Now let's get to basics:
- There are two siddhantas that decides shivaratri.
According to dharmasindhu, madhava and hemadris, and other great rishis. The chatrudasi tithi needs to be visible during the nissheta kala (8th/15th part of ratrimana), some say madhya ratri kala (3rd/5th) part of ratrimana.
What is nishita kala?
The 8th muhurtha of the night.
Example of Mahashivaratri calculation date:
Get the ratrimana and nishita kala, check if chaturdashi prevails during that time.
For example of 2008 Mahashivaratri in Mauritius: (You can perform the same calculations for any city. We just picked one. Note: I don't even live in Mauritius).
A) Subract sunrise of 6th - sunset of 5th = 11 hours and 41 minutes.
B) Now divide that by 15 as each muhurtha is of 2 ghatikas and there are 15 muhurthas in day and 15th in night. That gives you 46 minutes and 44 seconds.
C) Now 8th muhurtha is nishitha kala, means multiply that value by 7 giving you, 5 hours 27 minutes and 8 seconds.
D ) Add that to the sunset of 5th which is 18 hours 31 minutes.... giving you the start of nishitha kala. giving you 23 hours, 58 minutes and 7 seconds...
E) Now add 46 minutes and 44 seconds to that...... that's the total time of nishitha kala.
F) Now chaturdashi starts on march 5th at 24:01 midnight.... now this means chaturthi definately prevails during this time.
G) If you perform the same calculations for march 6th and march 7th, you'll observe that chaturdashi doesn't prevail during nishitha kala of march 6th, hence according to dharmasindhu school of thought (widely prevailing in india to 80%) Mahashivaratri is on March 5th in Mauritius. Even if you take the madhyaratri kala which is 3rd/5th of the ratri mana 11 hours and 41 minutes and adding to the sunset, it doesn't fall on March 6th. It clearly falls on March 5th according to 'ekadesha' vyapini rule.
- Nirnaysindhu school of thoughts: They endorse pradosha kala. means chaturdasi has to be prevailing 6 gatika after sunset, and that is 6 ghatika * 24 = 144 minutes. If it prevails during this time, take this day, otherwise take the previous. They don't give importantance to madhyaratri principle. According to this principle, in Mauritius it is not prevailing on March 5th during pradoshakala but it does prevail during the pradosha kala of march 6th, hence you can take the march 6th as a mahashivaratri. Remember, this rule is not widely used my most panchangkarta in india. Even Govt. of India's rastriya panchanga takes the 1st rule as i have explained before.... 90% of panchangkarta takes the first rule i have exaplained before.
Indian festivals are never observed according to indian dates. They are always regional like many rishies including surya siddhantakar himself has written into his books. According to me, If i was in mauritius i'll peform on March 5th, Not march 6th. Hindu panchangam is rather complex subject. Certain festivals are observed based on tithi prevailing during sunrise, and certain festivals are observed when certain tithi is seen during given kala. Another example: Karwa chauth is observed when the chaturthi tithi is seen during the moonrise, so even if tritiya is in morning but during the night at the time of moonrise, if chaturthi prevails, that's the day of kawa chauth, likewise same rule apply for samkastha chaturthi. For ganesh chaturthi the rule is chaturthi needs to visible during madhyahna kala (midday), Same rule is applicable for ramanavami.
Most websites that displays muhurtha or festivals (like wheniswhat, blah...blah....) they display festival dates for india only, and that also using the panchang they are located at. That's why when website says Ugadi (New year) is on April 6th, another one (whose location is in chennai) will proudly declare Ugadi is on April 7th. None of the printed panchang maker will say or ever say that their panchang and festivals are applicable to all the places in the world.
Note: By the way I have seen pictures of the beautiful temple in Mauritius. Here is the link http://www.abhivyakti-hindi.org/parva/alekh/2003/shivratri.htm. Beautiful temple isn't it? I wish I want to visit this beautiful place one day.
Pandit Mahesh Shastri
Seattle, WA, USA