Ninety degrees, twice a month
A square is a ninety degree separation, and for the Sun and Moon the working orb is about seven. The Moon passes through both squares to the Sun every lunar month, once on the way to full and once on the way back, so a square birth is a first quarter or last quarter birth.
Because there are two of these windows in each cycle rather than one, this is the most common of the Sun and Moon aspects. The Moon's speed means each window lasts a little over a day, and the phase is easy to confirm without any astrology at all. A half lit Moon in the evening sky at your birth is a first quarter square.
Which of the two squares matters. A waxing square, with the Moon ahead of the Sun, and a waning square, with it behind, are read differently by astrologers who work with phase, the first as building and the second as reviewing.
Friction as the mechanism
The Sun stands for what you set out to do and the Moon for what you fall back on. A square holds those two in signs of the same mode but different element, so they want the same tempo and disagree about everything else. Cardinal squares argue about which direction to start in, fixed squares about what to hold onto, mutable squares about what to adapt to.
Squares are usually described as friction, and friction in this tradition is not a fault. It is the thing that forces action. The aspect is read as producing people who are rarely at rest with themselves, who are aware of a cost to every choice, and who tend to build competence out of that dissatisfaction.
What it does not give is the automatic support that trines are credited with. Nothing here is effortless. What the square does supply is a reason to keep working on the problem, which is why it appears so often in the charts astrologers use as examples of sustained achievement.
The domestic reading
Older sources attach this aspect to a mismatch between the two parental figures, on the assumption that the Sun and Moon describe them. That is a claim about interpretation rather than astronomy and it is worth flagging as such. What can be said more carefully is that the aspect describes a person for whom the demands of self-direction and the demands of comfort were never obviously compatible.
In practice the useful question is which house holds which body. The Sun in the tenth square a Moon in the first is a very different pattern of pressure from a Sun in the fourth square a Moon in the seventh, even though both are the same ninety degrees.
Orb, mode and what softens it
Inside three degrees the square is a live feature of the chart. Between three and seven it is present but frequently outweighed by other configurations. A square that crosses a sign boundary loses the shared mode that gives the aspect its logic, and a Sun at twenty nine degrees Cancer with a Moon at two degrees Scorpio is technically square and reading much more like a trine in element terms.
Other contacts change it too. A planet that trines or sextiles both ends acts as an intermediary and the classic description softens considerably. Saturn touching either end tightens it. Jupiter tends to widen the tolerance without removing the friction.
The widget above reports the exact separation and both positions, which is what tells you whether this is a defining aspect of your chart or a footnote in it.
Questions people ask about this
What moon phase is Sun square Moon?
A quarter moon, either first or last. The Moon is ninety degrees from the Sun and appears half lit. Which quarter depends on whether the Moon is ahead of the Sun in zodiacal order or behind it.
Is Sun square Moon bad?
The tradition reads it as difficult rather than bad, and difficulty here means the aspect asks for effort instead of supplying ease. Sign, house and supporting contacts decide whether that effort turns into competence or into strain.
Why is this the most common Sun and Moon aspect?
Because the Moon squares the Sun twice per lunar cycle, once waxing and once waning, while it conjoins and opposes only once each. Two windows of roughly a day each means more births fall inside it.
Do my Sun and Moon share a mode?
Usually yes. A clean ninety degree square puts both bodies in the same mode, cardinal, fixed or mutable, in different elements. Wide squares that cross a sign boundary break that rule and read differently.
Does the waxing or waning square matter?
It does for astrologers who work with lunar phase. A waxing square is read as pressure to build something not yet formed, and a waning square as pressure to revise something already made. The ninety degree geometry is identical in both.
Can other aspects soften it?
Yes. A third planet making easy aspects to both the Sun and the Moon is traditionally read as a mediator between them. Look for trines and sextiles reaching both ends before you decide how sharp the square really is.