous, snake-like figure, sometimes three-layered, with 27 small circles arranged along the centre line referring to the ecliptic. The prescription for sow-ing seeds depends on the relative posi-tion of the moon with respect to Rāhu placed at the tail of the figure, the details of which are not of interest here. The number eighteen is of considerable importance in ancient Indian literature. This arose out of the knowledge of the 18-year eclipse cycle which can be easily traced to the mystical lunar number 3339 appearing in the Ṛgveda related to similar lunar eclipses⁸.

Sixty-year cycle

The sixty-year cycle arises as the product of the Vedic five-year cycle and the twelve-year sidereal cycle of Jupiter. The basis of this appears to be purely arith-metical. Rainfall description for a year within this cycle is given by Varāha-mihira in detail. It is doubtful whether this was put to any practical application. For example, Kṛṣiparāśara, the classical text on agriculture, does not recommend this cycle.

Complex variability patterns

It has been noted that the ancient variability descriptions were essentially qualitative, to be understood by personal experience. The word Cittalā used sev-eral times in the Kṛṣiparāśara, has the meaning of being moderate, similar to the modern average. The state of rainfall in other years was calibrated with respect to such a year. The Kṛṣiparāśara, apart from the seven-year cycle, lists yet another pattern which is quite complex. This has four states denoted as ativṛṣṭi, anāvṛṣṭi, suvṛṣṭi and cittalā. These four types of rainfall can be translated as excessive, deficient, sufficient and normal respectively. The rainfall for the monsoon season is decided by the position of moon (nakṣatra) at the spring equinox. The nakṣatra series started with Kṛttikā, including Abhijit as in the Vedic period (2nd–3rd millennium BCE). Normal rain-fall was expected only when the equinox moon coincided with either the Aśvini or the Svāti nakṣatra. It was deficient when moon at equinox was with Kṛttikā, Hasta, Anurādha or U. Bhādra. It was sufficient when moon was with Bharani, Citrā, Visākhā or Revati. In all the other 18 cases the rainfall would be excessive. Whatever be the basis of this prognosis, the interesting feature of this proposition is the inherent 18–19-year cycle of the four types of rainfall. This arises out of the coincidence of the lunar and solar cycles at the above period, with possible error of one nakṣatra position. This is similar to the Rāhu-chakra cycle of 18 years mentioned above. Whether or not the ancients knew the correct reasons for this cycle, the fact remains that a weak 18–19-year peak can be observed in the spectrum of many real geophysical time-series data^9,10.

Yet another variability pattern was based on the position of the moon when the first rains occurred at the start of the season. This is similar in spirit to the above four states of rainfall, but the details are different, since the quantity of seasonal rainfall is forecast in Drona measures. This pattern has been critically investigated with statistical details already and hence will not be detailed here¹¹.

