Basic principles underlying Dwi-graha Samyoga

There are instances where two or more planets occupy one sign in a Natal Chart. This phenomenon is referred to in Astrology as planetary conjunctions or Graha-Samyoga.

Sphere of influence created by rays of a planet
Every planet emits certain type of rays peculiar to it and creates its own sphere of influence in the sign he occupies or say transits. When another planet enters the same sign generating its own type of rays, the entire character of the sphere of influence in the sign would undergo change. 

Friendship and enmity
Most students of Astrology know that the rays of one planet would get either intensified or counteracted by those of another planet. If the Planet A intensifies the rays of the Planet B, planets A and B are considered friends in astrological parlance. If the Planet A counteracts the rays of the Planet B, they are considered enemies. 

The length of one Rashi or Sign  is 30 degrees and if the two planets  in the sign , say,  are  28 degrees far apart, the effect  their Samyoga  is able to produce is very little or negligible.  On the other hand, if there are two planets in neighboring signs near the cusp between them, their conjunction or Samyoga becomes more effective and would produce strong results good or bad depending on the nature of the two planets or the friendship or the enmity between them.

Effects varying according to sign and House
Effects produced by planetary conjunctions vary depending on the signs and the Houses where they occur as well as the strengths and weaknesses of the planets. Two signs coming under the same constellation or Nakshatra is too considered a conjunction. 

Influence of Sun and Jupiter on other planets  
Sun and Jupiter are considered the most powerful planets in the Zodiac. When another planet joins Sun or Jupiter in the same sign, they would not suffer diminution of their natural strength. However, other planets conjoining them would lose their strength to some extent.  Conjunction of other planets with either Moon or Mercury would cause the latter undergo a complete change in their character making way for the conjoining planet to become dominant.

Jupiter- Saturn Samyoga
Jupiter- Saturn Samyoga is considered very favourable because it leads to diminution of Saturn’s evil influence and a heightening of the benevolent character of Jupiter.  Moon- Saturn Samyoga produces an entirely different outcome.  Moon’s good characteristics disappear, and Saturn’s evil influence becomes dominant.  The results produced by this Samyoga are misery, suffering disease, and loss of money, lands, and residence.  However, this Samyoga endows the native with powers of endurance and resilience. 

Mars- Moon Samyoga
Mars- Moon Samyoga, which most Indian astrologers refer to as Chandra- Mangala Yoga is considered a favourable conjunction that makes the native virtuous and materially successful. The fact that Mars is the Rajayogakaraka for the native of Cancer ruled by Moon has turned this Samyoga into a favourable one, according to Indian astrologers.

Findings of Astrologer Hettigoda
However, veteran Sri Lankan astrologer Hendricks de Silva Hettigoda says that his experience is that Chandra-Mangala yoga is unfavourable for females. He also says that Indian Rishi Varahamihira says in his classical work ‘Brihat Jataka’, native born with the Mars- Moon Samyoga would turn out to be a bad son to his mother and also an unscrupulous trader.

According to Hettigoda, Jupiter- Moon conjuncture in certain instances has been found to be very unfavourable.  He says that in cases where Jupiter- Moon conjunction had occurred in Cancer in the 2nd House, marriage had been a failure. He says, however, despite Jupiter and Venus being ‘enemies’, their Samyoga produces very favourable results.