In Hindu traditional calender Their are 12 months (मास)
Chaitra (चैत्र), Vaisakha (वैशाख), Jyaistha (जेष्ठ), Asadha (आषाढ), Sravana (श्रावण), Bhadrapada (भाद्रपद), Asvina (अश्विन), Kartika (कार्तिक), Agrahayana (अग्रहायन), Pausa (पौष), Magha (माघ), and Phalguna (फाल्गुन).
Who have first given the names to Months?
What is the meanig of each Month's name?
Are their any stories related to these Names?
These 12 months are derived from the name of nakshtras. As I discuss in my answer here, the names of nakshtras are itself stated in Atharva Veda. In a particular month there is high probability of occurring full moon in particular nakshtra. Of course it may not occur exactly but it occurs near.
Maharshi Upamanyu in Vyaviya Samhita chapter 7 of Shiva Mahapurana also mentions relation between name of month and its relation with Nakshatras:
पौषे पुष्यनक्षत्रे कुर्यान्नीराजनं विभोः ॥ ७.२,२८.६ माघे मघाख्ये नक्षत्रे प्रदद्याद्घृतकंबलम् ॥ ७.२,२८.६ फाल्गुने चोत्तरान्ते वै प्रारभेत महोत्सवम् ॥ ७.२,२८.७ चैत्रे चित्रापौर्णमास्यां दोलां कुर्याद्यथाविधि ॥ ७.२,२८.७ वैशाख्यां तु विशाखायां कुर्यात्पुष्पमहालयम् ॥ ७.२,२८.८ ज्येष्ठे मूलाख्यनक्षत्रे शीतकुम्भं प्रदापयेत् ॥ ७.२,२८.८ आषाढे चोत्तराषाढे पवित्रारोपणं तथा ॥ ७.२,२८.९ (Shiva Purana chapter 7, Vyayaviya Samhita)
In the Pushya nakshatra in the month of Poush. Magha nakshatra in the month of Magha. Uttara (falguni) in month of Falguna. In the month of Chaitra on the full-moon day Chitra nakshatra. Vishakha nakshatra in the month of Vaisakha. Mula nakshatra in the month of Jestha (which is near Jyestha nakshatra). Uttarashadha nakshatra in the month of Ashadha.
The above reference is clear to establish that month names are directly related with Nakshatra names. Especially by the word "चैत्रे चित्रापौर्णमास्यां", Maharshi Upamanyu directly tells that in the month of Chaitra when Purnima is in Chitra.
On the basis of this 15 nakshtras were choosen and months were named on the basis of that name. For eg. There is high probability of occurring full moon in or near Visakha nakshtra in Vaisakha and so that full moon is termed as Vaisakh Poornima and the month is named Vaisakh. The above Shiva Purana chapter in some places differs with the name of Nakshatra in some places, like it says Mula in Jestha, but if we look closely Mula and Jyestha nakshatra are close, so if full moon occurs on Mula, it is closer to Jyestha, and so on.
So, the months are derived from these nakshtras:
Chitra Nakshtra = Chaitra month
Visakha Nakshtra = Vaisakha month
Jyestha Nakshtra = Jyestha month
Purva Aashaadha, Uttara Aashaadha Nakshtras = Aasadha month
Sravana Nakshtra = Sraavana month
Uttara Bhaadrapada, Poorva Bhaadrapada Nakshtras = Bhadra month
Asvini Nakshtra = Asvina month
Krittika nakshtra = Kaartika month
Mrigashira nakshtra = Maarghasira month
Pushya nakshtra = Pausa month
Maghaa nakshtra = Magha month
Uttara Phalguni, Purva Phalguni nakshtras = Phalguna month
The full moon either occurs in those nakshatras or near those nakshatras of a particular month and hence these months derive their name from those nakshatras. Also, refer to this Kamakoti article for further information on how the names of these Months got changed across various places and times.
The question is actually clearly answered by @Tezz. But there is another question from the user @optimus which not only concerns about how the names are derived but also depends on how the calander works.
The Shri-Surya-Siddhanta has a perticular rule to name the month. It places Nakshatras in group of three or two under a perticular month (See table). We name the month Whenever moon becomes full in a nakshatra of that perticular group.
This rule is presented in the 14th chapter of the Surya-Siddhanta.
नाक्षत्रमानम्- भचक्रभ्रमणं नित्यं नाक्षत्रं दिनमुच्यते । नक्षत्रनाम्ना मासास्तु ज्ञेया: पर्वान्तयोगत:॥ १५ ॥ कार्तिक्यादिषु संयोगे कृत्तिकादि द्वयं द्वयम् । अन्त्योपान्त्यौ पञ्चमश्च त्रिधा मासत्रयं स्मृतम् ॥ १६ ॥
The name of the month is to be known by the Nakshatra which is in conjunction with the moon at the time of Purnima-aanta (the end of Purnima). For the month's like Kartika etc. Group together nakshatras in order of twos starting with Krittika, however Group Nakshatras into three's for the Last, second last and fifth month.
I.e.
| Sr. No. | Nakshatras in which Full moon ends | Month |
|---|---|---|
| 1 | Krittika, Rohini | Kartika |
| 2 | Mrigashira, Ardra | Margasirsa |
| 3 | Punarvasu, Pushya | Pausa |
| 4 | Ashlesha, Magha | Maghaa |
| 5 | Purva Phalguni, Uttara Phalguni, Hasta | Phalgun |
| 6 | Chitra, Swati | Caitra |
| 7 | Vishaka, Anuradha | Vaisakha |
| 8 | Jyeshta, Mula | Jyestha |
| 9 | Purva Ashadha, Uttara Ashadha | Ashadha |
| 10 | Shravana, Dhanishta | Sravana |
| 11 | Shatabhishak, Purva Bhadrapada, Uttara Bhadrapada | Bhadrapada |
| 12 | Revati, Ashwini, Bharni | Ashvina |
But wait! try checking the calendar for a few months and you will see a violation of the above rule. For example, 8 Nov 2022 The full moon ends with Bharni however the month is named Kartika.
Why? In the translation of Shri-Surya-Siddhanta by Ebenezer Burgess, he writes
...owing to the incommensurability of the times of revolution of the sun and moon, as also to the revolution of the moon's line of apsides, full moon is liable to occur in succession in all the asterisms, and at all points of the zodiac; so that although, at the time when, the system of names for the months originated and established itself, they were doubtless strictly applicable, they would not long continue to be so....
However, today the name of the months entirely dependent upon the solar months. And they no longer regulate according to nakshatra of full moon. In the book, Indian Calender by Robert sewell and Sankara Balkrishna Dikshit, they write
- But the stars or groups of stars which give their names to the months are not at equal distances from one another; and as this circumstance,—together with the phenomenon of the moon's apparent varying daily motion, and the fact that her synodic differs from her sidereal revolution—prevents the moon from becoming full year after year in the same nakshatra...
- It is clear that this practice, though it was natural in its origin and though it was ingeniously modified in later years, must often have occasioned considerable confusion ; and so we find that the months gradually ceased to have their names regulated according to the conjunction of full moons and nakshatras, and were habitually named after the solar months in which they occurred.
The general rule has been that that amanta lunar month in which the Mesha sankranti occurs, is called Chaitra, and the rest in succession.
This is why we don't see the moon to go full in exactly that perticular Nakshatra but near to it. This picture will continue to deviate more and more.
To conclude, the names got their derivation from the Nakshatras. But due to small changes in the motion up in the sky, the names don't align well with the way they are derived.
