Same longitude, eight degrees of orb
A conjunction is a separation of zero degrees, read here with an orb of eight. Mars takes about twenty two and a half months to circle the zodiac against the Sun's twelve, so the Sun overtakes Mars roughly every twenty six months. That meeting is the superior conjunction, and it is the only kind Mars has with the Sun, because Mars orbits outside the Earth.
There is a plain astronomical consequence. At conjunction Mars sits on the far side of the Sun from us, at close to its maximum distance, faint and lost in solar glare. Anyone born under a tight Sun conjunct Mars was born at a time when Mars could not be seen at all.
Mars is also moving at close to its fastest apparent rate at this point, so the window is short relative to how long the pair spend apart. The aspect is neither rare nor common; it is periodic, and the period is a little over two years.
What the tradition makes of the pairing
The Sun in the tradition is the centre of the chart, the conscious sense of self and the direction a life points in. Mars is appetite in its active form: the willingness to push, to compete, to cut and to start. Both are hot in the old classifications, which is why the combination is treated as amplifying rather than balancing.
A conjunction fuses. It does not put the two in dialogue, as an opposition does, or set them at odds, as a square does. Whatever the shared sign is, it gets both the identity and the drive applied to it at once. The classic descriptions are courage, bluntness, a low tolerance for delay and a tendency to take initiative before consultation.
The counterweight in the older texts is that a fused Sun and Mars leaves nothing to hold the reins. There is no separate faculty standing back from the impulse, because the impulse and the sense of self are the same object. That is a structural observation about the aspect rather than a prediction about anyone.
Combustion, sign and house
Traditional astrology has a specific term for a planet within about eight degrees of the Sun: combust. Mars combust is read as a planet whose own significations are overwhelmed by the Sun's, which is an odd counterpoint to the modern reading of the aspect as extra force. Both views are in circulation, and it is worth knowing which one a given source is using before you take its conclusion.
Sign changes the whole picture. In Aries or Scorpio Mars is on home ground and the conjunction reads as concentrated. In Libra or Cancer Mars is traditionally weak and the same aspect reads as effort applied awkwardly. In Leo the Sun is at home and tends to set the terms.
House decides the arena. This conjunction in the tenth is a working life shaped by initiative. In the fourth it is the same energy turned on domestic ground, which reads very differently. Since houses depend on birth time, an uncertain time leaves that part of the reading open.
Orb, cazimi and what to check
Orb is the main variable. Within one degree the conjunction is unmistakable and, in the traditional system, approaches the condition called cazimi, in which a planet within about a sixth of a degree of the Sun is read as strengthened rather than burnt away. That is a very narrow band and few charts have it.
Between two and five degrees the aspect is a clear chart theme. Between six and eight it is a background note, and a conjunction that crosses a sign boundary loses much of its force, since the Sun and Mars are then working in different signs and the fusion is partial.
Mars retrograde is not a factor here. Mars only turns retrograde near its opposition to the Sun, so a Mars conjunct the Sun is always moving direct and usually quickly. The check above gives the separation, the shared sign and the house, which together decide how much of this page applies.
Questions people ask about this
Does Sun conjunct Mars mean Mars was invisible at my birth?
With a tight orb, effectively yes. At conjunction Mars is on the opposite side of the Sun from Earth and is lost in the glare for weeks either side. That is a fact about the sky rather than an interpretation.
How often does this conjunction happen?
Roughly every twenty six months, which is the interval between successive Sun and Mars conjunctions. With an eight degree orb the aspect holds for a stretch of weeks around each of those meetings.
What does combust mean here?
Combustion is the traditional term for a planet within about eight degrees of the Sun, and it is read as the planet's own significations being overwhelmed. Modern practice usually reads the same conjunction as reinforcement instead. The two readings pull in opposite directions, so check which one a source is using.
Can Mars be retrograde when conjunct the Sun?
No. Mars turns retrograde only in the months around its opposition to the Sun, so at conjunction it is always direct and moving at close to its fastest apparent speed.
Is this a difficult aspect?
The tradition does not settle that. It reads the conjunction as intensifying both bodies, which the same texts describe as courage in one place and rashness in another. Sign, house and dignity do more to shape the reading than the aspect alone.
Will my Sun sign and Mars sign be the same?
Usually, but not always. A conjunction spanning a sign boundary puts the Sun at the end of one sign and Mars at the start of the next, or the reverse. That version is read as weaker than a conjunction contained within a single sign.