Jupiter is the greatest benefic amongst all the planets. The first house in the horoscope is the most ideal position for Jupiter. He gets directional strength in this house and casts benefic aspects on the 5th house (signifying children, intellect etc), the 7th house (signifying wife, marriage, married life etc.) and the 9th house (signifying religious mindedness, bhagya, prosperity, father etc).
If Jupiter is in the first house in a horoscope, it indicates that the native is blessed with the grace of God. According to Jatak Desamarga, when Jupiter possessed of exceeding strength with clear bright rays, occupies the lagna, the child born should be declared to be free from all afflictions.
If Jupiter is in Sagittarius in the ascendant, he will be there as lord of the 4th and the ascendant. If Jupiter is in Pisces, he will be there as lord of the ascendant and the 10th house. If Jupiter is in Cancer (his sign of exaltation), the ascendant will be Cancer and he will be there as lord of the ninth. These are very beneficial dispositions for any horoscope.
The disposition of Jupiter in Sagittarius, Pisces or Cancer in the ascendant (Lagna-Kendra) also gives rise to Hamsa yoga (a Panch Mahapurusha Yoga), the effects of which have been given as under:
"The native's legs will have the markings of a conch, lotus, fish and ankusa. He will possess a handsome body; he will be liked by others; he will be righteous in disposition and pure in mind."
Jupiter in 1st House gives you the traits of the sign of Sagittarius, optimistic, lucky, and with a tendency to say the first thing that pops into your mind. That is why Sagittarius HAS to be the sign of good luck. If it weren’t, everyone with a Sagittarius Sun-sign, or rising sign, or Jupiter in the first house would be murdered before the age of 21 for saying outrageous things. If you have this placement, don’t get into the habit of relying too much on your "luck" because every now and then, it shuts off, and THEN you will be in real trouble. The other drawbacks of this position are a tendency to gain weight and no desire whatever to listen to any one else.
Other Views
Brikat Jatak- The person concerned will be learned.
Phaldeepika- The native will have a dignified appearance, will perform righteous deeds, will be long-lived and fearless. He will have happiness from children.
Saravali- The native will possess a beautiful body, will be long-lived, learned and tolerant.
Chamatkar Chintamani- The native will be respected for his good qualities. He will spend money on pilgrimages.