Intra-annual variability

As is well known, rainfall exhibits considerable month-to-month variation within given year. The texts Kṛṣiparāśara and Brhat-samhitā throw some light on how this issue was addressed in ancient India. Starting from the Vedic period, it was well known that the rainy season was dependent on the sun and not on the moon. Since the more ancient time-keeping was based on the moon, the annual rainfall cycle would have necessitated synchro-nization of the solar and the lunar year. The Sanskrit word varṣa means rainfall but denotes year also. Similarly, another popular word for year is abda, literally giver-of-water. Thus, naming the months and seasons with respect to the sun in addition to the already existing lunar months was introduced. This is quite evident in the Yajurvedic texts which name the twelve solar months as Madhu, Mādhava, Śukra, Śuci, Nabha, Nabhasya, Isa, Ūrja, Saha, Sahasya, Tapa and Tapasya. The word Madhu-māsa stands for the spring month in popular parlance even now, except that this is neither same as the lunar month Chaitra, nor the solar month Meṣa (Chittirai in Tamil), of the present-day religious almanacs. Lack of synchronization of the present Hindu solar calendar (souramāna pancānga) with the seasons is due to wrong inter-pretation of ancient texts. These were appropriate for their times, but now need correction due to precession of equinoxes. In addition, the relation between the so-lar zodiac defined with respect to the equinoctial point and the fixed sidereal zodiac (nakṣatra) of the Vedic period was misunderstood by some influential authors on astrology. Thus we see the Vedic solar uttarāyana or the winter sol-stice day being equated with makara sankrānti, which in turn is wrongly shown to be occurring on or about 14 January, whereas the correct date should have been around 22 December, the shortest day in the civil calendar. Hence, to map ancient seasonal information as given in Sanskrit texts to modern times, a correction of about three weeks is neces-sary. This is essential to appreciate the within-year rainfall pattern as described by ancient authors in terms of the solar nakṣatra notation, which was formulated when the sun at vernal equinox (0° longitude) was stationed near the visible star aśvini (Beta-Arities?) in the 2nd century CE. Presently, the equinox in the civil calendar is on 22 March and hence if we talk of aśvini-rain, according to ancient practice, it has to be during the dates 21 March–3 April and not 13 April–26 April, as shown in the traditional alma-nacs¹². Varāha-mihira was well aware of the connection between the monsoon season and the position of the sun. In chapter 28, verse 20 of his Brhat-samhitā, rains are said to be certain when the sun passes through the asterism ārdra. If the vulgate almanacs are followed, this cor-responds to 22 June–5 July, whereas the observational results of the ancients cor-respond presently with 30 May–11 June. This is well known to be the period of onset of the southwest monsoon over large parts of the subcontinent. Such considerations become particularly im-portant when villagers use their folkloric knowledge of proverbs such as ‘if it does not rain in the Hasta (asterism), one’s mother will also not give food’. This should be taken to correspond to 4–16 September, whereas the pancāngas mark this for 27 September–10 October. The risk associated with wrong identification of the position of the sun among the nakṣatra increases when seed-sowing operations are undertaken based on an-cient beliefs. For example, in some areas of Karnataka, farmers believe that sow-ing during punarvasu rains leads to a rich harvest of groundnuts. This period should be taken as 12–25 June and not 6–9 July as shown in the pancāngas.

Discussion

Year-to-year variation in the amount of rainfall during the monsoon season is of considerable current interest to people in several walks of life. That this was so even in ancient times should not be sur-prising, considering the strong depend-ence of agriculture on rainfall. What turns out to be remarkable is that some significant oscillatory features in mon-soon rainfall as we know them today, were already known to the ancients. How they were able to decipher these signa-tures is a matter of conjecture. We have to surmise that even if records were not maintained as in modern days, at least those kingdoms and provinces that fol-lowed the Arthaśāstra of Kautilya had a method for measuring rainfall. The near three-year cycle known presently as the ENSO (El Nino Southern Oscillation) was clearly known to Kautilya in terms of a proxy, namely the visibility of Venus. This does not mean that Kautilya was the originator of this concept. Utpala (9th– 10th century CE) in his commentary on the Brhat-samhita of Varāha-mihira (5th– 6th century CE) quotes extensively from the works of Parāśara, who describes an ancient observational tradition originat-ing around 1400 BCE¹³. An important property ascribed to Venus by Parāsara is arka-varṣa-nigraham or control of sun-induced rains. Thus, it is quite likely that the three-year rule associated with Venus visibility was known in India since an-cient times. Failure of this rule in several years could have encouraged detailed ob-servations, eventually leading to empiri-cally propose 5, 7 and 18-year cycles for the summer monsoon rainfall. The basis for these cycles is not available in the literature, even though from modern analysis these cycles are known to be realistic.

Within the year rainfall also exhibits fortnightly, monthly and seasonal fluc-tuations. These are generally described in the texts in terms of the position of the sun and moon among the nakṣatras. The position of the sun with respect to the fixed stars, as observed from the earth, changes over long periods of time due to precession. Hence matching of time-marking statements and folkloric prov-erbs with modern-day civil calendar dates has to be done after correcting the dates given in the traditional almanacs. An objection to this may be to raise the issue of whether in ancient times the sol-stices were observed on the correct dates. That the ancients observed this within an error band, better than at present, is evi-denced by stone and copper-plate inscrip-tions with dates for the winter solstice available in the volumes of The Indian Antiquary¹⁴. For example, the Śravana-belagola Kannada inscription of Hoysala Vīraballāla records the winter solstice in the Śaka year 1104 to be on pusya bahula tadige śukravāra. This corresponds to Friday, 25th December 1181 CE. Similarly, according to the Terawan copper-plate inscription of Kalyāna, the winter solstice was observed in the Śaka year 1182 on puṣya vādi saptamī śanidina, correspond-ing to Saturday, 25 December 1260 CE. As recently as in the 18 century, the Melkote inscription of Krishnaraja Wodeyar of Mysore, records uttarāyana-makara sankrānti on 29 December 1724 CE.

