Detailed Explanation of 'Adhika masa' (intercalary month), 'tithi' or 'paksha'

The calculation of lunar months is totally dependent on the movement of sun. As we have already told any month which starts when moon is in nirayana Pisces is Chaitra. Because the lunar year can be co related with solar year. It also means that the number of solar years and lunar years are always going to be same in a specific time interval. Thirty lunar days consists of a lunar month. Three hundred sixty and sixty lunar days consist of a lunar year. In one solar year there are 371 lunar days. So in every solar year there are more 11 lunar days. When this sum goes to 30 days one more month is formed. In that case the lunar month is of 13 months and lunar year is of 390 days. This extra month is called as 'adhika masa' or intercalary month.

          Let us take an example. The month in which there is no transition of sun into another rasi happens to be an 'adhika masa'. If a month starts when sun is in Aries and till this month ends there is no transition of sun in another sign. After starting a new lunar month sun enters in to Taurus. So both the months start when sun is in Aries so both of them should be called as 'Vaishakha'. In this case the first is called as 'Adhika Vaishakha' and the second is called as 'Nija Vaishakha'. In adhika vaishakha there will not be a transition of sun into other sign.

          Sometimes no lunar month starts when sun is in particular sign. In that case as per our rule one month is missed it is called as 'Kshya Masa'.

          Let us take an example. Suppose a margashirsha month is started when the Sun is in Scorpio. Later sun enters into Sagittarius and in the same lunar month the Sun also enters into Capricorn. Later a new lunar months starts. This new month is called as magha because it starts when sun is in Capricorn. In this case as no month starts when sun is in Sagittarius, 'pausha' month is lost. Even though Pausha month is missing margashirsha is considered as a kshaya masa and not Pausha. This is because sun enters two signs in the same lunar month. Kartika, Margashirsha and Pausha are the three months which could happen to be kshaya masas. This is because the relative rotation speed of sun in these months is more. In any year whenever there is a kshaya masa there are also two adhika masas in the same year, one before kshaya masa and one after kshaya masa. The kshaya masa are generally observed after 141 years. Sometimes it happens to be after 19 years.

Just like months the tithis are also happen to be kshaya and vriddhi. In one day ( From one sunrise to another sun rise) if the starting of three tithis happen then the middle tithi is called as 'kshaya tithi'. If one tithi happens to be on 3 days it is called as 'vriddhi tithi'.

When there are two kshaya tithis in a paksha it is called as kshaya paksha. This is not considered as a good omen.