Nakshatra

History

Historically, nakshatra referred to stars or star-groups associated with the Moon's path. In later jyotisha, the term also came to denote 27 equal divisions of the ecliptic. In this equal division system, each nakshatra spans 13° 20'.

The Atharva veda contains a list of 28 nakshatras. Other ancient Indian scriptures like the Taittiriya Samhita (4.4.10.1-3) and the Shatapatha Brahmana (10.5.4.5) record 27 nakshatras.

In the Atharvaveda

In the Atharvaveda (Shaunakiya recension, hymn 19.7) a list of 27 stars or asterisms is given, many of them corresponding to the later nakshatras: Sakamoto-Goto identifies it as a hymn used for the daily worship of the nakshatras. The text lists 28 nakshatras, beginning with the Krttikās and including intercalary asterism Abhijit.

1. Ashwini (Sheratan and Mesarthim)
2. Bharani (41 Arietis)
3. Kṛttikā (the Pleiades)
4. Rohinī (Aldebaran)
5. Mrigashīrsha
6. Ārdrā (Betelgeuse)
7. Punarvasu (Castor and Pollux)
8. Pushya
9. Asleshā (ε Hydrae)
10. Maghā (Regulus)
11. Purva Phalguni (Zosma and Chertan)
12. Uttara Phalguni (Denebola)
13. Hasta
14. Chitrā (Spica)
15. Svāti (Arcturus)
16. Vishākhā
17. Anurādhā (Acrab, Dschubba, Fang and Iklil)
18. Jyeshthā (Antares, Alniyat, and Paikauhale)
19. Mūla
20. Purva Ashadha (Kaus Media and Kaus Australis)
21. Uttara Ashadha (Nunki and Ascella)
22. Shravana (Altair, Alshain, and Tarazed)
23. Dhanishta (γ¹ Delphini)
24. Shatabhisha (λ Aquarii)
25. Purva Bhadrapada (Markab and Scheat)
26. Uttara Bhadrapada (Algenib and Alpheratz)
27. Revati (ζ Piscium)

This 27-day cycle has been taken to mean a particular group of stars. This has to do with the periodicity with which the Moon travels past the specific star fields called nakshatras. Hence, the stars are more like numbers on a clock, through which the hands of time (the moon) pass. This concept is described by J. Mercay (2012) in connection with Surya Siddhanta.

Astronomical derivation of nakshatras

The choice between 27 or 28 nakshatras comes down to how ancient calendars calculated the speed of the Moon. During the Bronze Age (around 2000 BCE), astronomers used a calendar of 360 days, where the sun moved 1 degree per day and the moon moved an average of 13 degrees per day. Dividing the 360-degree sky by the Moon's 13 degree daily path gives an orbital period of 27.69 days .The astronomers rounded it to 28 nakshatras. Later, when a 365 day solar calendar was adopted, the Moon's updated daily speed of 13.2 degrees resulted in a shorter orbit of 27.27 days. Astronomers then rounded this down to 27 nakshatras and dropped the star Abhijit.

Vedic Calendar

The Vedic calendar was based on a 360-day solar year composed of twelve 30- day months. Its earliest mention is found in the Rigveda texts. It arose from a ritual need to identify the exact nights of the new moon (amāvāsyā) and full moon (puranmāsī) for sacred sacrifices, which were otherwise difficult to determine by naked eye observation.

The priests tracked the Moon's night-by-night movement against the fixed stars, establishing a system of 28 nakshatras(lunar stations) representing the 28 nights the moon was visible during a synodic month. Early astronomers derived the name of each month directly from the specific nakshatra in which the full moon shone during that month.

Vedic origin of tithi

The tithi is a lunar day in the Hindu calendar. In the 360-day vedic calendar, the Moon travelled an average of 13 degrees per day, while the Sun travelled 1 degree per day. As a result, the Moon separated from the Sun by 12 degrees (13 minus 1) every 24 hours. This time that the moon takes to pull 12 degrees ahead of the Sun, is called a tithi.

List of Nakshatras

In Hindu astronomy, there was an older tradition of 28 nakshatras which were used as celestial markers. When these were mapped into equal divisions of the ecliptic, a division of 27 portions was adopted since that resulted in a clearer definition of each segment, subtending 13° 20′ rather than the 12° 51+3⁄7′ that 28 segments would give. In the process, the nakshatra Abhijit was left out without a portion. However, the Abhijit nakshatra becomes important when deciding on the timing of an auspicious event. The Surya Siddhantha concisely specifies the coordinates of the 27 nakshatras.

It is noted above that with the older tradition of 28 nakshatras each equal segment would subtend 12.85 degrees or 12° 51′. But the 28 nakshatra were chosen at a time when the Vedic month was recognised as having exactly 30 days. In India and China the original 28 lunar mansions were not equal. Weixing Nui provides a list of the extent of the original 28 nakshatras expressed in Muhurtas (with one Muhurta = 48 minutes of arc). Hindu texts note there were 16 nakshatras of 30 Muhurtas, 6 of 45 Muhurtas, 5 of 15 Muhurtas and one of 6 Muhurtas.

The 28 mansions of the 360° lunar zodiac total 831 Muhurtas or 27.7 days. This is sometimes described as an inaccurate estimate of our modern sidereal period of 27.3 days, but using the ancient Indian calendar with Vedic months of 30 days and a daily movement of the Moon of 13 degrees, this early designation of a sidereal month of 831 Muhurtas or 27.7 days is very precise. Later some Indian savants dropped the nakshatra named Abhijit to reduce the number of divisions to 27, but the Chinese retained all of their original 28 lunar mansions. These were grouped into four equal quarters which would have been fundamentally disrupted if it had been decided to reduce the number of divisions to 27.

Irrespective of why early Indian astronomers followed a Vedic calendar of exactly 12 months of 30 days it was this calendar and not a modern calendar of 365 days that they used for the astronomical calculations for the number of days taken for the Moon to complete one sidereal cycle of 360°. This is why initially they named 28 nakshatras on their lunar zodiac.

Each of the 27 nakshatras, starting with Ashvini and ending with Revati, has its own symbol, an associated mythological deity, and a planetary ruler (navagraha).The following list of nakshatras gives the corresponding regions of sky, as per Basham (1954).

Padas (quarters)

Each of the 27 nakshatras covers 13° 20’ of the ecliptic. Each nakshatra is also divided into quarters or padas of 3° 20’, and the table below lists the syllables traditionally used as the first sound of the name given to a child born under each pada. The 27 nakshatras, each with 4 padas, gives 108. 108 is also the number of beads in a japa mala, representing all the elements (ansh) of Vishnu:

Astrological Significance

In Hindu astrology, nakshatras play an important role. Finding a person's nakshatra requires mathematical calculations using the exact time, date and coordinates of their birth. Astrologers use both the nakshatras and rashis(zodiac signs) to analyze a person's character, physical health, marriage prospects, career path etc. Checking these signs for marriage compatibility is a common Hindu tradition.