In Indian astronomy, yoga (also called nityayoga) is a period of time, of varying lengths, during which the sum of the nirayana longitudes of the Sun and the Moon increases by an amount of 13 degrees 20 minutes (or, equivalently, 800 minutes).[1] While considering the sum, when the sum is 360 degrees or more, then the angle 360 degrees is subtracted from the sum to make the sum an angle between 0 degree and 360 minutes. Consider a moment T1 when the sum of the longitudes of the Sun and the Moon is 0 degree and let T2 be the next immediate moment when the sum of the longitudes of the Sun and the Moon is 13 degree 20 minutes. The duration of time between the moments T1 and T2 is the first yoga. Similarly, let the next immediate moment when the sum of the longitudes of the Sun and Moon is 26 degrees 40 minutes. The duration of time between the moments T2 and T3 is the second yoga. The third, fourth and higher yoga-s are defined in a similar way. Since 27 X 13 degrees 20 minutes = 360 degrees, at the end-moment of the 27th yoga, the sum of the nirayana longitudes of the Sun and Moon would be 0 degree. The numbering of the yoga-s then starts afresh from that point. It appears that the astronomical yoga-s are in no way related to any astronomical phenomena.[2] S. B. Dikshit in his Bhāratīya Jyotiṣ Śāstra observes: "It is not known what planetary position in the sky is indicated by yoga, and it is useful only in astrology."[3]
The astronomical yoga as defined above is not to be confused with the concept of yoga widely used in Indian astrological traditions. In Indian astrology, the term yoga has been used to indicate luni-solar distances and planetary situations, associations, and combinations. When one planet or house is related to another by placement, aspect or conjunction in a particular way then it is said that the planets and houses are in a particular yoga.
In the traditional Indian calendars or almanacs, that is in Pañcāṅg-s, YogaorNityayoga is one of the five elements or organs or limbs that constitute the Pañcāṅg-s, the "five organs" in the literary meaning of the term Pañcāṅg. The other four elements are Nakṣatra, Tithi, Vāra and Karaṇa.
In Indian astronomical literature, each of the 27 yoga-s has been given unique name as below. The literal meanings in English of the names are given in parentheses.
Nityayogaoryoga has a prominent place in traditional Indian almanac known as Pañcāṅg. It is one of the five elements that constitutes the pañcāṅg-s, that is, the five elements that define a Pañcāṅg. However, the yoga-s entered the Pañcāṅg calculations only several centuries after the other four elements became parts of the Indian alamanac. Pancha-siddhantika, a text on astronomy composed around 505 CE by Varāhamihira gives the methods of calculating nakṣatra-s and tithi-s but does not give any method for calculating yoga-s. Similarly, Bṛhat Saṃhitā, a work on astrology also by Varāhamihira has long discussions on the effects of nakṣatra-s but is silent on the effects of yoga-s. These facts indicate that the concept of yoga-s did not exist at the time of Varāhamihira. The currently available Brāhmasphuṭasiddhānta composed by Brahmagupta in c.628 CE has just one verse containing a reference to yoga, but all internal evidences point to the possibility of the verse being a much later interpolation. Khaṇḍakhādyaka, another treatise composed by Brahmagupta in c.665 CE has a couplet of verses referring to yoga. These verses have also been determined as later interpolations. All these point to the fact that the concept of yoga in astronomy arose post Brahmagupta. Lalla (c. 720–790 CE) in his Śiṣyadhīvṛddhidatantra mentions the yoga-s in detail. Surya Siddhanta, the founding text of the Saura-pakṣa in Indian astronomy, of undetermined authorship believed to have been composed in the 4th-5th century CE but again believed to have undergone a substantial revision in around 800 CE presents the list of all the 27 yoga-s as they are used in modern Pañcāṅg-s and also methods of calculating the yoga-s. All these evidences suggest that the concept of yoga arose sometime around 700 CE and became an integral part of the Pañcāṅg-s only after around 700 CE.[3]
The word vyātīpāta occurs in two verses in Brāhmasphuṭasiddhānta, but from the context of the occurrence of the word, it is clear that it is not referring to the vyātīpāta that occurs as the 17th yoga in the list of 27 yoga-s. It is referring to one of two mahāpāta-s which occur when the Sun and the Moon are in parallel declination and this happens when the sum of the longitudes of the Sun and Moon is 180°. There are two moments in every lunar month when this happens. One of them is called the vyātīpāta and other vaidhṛti. In order to find these mahāpāta-s one has find the sum of the longitudes of the Sun and the Moon. This must have led to the idea of finding yoga-s by finding the sum of longitudes just as tithi is determined by the difference of longitudes.[3]
There is another theory regarding the origin of the yoga concept in Indian astronomy. According this view, the astronomical yoga came into being in attempts to predict the phenomena of eclipses.[4]
Robert Sewell's The Indian Calendar contains a section which explains in meticulous detail how the Yoga at sunrise on a day specified by a date in the Common Era can be determined. The procedure also explains how to find how much time has elapsed at the moment of sunrise since the beginning of the Yoga.[5]
The lengths of the various toga-s varies from yogatoyoga. The following table gives the mean length, the greatest length and the least length of the yoga-s.[6] It follows that the total length of a yoga-cycle consisting of 27 yoga-s is 26 days 10 hours 12 minutes 47 seconds.
| Hours | Minutes | Seconds | |
|---|---|---|---|
| Mean | 22 | 35 | 44.7 |
| Greatest | 24 | 36 | 24 |
| Least | 20 | 52 | 58 |
There is a different system of yoga-s in use in India. This system consists of 28 yoga-s, in contrast to 27 yoga-s in the system already explained, and the names and the rules for the determination of these yoga-s are different from the ones given earlier. In this system, the succession of the yoga-s depends the day of the week. Hence it has absolutely no connection whatsoever with any astronomical phenomena as the week has no definable relation to the motion of the moon or the sun. This system of yoga-s do not find any mention in Sūrya-sddhānta. In some Hindu calendars yoga-s of this system are also given for each day of the month. But these yogas are only of astrological interest.
The names as well as the rules for the determination of the 28 yoga-s as given in Śrīpati's Jyotiṡa Ratnamāla are given below.[7][8]
The rule assumes a cycle of 28 nakṣatra-s which includes the 28th nakṣatra, namely, Abhijit. The 28 yoga-s are assigned as follows:[7]
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