On what basis the sixteen Tithis are divided into five groups and why are the groups so called?

Question

It is well known, that a Hindu day has five limbs (the panchAngas) among which the Tithi is one. The panchAngas form the basis for calculations in Jyotisha.

From Wikipedia :

In Vedic timekeeping, a tithi (also spelled thithi) is a lunar day, or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12°. In other words, a thithi is a time-duration between the consecutive epochs that correspond to when the longitudinal-angle between sun and moon is an integer multiple of 12°. Tithis begin at varying times of day and vary in duration from approximately 19 to approximately 26 hours.

We have sixteen Tithis, from pratipada to amAvasyA.

Now, these Tithis are also divided into five groups as follows:

NandA Tithis

Pratipada, Shashthi and EkAdashi.

BhadrA Tithis

DwitiyA, Saptami and DwAdasi.

RiktA Tithis

Chaturthi, Navami and Chaturdashi.

JayA Tithis

TritiyA, Ashtami and Trayodashi.

PurnA Tithis

Panchami, AmAvasyA and PurnimA.

• On what basis the above classifications/divisions are made?

• What are the reasons behind calling one group RiktA and another group PurnA and so on for every other groups?

Answer 1

This classification is based on five elements (Pancha Mahhabhuta). Though I don't have much reliable source (i.e book of Jyotisha) to cite, however, according to the book from British Library: Personal Panchanga and five source of light by Komilla Suttom, published by The Wessex Astrologer Limited, England (ISBN 9781902405261) :

Tithi Bhuta

The main tithi bhuta is Apas, the water element. Tithi shows our emotional stability and happiness. The water element makes this important for relationships, love and how we deal with individuals, as Venus is the primary ruler of the tithi. The strength or weakness of Venus in the natal chart would add or detract from the quality of the tithi. A tithi can also show compatibility – often people of same tithi get on well together. Each tithi also has its own bhuta ruler as well.

Five Sets of Tithis and their Sub-bhutas

There is a sub classification of tithis, which is linked to the panchamahabhuta. Tithis are divided into sets of five that repeat themselves over six times, thrice in the waxing phase and thrice in the waning. In this classification each set of three tithis are known as nanda, bhadra, jaya, rikta and poorna. Nanda tithi are 1, 6, 11 tithis; Bhadra are 2, 7, 12 tithis; Jaya are 3, 8, 13; Rikta are 4, 9, 14, and Poorna are 5, 10, 15 or 30 tithis.

Type      Tithi      Bhuta
---------------------------
Nanda     1,6,11     Agni
Bhadra    2,7,12     Prithvi
Jaya      3,8,13     Aakash
Rikta     4,9,14     Apas
Poorna    5,10,15    Vayu

The effects of Bhuta are also explained in next paragraphs, I'm here just quoting the purpose of this classification for the prediction point of view:

• Apas tithi (rikta) are good at relationships but as they are rikta tithis, their inner insecurities can create problems for them. They need partners who support them as their emotional neediness is at times too much for others to manage.

• Prithvi tithis (bhadra) sustain relationships. They want relationships that last.

• Vayu tithis (poorna) can easily churn up their emotions. They need steadiness from relationships or else worry and stress can cause them problems.

• Agni tithis (nanda) burn up relationships and are the worst offenders for selfish and short-term relationship

• Aakash tithis (jaya) protects relationships.

_{Some blogs like this also provide similar information.}

Note: answer providing references from reliable sources i.e scriptures of Jyotisha would be more useful.