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Eventu1ally, 12 more or less equally spaced nakshatra were chosen among the 27 and selected as month names:
Month NameNakshatra NameNakshatra Number
CaitraCitra14
VaisakhaVisakha16
JyaisthaJyestha18
AsadhaAsadha2021
SravanaSravana22
BhadraBhadrapada2526
AsvinaAsvini 1
KartikaKrttika 3
MargasirsaMrgasiras 5
PausaPusya 8
MaghaMagha10
PhalgunaPhalguni1112

9.6.1.2.2.2
The Tithi
A second concept, very important in Hindu astronomy, is the tithi. There are 30 tithi in one lunar month. A tithi is completed when the Moon advances ahead of the Sun by 12 degrees or an integral multiple of that value.
The tithis thus occur in two groups, one of 15 for the waxing moon and one of 15 for the waning moon. The first tithi in the waxing half starts with the Moon in conjunction with the Sun and ends when the Moon is 12 degrees ahead of the Sun. The tithis are numbered 1 to 15. The 15th tithi ends with full moon. The tithis of the waning half of the moon then begin, and the 15th ends with the next new moon.
The average duration of one tithi is thus (29.5305/3024 =) 23.62 hours but varies from 20.0 to 26.8 hours in duration. Again, Indian astronomy has latched upon an irregular time period.
The tithi implies precise measurement of angles. The Rig Veda does not mention tithis; they are first referenced in the Yajurveda.
9.6.1.2.3
The Vedanga Jyotisha Calendar
An account of the calendric rules followed in several existing treatises is given by Varahamihira in his Panca Siddhantika, Chapter 12, where the rules are collected under the rubric of Paitamaha Siddhanta. In this discussion, the construction of a five-year calendar is elaborated. Six main points are elaborated in the form of rules.

 
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