The phenomenon of Janma Nakshatra coinciding with English Birthday once every 19 years!

In the South of India and also in countries like Sri Lanka, Malaysia and Singapore, the Janma Nakshatra of Moon is given high prominence. It is a tradition to tell the constellation of Moon star and offer prayers to God for one's well being. Birthdays are mostly celebrated on the rising birth star during the respective solar month in which one is born.

It is generally witnessed that every 19th year in one's life the Nakshatra at birth and the English Birthday date seems to coincide and is known to be pretty auspicious. There is however a mathematical computation as well as a reasoning based on why such a phenomenon usually occurs.

One needs to know a couple of basics before going into the specifics. First up is about the Sun which takes 365.25 days to cover it's full round of the 360 degree zodiac in order to return back to its natal solar position. A calendar year however consists of 365 days and every 4th year happens to be a leap year consisting of 366 days. The reason for the leap year is because the earth's orbit around the sun takes 365.25 days approx and to factor that 0.25 days, an extra day is added once in every 4 years.

Now coming to the moon, it takes approx 27.32 days to cover all the 27 Nakshatras or in essence you could say the 360 degrees of the zodiac. A nakshatra covers a distance of 13 degrees 20 minutes of the 360 degree band zodiac belt making the total nakshatra count as 27 (360 degrees divided by 13 degrees 20 minutes). The average daily motion of the moon is 13 degrees 10 minutes and 36 seconds. Converting this in zodiac space minutes, it traverses 790.60 minutes (13*60+10+36/60) in a day and the total nakshatra span happens to be 800 minutes (13*60+20). Hence a Nakshatra remains in the sky for a little over a days time or you can say 24 hours 17 mins and 7 seconds to be more precise (Please note this is also approximate as its based on mean daily motion). In effect to cover all the 27 nakshatras it takes roughly 27.32 days [648 hours (27 nakshatras*24 hours), 459 minutes (27 nakshatras*17 minutes) and 189 seconds (27 nakshatras*7 seconds)].

Thus within a calendar year of 365 days, moon makes 13 full rounds which will be 27.32*13=355.16 days which we shall approximate to 355 days. Thus in a way we can say that a solar year is 365 days and moon lunar year is 355 days. On the date of our birth or 0th year, the constellation which is rising in the skies happens to be our Janma Nakshatra. We celebrate our 1st birthday after the completion of 365 days but the moon completes 13 rounds in 355 days and we see a gap of 10 days between our English Birthday and our star birthday. In the 2nd year the gap becomes 20 days, 3rd year it becomes 30 days but within that span of 30 days, the Moon completes a set of Nakshatras in 27.32 days making the reminder as 3. In the 4th year you see 40 days difference and reminder is 13 and for 5th year reminder will be 23. In the 6th year when the difference heads to 60 days (365*6 less 355*6), moon will complete 2 rounds of 27.32 days which is roughly 55 days and still you would see a reminder of 5 days. This keeps happening until the 19th year when the difference becomes 190 days (365*19 less 355*19). When you divide this figure by 27.32, the reminder becomes 0 with the point difference being very minimal. (190÷27.32 equals approx 7 with fraction reminder). Hence most likely only once in every 19 years, your original Nakshatra at birth ends up arriving on the date of your birthday.

Additionally, it is often seen that the first star birthday is celebrated in a prominent way with the event being termed 'Ayush Homam'. This day will always end up being either 10 days before the actual English Birthday or 17 days after the English Birthday. As you would have guessed 10+17=27 which is the number of days for one nakshatra cycle to get covered. The reason it occurs on either 10 days before or 17 days after is because it also has to coincide with the solar month in which one has taken birth.

Important Facts and Information with respect to Janma Nakshatra:
- The Janma Nakshatra should be celebrated based upon the Luni-Solar month
A lunisolar calendar is a calendar whose date indicates a mixture of both the phase of the Moon and the time of the solar year, hence a combination of the lunar as well as the solar calendar. As an illustration let's say someone were born in the constellation of Rohini on Jan 21st in the solar month of Thai which usually runs from around mid of January to mid of February. Hence while celebrating birthday based on Janma Nakshatra, both these factors need to be ensured. There could be a case of Rohini Nakshatra to occur on say Jan 13th and at that time the solar month of Thai wouldn't have begun yet. Thus in such a case, celebration should be deferred until around Feb 9th or 10th when Rohini Nakshatra would coincide with the Tamil month of Thai. In such a case, the Janma Nakshatra Birthday could be almost 20 days away from one's actual English Birthday but the rule of Nakshatra coinciding with the solar month should always be adhered to.

