What Jupiter governs
Jupiter covers expansion, education beyond the immediate, law, travel that changes the frame of reference, and the beliefs a person holds about how the world works. Traditional practice calls it the greater benefic, meaning it is read as easing whatever it touches.
That reputation needs qualifying. Jupiter enlarges without discriminating, so it enlarges problems as readily as opportunities. A Jupiter closely tied to a difficult placement does not fix it, it makes more of it. The honest reading is growth, and growth in the wrong place is called excess.
A year in a sign
The twelve-year orbit means about twelve months in each sign, with variation caused by retrograde loops near the sign boundaries. Everyone in a given school year tends to share the Jupiter sign, which puts it in an intermediate category. It is not generational in the way Neptune is, but it is not personal in the way Mercury is either.
The practical consequence is that the sign explains something about a cohort. What separates one person in that cohort from another is the house Jupiter occupies and the aspects it makes to the faster bodies.
Rulership and dignity
Jupiter rules Sagittarius, and in traditional astrology it also rules Pisces, which modern practice reassigns to Neptune. It is exalted in Cancer, in detriment in Gemini and Virgo, and in fall in Capricorn.
The detriments are both Mercury's signs, and the tension is a real one: Jupiter works by generalising and Mercury works by dividing into cases. The fall in Capricorn is the same problem in a different form, since Capricorn's function is to limit and Jupiter's is to extend.
Retrograde for four months a year
Jupiter turns retrograde once a year and stays that way for about four months, which means roughly a third of all charts have it. At that frequency it cannot be read as a mark of anything unusual.
The standard reading is that the expansion turns inward: belief is examined rather than assumed, and opportunity is more often generated than handed over. This is a mild distinction, and it should not be given more weight than the sign, the house or the aspects.
The house is where the growth lands
Because the sign is shared so widely, the house does most of the individual work. Jupiter in the second house expands income and appetite for spending; in the ninth it expands study and travel; in the twelfth it operates in private and is often unnoticed by the person until much later.
All of that requires a birth time. Without one, a Jupiter reading reduces to a statement about everybody born that year, which is true but not useful.
Questions people ask about this
How long is Jupiter in one sign?
About a year, since it takes roughly twelve years to travel the whole zodiac.
Is Jupiter always lucky?
No. It enlarges what it contacts without selecting for benefit, so it can produce excess as readily as opportunity.
Which signs does Jupiter rule?
Sagittarius, and Pisces in traditional practice. Modern astrology gives Pisces to Neptune instead.
How common is Jupiter retrograde in a birth chart?
It is retrograde about four months every year, so roughly a third of charts have it.