Regarding which 'tithi' (hindu date) is today, it is said that whatever tithi is seen by the rising Sun, is the tithi for the day.
e.g. if 'pournima' had started yesterday morning 'after sunrise' and ends today after sunrise, then today is pournima. (and yesterday was not pournima)
Is this right? If not, what is the rule?

• @Sarvabhouma - thanks for adding the correct tag (panchang). I was looking for one, but could not find a suitable one. Sep 25, 2018 at 9:31

Let's start with the below table for Sukla Paksha (brighter fortnight) and Krishna Paksha (darker fortnight).

``````Sukla Paksha    Krishna Paksha    Name of Tithi
-------------------------------------------------
2nd             17th           Dwitiya
3rd             18th           Tritiya
4th             19th           Chaturthi
5th             20th           Panchami
6th             21st           Shashti
7th             22nd           Saptami
8th             23rd           Ashtami
9th             24th           Navami
10th             25th           Dasami
13th             28th           Trayodasi
14th             29th           Chaturdasi
15th              -             Poornima
-              30th           Amavasya
``````

The tithis in the above table start from Sukla Paksha, i.e, the lunar month ends on Amavasya, this method is known as Amanta followed by states in South India, North Indian states follow a different lunar month calculation in which lunar month starts from Krishna Paksha and ends on Poornima (full moon day) and hence called Purnimanta.

The above numbers are index which we will see how to calculate below, they are used to ascertain the running tithi. A lunar day, also known as tithi, means the difference between the longitude of sun and moon changes by exactly 12 degrees, when it happens, a new lunar day or a new tithi starts. A lunar month consists of 30 tithis.

## How to figure out Sukla Paksha?

When Moon is ahead of Sun by an amount between 0 degrees and 180 degrees, it is known as Sukla Paksha or brighter fortnight. This is the time when moon is waxing. It is calculated by finding the difference between the longitude of Moon and Sun.

## How to figure out Krishna Paksha?

When Moon is ahead of Sun by an amount between 180 degrees and 360 degrees, it is known as Krishna Paksha or darker fortnight. This is the time when moon is waning. It is calculated by finding the difference between the longitude of Moon and Sun.

## Moon & Sun Longitude Difference

At the end of a lunar month, the difference between the longitude and latitude will be 360 degrees (30 x 12), it means at the start of the month, both sun and moon will be at the same longitude and after 30 lunar days advancing 12 degrees a day, moon will be back at same longitude as sun. At 360 degrees, one cycle is complete and a new cycle starts.

# Procedure to Calculate Tithi

1. Find the longitude of Moon and Sun and calculate the difference. If the result is negative, add 360 to it as we need a value between 0 and 360 and we can add 360 because 360 means one complete cycle.
2. Divide the result by 12, we do this because a tithi changes whenever there is 12 degree change in the longitude difference between Moon and Sun. The quotient you get is the value we need for further calculation. Ignore the remainder.
3. Add 1 to the quotient and the value you get is between 1 and 30 and now you can refer the above table, look for the number you just got and you will know the running Tithi. For example, if you get the number 4, we can see from the above table that it falls in Sukla Paksha and the Tithi is Chaturthi so we call it Shukla Chaturthi, the number 19 also corresponds to Chaturthi so if you get 19, we call it Krishna Chaturthi. We first write the fortnight name followed by tithi (Sukla Ekadasi, Krishna Ashtami etc..)

# Example

I use Stellarium Software to find out the current positions of Sun and Moon (and pretty much every other planet / star / few galaxies in universe). At the time of writing this answer, the longitude of Moon and Sun is

Moon: +10 degree 08 minute

Sun: +182 degree 24 minute

For illustration purpose, I will be using decimal (.) in place of degree from now on so if I say +10.08, read it as 10 degrees and 8 minutes.

Now let's follow Step 1 and find out the difference between longitude of Moon and Sun.

+10.08 - (+182.24) = -172.16

We can determine by the above value that Sun is at a higher longitude than Moon which is why we are getting negative value. As per Step 1, we'll have to add 360 to it because number is negative so adding 360 to -172.16 gives 187.84

Now we need to convert this value to decimal degrees. The formula to do this is

Decimal Degrees = degree + (minute / 60) = 187 + (84/60) = 188.4

Now as per Step 2, the above value has to be divided by 12 so 188.4 / 12 is 15.7, we can ignore the remainder and the quotient is 15, as per Step 3, we add 1 to it and the final value we get is 16.

We now refer the above table and find that number 16 means the current tithi is Krishna Pratipada!

The tithi on 24th September was Poornima which started after Sunrise and Poornima was on the same day as well and not on 25th!

• Excellent explanation @Chinmay Sarupria . Thanks. I was not knowing this in depth. From this, I gather that the notion of "just rising Sun's tithi is the tithi for that day" is incorrect. Whatever comes as per mathematical calculations, is the tithi for that day. Am I right? Sep 26, 2018 at 3:36
• In Ur answer Thithis start with Shukla paksha so I believe it's Amanta Panchangam and not Purnimanta..can u pls mention that in answer bcz in most of the Indian states Thithis start with Krishna paksha so it might be confusing for them..
– YDS
Sep 26, 2018 at 4:14
• @YDS Ok have added a note below the tithi table. Sep 26, 2018 at 9:38
• @Vineet Tithi changes happen after sunrise as well. For example, tithi may be Ashtami at the time of sunrise but it could be Navami after 2 hours. So yes, that sunrise tithi notion is incorrect. Whatever tithi you get by mathematical calculation is the correct one and all tithis last for a set duration, i.e, 12 degrees longitude change between Moon and Sun. Sep 26, 2018 at 10:00
• In astrology, nature of anything is determined by the time it is born. A solar day is born at sunrise time. Therefore the thithi at sunrise is treated as having a significant bearing on the nature of that solar day. So even if only a small portion of the solar day is pournima, if solar day started on pournami tithi, that day will get the properties of pournami from an astrological assessment perspective for that day specifically. Sep 26, 2018 at 12:20