- Birth Star occurring twice in a Solar month, only second one should be considered
If the Janma Nakshatra occurs twice within a given solar month, only the second occasion needs to be pigeonholed as Janma Nakshatra Birthday. Let's say someone was born in Revathi Nakshatra during the month of Chaitra which normally runs from Apr 14th to May 14th. Suppose Revathi star occurs twice within this phase for instance both on Apr 15th and May 12th, only the latter date of 12th May needs to be considered for the purpose of celebrating Birthday based on Janma Nakshatra

- Birth Star falling in 2 consecutive Solar Days, only second one to be considered
There can be instances of one's Nakshatra falling across 2 days. In such a case only the second one needs to be considered. The important thing to note is that Sunrise should be prevalent and hence the second date gains prominence. Even in cases where the birth star runs before Sunrise on Day1 and ends after Sunrise on Day2, the second day only should be taken into account. Only in such a scenario where the sunrise lasts for less than a third of the Nadika or a sixth of a Muhurtha which is just about 8 minutes on the second day, on that basis alone the first day can be considered presuming the Nakshatra started before Sunrise on Day1. However in extreme cases of Sunrise not being prevalent on both days, then the Janma Nakshatra day should be put on hold and be taken into consideration only in the next Solar month though it would end up being different from the solar month in which one were born.

Note:
The below is to understand how the calculation works. For simplicity we are taking 365 Days as Solar Year and Moon Year as 355 Days. But in actuality the Solar year is around 365.25 Days and a Moon's rotation within a complete solar year of 360 Degrees is around 13 times and takes 355.16 Days. You would see that in the 19th year the reminder is just 1 only because we have not considered fractions and likewise for 38th year it is 2, again on account of not taking the fractions which if taken ends up being close to nil and makes the English Birthday and Star Birthday tally exactly.

Year 1: Solar Cycle 365 Days Moon Cycle 355 Days Difference = 10 Days

Year 2 : Solar Cycle 365*2 Days Moon Cycle 355*2 Days Difference = 20 Days

Year 3 : Solar Cycle 365*3 Days Moon Cycle 355*3 Days Difference = 30 Days but one extra cycle formed by Moon of 27 Days, hence net difference = 3 Days (30-27)

Year 4 : Solar Cycle 365*4 Days Moon Cycle 355*4 Days Difference = 40 but one extra cycle formed by Moon of 27 Days, hence net difference = 13 Days (40-27)

Year 5 : Solar Cycle 365*5 Days Moon Cycle 355*5 Days Difference = 50 but one extra cycle formed by Moon of 27 Days, hence net difference = 23 Days (50-27)

Year 6 : Solar Cycle 365*6 Days Moon Cycle 355*6 Days Difference = 60 but two extra cycle formed by Moon of 27 Days, hence net difference = 6 Days (60-54)

Year 7 : Solar Cycle 365*7 Days Moon Cycle 355*7 Days Difference = 70 but two extra cycle formed by Moon of 27 Days, hence net difference = 16 Days (70-54)
Year 8 : Solar Cycle 365*8 Days Moon Cycle 355*8 Days Difference = 80 but two extra cycle formed by Moon of 27 Days, hence net difference = 26 Days (80-54)
Year 9 : Solar Cycle 365*9 Days Moon Cycle 355*9 Days Difference = 90 but three extra cycle formed by Moon of 27 Days, hence net difference = 9 Days (90-81)
Year 10 : Solar Cycle 365*10 Days Moon Cycle 355*10 Days Difference = 100 but three extra cycle formed by Moon of 27 Days, hence net difference = 19 Days (100-81)
Year 19 : Solar Cycle 365*19 Days Moon Cycle 355*19 Days Difference = 190 but seven extra cycle formed by Moon of 27 Days, hence net difference = 1 Days (190-189)

Year 30 : Solar Cycle 365*30 Days Moon Cycle 355*30 Days Difference = 300 but seven extra cycle formed by Moon of 27 Days, hence net difference = 3 Days (300-297)

Year 38 : Solar Cycle 365*38 Days Moon Cycle 355*38 Days Difference = 380 but fourteen extra cycle formed by Moon of 27 Days, hence net difference = 2 Days (380-378)

It should also be noted that every once in 3 years an extra month called Adhik maasa (365 solar year less 355 day lunar year = 10 days a year*3 years = 30 days or 1 month) is added as a technical correction in order to bridge the difference and synchronize the lunar and solar calendar. However this has no bearing with respect to the Moon's revolution across the 27 Nakshatras or 360 degree zodiacal orbit path.