The angle and the position of Mars
A square is a separation of 90 degrees, read here with an orb of seven. Both the waxing square, with Mars ahead of the Sun, and the waning one, with Mars behind, count. In observational terms this is quadrature: Mars sits ninety degrees from the Sun in the sky, rising around midnight or setting around midnight depending on which square it is.
Mars at quadrature is direct in its motion. Retrogradation only happens near opposition, where the elongation is around a hundred and thirty five degrees or more, so a square never coincides with it. Anyone reading a natal Sun square Mars can state confidently that Mars was moving forwards.
The Sun and Mars pass through both squares in each of their roughly twenty six month cycles. With a seven degree orb, each pass covers a window of a few weeks, and Mars is moving at a moderate apparent speed rather than its fastest or slowest.
Friction as the mechanism
The Sun is the conscious direction of a life. Mars is the willingness to fight for something, and the physical push behind any effort. A square joins signs of the same mode but incompatible elements, so the two are working at the same tempo on incompatible terms.
The tradition reads that as force that has to be discharged. Unlike an opposition, which can be watched and negotiated across a distance, a square is described as insisting on movement. The standard descriptions run to impatience, a competitive streak, a habit of acting first and considering afterwards, and a certain amount of self generated obstruction where the intention and the method work against each other.
The same texts note the other side: this is one of the aspects most often found where sustained effort is required, because the friction supplies its own fuel. Whether either description fits any particular chart is a question the general article cannot answer.
Mode, dignity and house
Cardinal squares, joining Aries, Cancer, Libra and Capricorn, are read as quick to erupt and quick to move on. Fixed squares, in Taurus, Leo, Scorpio and Aquarius, are read as slow to build and slow to release. Mutable squares, in Gemini, Virgo, Sagittarius and Pisces, are read as scattering the pressure across several fronts.
Mars in its own signs of Aries and Scorpio operates with full resources, and the square reads as effective aggression rather than frustrated effort. Mars in Cancer or Libra is traditionally considered weak, and the same square reads more like being at odds with one's own methods.
The house pair supplies the subject matter. Sun in the ninth square Mars in the twelfth is one thing. Sun in the second square Mars in the fifth is another entirely. Two charts with the same square in the same signs can describe completely different lives once the houses are added.
Orb, and using the check rather than the summary
The tighter the square, the more literally the description applies. Inside one degree it is a defining feature and people who have it tend to recognise it immediately. At three or four degrees it is a real but manageable strand. At six or seven it is faint enough that anything else in the chart will speak louder.
Direction of application is worth a glance too. An applying square, where the faster body is still closing on the exact angle, is read as gathering force. A separating one is read as force already spent. That distinction comes from traditional practice and is worth what you decide it is worth.
The widget above reports the separation in degrees, both signs, both houses and whether the aspect is applying or separating. Read that first, then decide how much of the general description belongs to your chart.
Questions people ask about this
Can Mars be retrograde in a square to the Sun?
No. Mars only turns retrograde near opposition, at an elongation well beyond ninety degrees. A square to the Sun always finds Mars in direct motion, which is one of the few things about this aspect that can be stated as fact rather than interpretation.
What orb is used for a square?
Seven degrees either side of an exact ninety degree separation. Inside two degrees the aspect is strong. Towards the outer edge it is usually a minor influence compared with tighter contacts elsewhere in the chart.
Is Sun square Mars an aggressive aspect?
The tradition associates it with force and impatience rather than aggression as such, and it also associates it with stamina. The sign, the dignity of Mars and the houses do more to set the tone than the angle by itself.
How often does this aspect form?
Twice in each cycle of roughly twenty six months, once with Mars ahead of the Sun and once behind. Each pass lasts a few weeks within the seven degree orb.
Does an applying square differ from a separating one?
In traditional practice yes. An applying aspect, still closing on exactness, is read as building. A separating one is read as having already discharged. Many modern astrologers weigh the distinction lightly, so it depends on the method you follow.
What is the difference between this and the opposition?
An opposition is a hundred and eighty degrees and is read as a dialogue across an axis, often experienced through other people. A square is ninety degrees and is read as internal friction that pushes towards action. The opposition also puts Mars close and retrograde, which the square never does.