Graha Yuddha is a novel concept of Hindu astrology. Two
planets are said to be in yuddha of conflict, when they are in conjunction and the
distance separating them is less than one degree. All the planets except sun (surya)
and moon (Chandra) may enter into a war. The vicious planet is the one whose
longitude is less than its magnitude.
It is essential to evaluate the various balas (strengths) of
the planets to arrive at the results of such a yuddha. In order to decide the victor
and the vanquished, the aggregate of the various strengths is considered. The
smaller value is subtracted from the larger. The difference thus worked out is
divided by the difference between the diameters of the discs of the two
fighting planets. The resulting quotient (yuddhabala) must be added to the kalabala
(strength of time) of the victorious planet and subtracted from the total
kalabala of the vanquished planet. The result in either case indicates the
kalabala after the graham yuddha.
The diameters of the discs of planets are called the bimba
parimanas (they are 9.4 of the arc for Mars (Chowa or Angarak), 6.6 for Mercury
(Budha), 190.5 for Jupiter (Guru or Brihaspati), 16.6 for Venus (Shukra ) and
158.0 for Saturn (Shani).)
According to Varahamihira, the planet with a northern displacement
would be the winner in a Graha Yuddha. Also, the sun and the moon are more
powerful in Uttarayan; Mars, Mercury, Jupiter, Venus and Saturn become
powerful when in retrograde.
The planetary war is of four types – bheda, ullekha,
amsumardhana and apasavya. Occultation is called bheda. The halos of planets,
if they intersect, yield ullekha. When two planets are close together, the
light rays emitted by one would be intersected by those of the other, and this
is called amsumardhana. The planet in the north is referred to as being in
apasavya. All these details are described in Brihat Samhita.
Source - notes taken from Encyclopedia of Hinduism Volume IV - IHRF