Summary and conclusion

IAV patterns of monsoon rainfall as des-cribed in ancient Indian texts have been presented here. It is interesting to note that the dominant periods were taken to be 3, 5, 7, 18 and 60 years. Time-series analysis of actual seasonal rainfall data of the past 100 years shows that near the above periods the spectrum has signifi-cant peaks. It is now known that the ENSO oscillations of 2–3 year period explain maximum variance of the year-to-year fluctuations². The Venus visibil-ity portent of Parāśara, stated also in the Arthaśāstra appears to be a proxy for this ENSO signature.

For understanding the within-year variability of rainfall, ancient texts have to be interpreted after making correction for the precession of equinoxes. Blind following of the texts, in letter but not in spirit, has led to marking of the date of winter solstice (makara sankrānti) as 14 January in the pancāngas. Hence the tra-ditional dates of expecting rainfall de-pending on the sun’s nakṣatra as given in the pancāngas, are to be advanced by about three weeks for practical use in agricultural operations. It is known that astronomically wrong information, parti-cularly of the solstices and equinoxes, has crept into several of the printed pancāngas¹⁵. This calls for serious intro-spection and discussion on the part of socio-religious leaders with scientific temperament to reform the traditional almanacs, so that culturally important as-tronomical events such as equinoxes and solstices are observed on the naturally correct dates of the civil calendar.

References

1.  Jain, J., Life in Ancient India as Depicted in Jaina Canons, Munshiram Manoharlal Publication, New Delhi, 1984.

2.  Iyengar, R. N. and Kanth, S. T. G. R., J. Meteorol. Atmos. Phys., 2005, 90, 17–36.

3.  Narasimha, R. and Kailas, S. V., Proc. Indian Natl. Sci. Acad., 2001, 67, 327– 341.

4.  Shamasastry, R. (ed.), Arthaśāstra of Kautilya, Oriental Library reprint, My-sore, 1988, 9th edn.

5.  Vedānga Jyotisa, Text with Translational of T. S. K. Sastry, INSA, New Delhi, 1984.

6.  Bhat, R. K. (ed.), Brhat-samhitā of Varāha-mihira, Motilalal Banarsidas, New Delhi, 1981.

7.  Krsi-parāśara, Agri-history Bull. No. 2, Asian Agri-history Foundation, Secun-derabad, 1999.

8.  Iyengar, R. N., Indian J. Hist. Sci., 2006, 40, 139–152.

9.  Campbell, W. H., Blechman, J. B. and Bryson, R. A., J. Climate Appl. Meteo-rol., 1983, 22, 287–296.

10. Currie, R. G. and O’Brien, D. P., Int. J. Climatol., 1990, 10, 179–189.

11. Iyengar, R. N., Curr. Sci., 2004, 87, 531– 533.

12. Pancāngas: Vontikoppal Pancānga (Kannada), Mysore; Vākya Pancāngam (Tamil), Srirangam; Gantala Panc āngamu (Telugu), Rajamundry; Kālanirnaya (Marathi), Mumbai.

13. Iyengar, R. N., Indian J. Hist. Sci., 2008, 43, 1–27.

14. Keilhorn, F., Indian Antiquary, 1–17 January 1895, XXIV.

15. Krishnaswamy, G. V., Isis, 1930, 14, 403–410.

ACKNOWLEDGEMENTS. Thanks are due to Dr B. P. Radhakrishna for encouraging me to write this article. Award of the Raja Ramanna Fellowship by the Department of Atomic Energy and support of the Indian National Academy of Engineering under the Indian Engineering Heritage (Civil) Group are acknowledged. 

Prof. R N Iyengar is currently the Director of Centre for Ancient History & Culture, Jain University. 

Picture Credits: 

KV 192, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

 

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