The fundamental concepts of Indian / Hindu / Vedic astrology (Jyotish Shastra) have roots in Hindu religion and its scriptures. To put it differently, you could consider the hindu / vedic / puranic scriptures to be the fertile ground from which the massive tree of astrology has grown.
Therefore, if one has any doubt regarding the basic astrological concepts or rules, the scriptures need to be referred to for answers.
The 7th house in the rasi chart indicates marriage / spouse / partner and the 9th house represents dharma / father / religion, etc. Logically, one can put forward the following arguement:
1st house represents the person - The rasi chart overall represents the physical aspects of the person
2nd house represents wealth - The 2nd divisional chart (Hora) is studied to determine wealth
3rd house represents sibilings - The 3rd divisional chart (drekkana) is studied to determine details of siblings
.... and so on.
Therefore, since 7th represents marriage .. the 7th divisional chart (saptamsa) must be considered to determine details of marriage and spouse. However, it has been clearly defined in the astrological texts that the 9th divisional chart (navamsa) needs to be seen for marriage and spouse. However, the 9th house in rasi chart has nothing to do with spouse. Why is it different for marriage? Why doesn't the above logic work?
To answer the above question, let us begin with trying to understand what the 9th house signifies. The 9th house is called dharma-sthana and therefore represents dharma. So what is dharma? Righteousness, rules that need to be followed to be righteous, religion, rules of religion that need to be followed - all these mean dharma.
Now let's see what are the rules that one needs to follow as a Hindu, to uphold dharma? To know this, look at the 4 ashramas that a hindu has to go through in his life. They are Brahmacharya (bachelorhood, student, etc), grihasta (house holder, married person), vanaprastha (retired, simple life with minimal attachments), sanyasa (renunciation of worldly life). Grihastha ashrama or grihastha dharma is the religious duty of the hindu to marry and live the life of a householder - to procreate and thereby perpetuate one's lineage (vamsa).
Therefore, for the followers of Hinduism, marriage is a dharmic duty and one's spouse is one's partner in the practice of religious duty. To ensure that one gets the best matched partner to follow one's religious duty, we have arranged marriages which are based on horoscope matching.
The 7th house (marriage) is the 11th (gain) from the 9th (dharma). Therefore, marriage is the gain of dharma.
The navamsa (9th divisional chart) has to be studied in order to predict regarding one's dharma and since marriage amongst hindus is primarily a dharmic-duty, the maharishis have recommended the study of navamsa for marriage.
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