Ninety degrees, and how long it lasts
A square is a quarter of the circle, ninety degrees, with a working orb of seven either side. The Sun travels about a degree a day and Jupiter about a twelfth of a degree, so the fourteen degree window takes a fortnight or so to cross.
Like the trine, the square happens twice per cycle, once as Jupiter falls behind the Sun and once as it moves ahead, which means roughly a month of births a year carry it. Jupiter can be direct or retrograde at either pass, and since it spends about four months of every year retrograde, neither condition is worth much on its own.
The two bodies land in signs of the same mode, cardinal, fixed or mutable, and different elements. That combination of shared urgency and mismatched material is exactly what the square is usually said to describe.
Appetite against judgement
The Sun is what you deliberately are. Jupiter is the impulse to make things bigger and the assumption that there is room to do so. A square sets those against each other in a way that cannot be settled by preference, because each keeps claiming the same ground from a different angle.
The traditional description is overcommitment. Yes is given before the cost is checked. Projects are begun at a scale that would have been sensible with more time or more help. What tends to follow is a life with a wider range than the cautious version would have had, and a longer list of things abandoned partway.
The counterweight is that squares generate action. Nothing about this configuration lets a person coast, which is the standing complaint about the trine. Charts that produce a great deal of visible output very often have squares doing the pushing, and this one pushes towards scale.
Signs, dignity and the house pair
Look at which body has the stronger position. Jupiter in Sagittarius or Pisces square a Sun in Virgo or Gemini is a square where the Jupiter side wins most arguments, and the person will describe a life of large gestures with the tidying up left to others. Reverse the dignities and the Sun side keeps a lid on it, and the same aspect reads as ambition repeatedly talked down.
Then the houses, which need a birth time. A square from the second to the fifth is a specific and rather concrete story about money and pleasure. One from the sixth to the ninth is about work against travel or study. The general description of the aspect is nearly useless until it is pinned to a pair of houses.
The check above supplies both, along with the orb, which is the number that decides how much of this applies at all.
The honest limits
Everyone born in the same fortnight has this aspect. It is a cohort marker before it is anything personal, and any reading that promises a specific life from it is overreaching.
Orb narrows that quickly. A square inside a degree and a half tends to be something the person recognises immediately when it is described. Past five degrees it is a minor note, and it will be outranked by any tighter aspect to the Sun.
The other thing to check is whether a third body opposes either end. That turns the square into a T-square, and the empty leg of a T-square is usually a better description of what a person actually spends their time on than any single aspect in the chart.
Questions people ask about this
Is Sun square Jupiter a bad aspect?
No, though it is often written up that way. It describes expansion arriving faster than judgement can check it, which produces both overcommitment and an unusually wide range of experience. Squares generate action, which is something the easier angles do not do.
What signs will the two be in?
Signs of the same mode and different elements. A Sun in Aries square Jupiter puts Jupiter in Cancer or Capricorn, both cardinal. Wide squares can cross into a neighbouring sign, and that version behaves more weakly than the degree count suggests.
How common is it?
The square forms twice in every thirteen month cycle and each pass holds for about a fortnight at the seven degree orb. That means roughly a month of births each year carry it, so it is one of the more common aspects.
Does Jupiter retrograde change the square?
Not by much. Jupiter is retrograde for about four months in each year, so having it retrograde at birth is ordinary. It affects when the aspect perfects rather than what it means, and it should not be treated as a weakness.
What is a T-square and does it matter here?
A T-square is a square where a third body opposes one of the two ends. It concentrates the pattern on the empty point across the chart, and where one exists it usually describes a person's actual preoccupations better than the isolated square does.
How do I tell which planet dominates?
Compare the dignity of each. Jupiter in Sagittarius or Pisces has the upper hand over a Sun in a sign Jupiter dislikes, and the reverse holds too. The stronger body sets the tone and the weaker one supplies the friction.