There are instances where two or more planets occupy one Zodiacal sign. This occurrence is referred to in Astrology as planetary conjunctions or Graha Samyoga.

Every planet except the nodes Rahu and Ketu emits certain type of rays and creates its own sphere of influence in the Zodiacal sign it occupies or transits. When another planet enters the same sign radiating its own rays the entire character of the sign would undergo change.

Friendship and enmity between planets
Rays of one planet would get either intensified or counteracted by those of another planet entering its longitude. If Planet A intensifies the rays of Planet B, Planet A and Planet B are considered `friends. If C counteracts the rays of D, Planets C and D are considered `enemies’.

The length of one Rashi or sign in the Zodiac is 30 degrees. And if the two planets in one sign are say 28 degrees far apart, the effect their Samyoga or conjunction can produce is very little or negligible.

On the other hand, if there are two planets in two neighboring signs near the cusp between them, their conjunction or Samyoga becomes more effective and it would produce stronger results, good or bad depending on the nature of the two planets or the `friendship’ or the `enmity’ between them.

Two planets under one constellation
Effects planetary conjunctions produce vary depending on the sign and the House or Bhava where they occur as well as strengths and weaknesses of the planets.  Two planets coming under one and the same constellation too are considered to form a conjunction.

Planets losing or gaining strength in conjunctions 
Sun and Jupiter are the most powerful planets in the Zodiac. When another planet joins Sun or Jupiter in the same sign, they would not suffer a diminution of their strengths. However, other planets conjoining them would lose their strength and characteristics to some extent. Mercury and Moon are the two planets who are completely overpowered by other planets. These two planets undergo a complete change in their nature and characteristics when another planet conjoins them. 

Jupiter-Saturn conjunction
Jupiter-Saturn Samyoga is considered very favourable, because it leads to diminution of evil influences of Saturn and intensification of Jupiter’s benevolent character.

Moon-Saturn conjunction
Moon-Saturn Samyoga produces an entirely different result. Moon’s good characteristics disappear and Saturn’s evil qualities become dominant in such a union. The results that this Samyoga produces are misery, suffering, disease, and loss of money, lands and even the good name. However, this Samyoga endows the native with the qualities of endurance and reliance.

Exceptions to the general rule
Mars-Moon Samyoga, which Indian astrologers hail as Chandramangala Yoga is considered unfavourable for females, according to veteran Sri Lankan astrologer Hendrick de Silva Hettigoda. Indian Rishi Varahamihira says in his celebrated work `Brihat Jataka’ that native born with Moon-Mars conjunction would eventually turn out to be a bad son to his mother and an unscrupulous trader.

Hettigoda says he has found through experience that Moon-Jupiter Samyoga too is not favourable in certain instances. He says that he had found marriage had been an utter failure in cases where Moon-Jupiter conjunction had occurred in the 2nd  or the 7th  House.

Hettigoda says that despite Jupiter and Venus are called `enemies’ in Astrological parlance, their conjunction produces very favourable results.