The angle and the arithmetic
A trine is a hundred and twenty degrees of separation with a working orb of seven, so the window is fourteen degrees wide. The Sun moves about a degree a day and Jupiter roughly a twelfth of that, which means the pair passes through the window in a little over a fortnight.
It happens twice in each thirteen month cycle, once with Jupiter ahead of the Sun and once behind, so a trine is about twice as common as a conjunction or an opposition. Between them, the two trines account for around a month of births each year.
Because the two bodies are a third of the circle apart, they land in signs of the same element. A Sun in Leo trine Jupiter will have Jupiter in Aries or Sagittarius. That elemental agreement is doing a good deal of the interpretive work, and it is worth checking before you credit the angle itself.
Why easy is not the same as useful
The Sun is deliberate identity. Jupiter is enlargement, faith that there is more where this came from, and a tolerance for risk that other parts of the chart do not share. A trine connects them without friction, so the enlargement is simply available.
What that produces, in the usual accounts, is a person who does not have to fight for confidence and therefore rarely examines it. Opportunities appear and are taken or not taken with equal ease. The classic complaint about trines, and it applies here more than most, is that nothing in the configuration forces the issue. A square would.
So the reading is best stated as a resource rather than an outcome. The chart has capacity for scale in whatever the shared element is. Whether any of it gets built depends on the squares and oppositions elsewhere, which supply the pressure a trine deliberately withholds.
Element, house and the part that varies
Element is the first thing to look at. In fire the trine reads as straightforward nerve and appetite for the new. In earth it becomes patience and a long tolerance for slow accumulation. In air it is breadth of interest and an unusual amount of contact with people. In water it is read as sympathy, and traditionally as good instincts about when to move.
House is the second. Both bodies in cadent houses gives you a trine that mostly runs in the background. One of them on an angle, particularly the Ascendant or the Midheaven, drags the whole configuration into ordinary daily view and changes it from a latent resource into something visible in how the person operates.
Jupiter may be retrograde or direct here, since a trine can form on either side of the retrograde loop. It is retrograde roughly four months in twelve, so neither state is unusual and neither needs explaining away.
How much weight to give it
This aspect is shared by everyone born in a two week window, twice a year. That is a large group of people with nothing else in common, which is the reason to be careful about how much you load onto it.
The orb from the check above is the first filter. Inside two degrees a Sun to Jupiter trine is genuinely noticeable. At six or seven it is a mild background note and any tighter square in the chart will speak louder.
The second filter is whether the trine is part of something larger. A third body trine both ends makes a grand trine, which the tradition reads as a closed circuit that is comfortable and self-sufficient and rather hard to get out of. That configuration says considerably more about a life than the single trine does.
Questions people ask about this
Is Sun trine Jupiter a lucky aspect?
The tradition reads it as ease rather than luck, and ease is not an outcome. It supplies confidence and appetite for scale without effort, which is why it is so often left unused. What gets built usually depends on the harder aspects elsewhere in the chart.
What signs will my Sun and Jupiter be in?
Signs of the same element, in the ordinary case. A trine spans a third of the zodiac, so a Sun in Taurus trine Jupiter puts Jupiter in Virgo or Capricorn. A wide trine can spill into an adjacent sign, and that version reads more weakly.
How often does Sun trine Jupiter happen?
Twice in each thirteen month cycle, once on each side of the conjunction, and each pass holds for about a fortnight at the seven degree orb. That makes it roughly twice as common as the conjunction or the opposition.
Does it matter if Jupiter is retrograde?
Very little here. Jupiter is retrograde for around four months of every year, so a natal retrograde Jupiter is the ordinary case rather than a special condition. It says almost nothing on its own and should not be read as a weakening of the trine.
What is the orb for a trine?
Seven degrees is the standard working orb. The strength falls off sharply across that range, so treat anything inside two degrees as a real feature of the chart and anything beyond five or six as a background colouring.
Is a trine better than a square?
Neither is better. A trine gives access without pressure and a square gives pressure without comfort, and charts that achieve anything usually have both. Judging one as good and the other as bad is a habit worth dropping early.