The angle, the orb and how often it comes round
A conjunction is two bodies at the same ecliptic longitude, and for a pair involving the Sun the usual working orb is eight degrees either side. The Sun covers roughly a degree a day. Jupiter takes about twelve years to circle the zodiac, so it crawls along at about a twelfth of that.
The two of them therefore close and open the sixteen degree window at just under a degree a day, which keeps the aspect in force for something like seventeen or eighteen days. Because the Sun laps Jupiter once every thirteen months or so, that window comes round a little over once a year.
Nothing here is rare. Roughly one birth in twenty has it inside the full orb. What separates one chart with it from another is the orb itself, the sign, and above all the house.
What the two bodies stand for
The Sun in the tradition is conscious purpose: what you identify with, what you are trying to be seen as, the part of the chart you would defend if challenged. Jupiter is the principle of increase, of taking a thing and making it larger, more general, more optimistic and less careful.
A conjunction fuses rather than balances. It does not argue with what it touches, it amplifies it, and the classical description of this pair is a person whose sense of self runs a size bigger than the facts strictly support. That is read as generosity and nerve when it works and as overreach when it does not, and the same chart will produce both at different times of life.
The older sources call Jupiter the greater benefic and stop there. That is worth resisting. What the aspect reliably supplies is scale and appetite. Whether scale is useful depends on what it is applied to, which is a question about the rest of the chart rather than about Jupiter.
A cohort of about two and a half weeks
Everyone born inside that seventeen day window has this aspect. It is not personal in the way an angle to the Ascendant is personal, and it does not need a birth time to detect. That makes it a weak individual signature on its own, which most descriptions of it politely skip over.
What individualises it is the house. Without a birth time, the conjunction sits somewhere in the zodiac and says something general. With one, it sits in the second house or the ninth or the twelfth, and the difference between those three lives is enormous. The same is true of any contact the conjunction makes to a personal point, since a Sun and Jupiter both square the Ascendant is a far louder thing than the pair alone.
Jupiter is retrograde for about four months in every year, so it may well be retrograde in your chart. At a conjunction with the Sun it cannot be, because that is the moment Jupiter is behind the Sun rather than opposite it, and Jupiter turns direct near opposition. If your chart shows this aspect, Jupiter is moving forward.
Orb, sign and what actually shifts the reading
Inside a degree the description above is unmistakable to anybody who knows the person. At six or seven degrees it is a colouring rather than a feature, and it will lose to any tighter aspect in the same chart. The check above gives you the number, which is the part a general article cannot know.
Sign changes the flavour completely. In Sagittarius, Jupiter's own sign, the pair reads as a straightforward enlargement of confidence. In Virgo or Gemini, where Jupiter is traditionally uncomfortable, the same fusion tends to show up as scattering, taking on more than can be finished, or an appetite for detail that never quite closes.
Then there is everything else touching those degrees. A Saturn square to the conjunction is the classic corrective, and charts that have it usually describe the Jupiter part as something they learned to ration rather than something that came free.
Questions people ask about this
How rare is Sun conjunct Jupiter?
It is not rare. The alignment comes round a little over once a year and holds for roughly seventeen days at the standard eight degree orb, so about one person in twenty has it. Tight orbs of a degree or less are the genuinely uncommon version.
Does Sun conjunct Jupiter mean good luck?
The tradition calls Jupiter the greater benefic, but that is a label rather than a finding. What the aspect supplies is scale and appetite applied to the sense of self. Whether that reads as luck depends on the house, the sign and what else in the chart holds it in check.
Can Jupiter be retrograde in this aspect?
No. Jupiter is retrograde for around four months a year, but the retrograde period is centred on its opposition to the Sun, not its conjunction. If Jupiter is conjunct your Sun it was moving direct when you were born.
Is everyone born in the same fortnight the same?
Clearly not, and that is the honest limit of this aspect. It is shared by a cohort of about two and a half weeks. The house placement, which needs a birth time, and the orb are what make one chart's version different from another's.
What is the difference between this and Sun trine Jupiter?
A conjunction fuses the two so that Jupiter's scale is inseparable from the identity itself. A trine keeps them in separate signs of the same element, offering the same expansion as something available rather than something built in, and therefore easier to leave unused.
Does the sign matter more than the aspect?
Often, yes. Jupiter in Sagittarius or Pisces behaves quite differently from Jupiter in Gemini or Virgo, and the conjunction inherits that. Read the sign first, then the angle, then the house.