A hundred and eighty degrees, and what the sky was doing
An opposition is a separation of 180 degrees of ecliptic longitude, read with an orb of eight degrees on this site. Because Mars orbits beyond the Earth, an opposition to the Sun means the Earth is directly between the two, and Mars rises as the Sun sets.
These oppositions come round about every twenty six months, the same interval as the conjunctions and for the same reason. Not all of them are equal: Mars has a noticeably elliptical orbit, so some oppositions bring it much closer and much brighter than others. The most favourable ones fall roughly every fifteen or seventeen years.
The detail that matters for a natal chart is retrogradation. Mars turns retrograde only around opposition, for about ten weeks in each cycle, centred close to the exact opposition. So a Sun opposite Mars inside a modest orb almost always has Mars retrograde, and that is the single most reliable extra fact this aspect carries.
Two bodies in permanent dialogue
The Sun is read as conscious purpose and the direction of a life. Mars is read as the drive to act, and as the capacity for conflict. An opposition does not merge those or grind them together. It sets them at opposite ends of an axis, each visible to the other, neither able to leave.
The traditional descriptions revolve around a sense that effort and intention are pulling against each other, and that other people frequently turn up as the carriers of one side. Oppositions in general are read as the aspect most often projected outward: whatever you do not own at your end of the axis tends to arrive wearing someone else's face. With Mars involved, that usually means conflict shows up as something happening to you before it is recognised as something you are part of.
None of this is a verdict. It is what the tradition reads into a 180 degree separation between these two bodies, and how much weight it carries depends on the specifics below.
Retrograde Mars, signs and the axis
A retrograde Mars is not moving backwards in space. It is the Earth on its faster inner orbit overtaking Mars, which makes Mars appear to loop against the background stars. Interpretively, most practitioners read a natal retrograde Mars as drive that is redirected inward or expressed on a delay rather than absent. Since almost every Sun opposite Mars comes with it, the two readings are hard to separate in practice.
Oppositions join signs of the same mode and opposite elements, fire with air or earth with water. Sun in Aries opposite Mars in Libra is a cardinal fire and air axis that argues quickly. Sun in Taurus opposite Mars in Scorpio is fixed earth and water, and it does not let go.
The house axis names the subject. Across the sixth and twelfth it reads as work against retreat. Across the third and ninth it reads as immediate surroundings against distance. As always the houses depend on your birth time.
Orb and how much of this to keep
Inside two degrees you were born within days of the exact opposition, with Mars at or near its brightest and firmly retrograde. That is the strong form of the aspect and the one the descriptions above were written for.
Between four and eight degrees the picture loosens. Mars may have already stationed direct or not yet turned, and the aspect is one factor among several rather than a chart theme. An opposition that spans a sign boundary is weaker again, since the neat mode and element pairing breaks down.
The widget above reports the exact separation, both signs, both houses and whether Mars was retrograde. That last item is the one people most often get wrong when reading this aspect from a general article, and it is worth checking rather than assuming.
Questions people ask about this
Is Mars always retrograde when opposite the Sun?
Very nearly. Mars turns retrograde only around its opposition, for roughly ten weeks centred close to the exact aspect. A tight Sun opposite Mars will have Mars retrograde. At the wide edge of an eight degree orb it may have already stationed direct.
How often does Sun opposite Mars occur?
About every twenty six months, the same rhythm as the conjunction. The oppositions vary in how close Mars comes, because its orbit is markedly elliptical, and the closest ones fall about every fifteen to seventeen years.
Does an opposition mean conflict with other people?
The tradition reads oppositions as the aspect most likely to be experienced through others rather than internally, and Mars is the body associated with conflict. That is a description of a pattern, not a forecast. The houses involved decide what area of life it is even about.
What was visible in the sky at my birth?
With a tight orb, Mars rising as the Sun set, and staying up all night. At opposition Mars is at its closest to Earth and among the brightest objects in the night sky, which is not true at any other point in its cycle.
Which signs does this aspect join?
Signs of the same mode and complementary elements, so fire with air or earth with water. Aries with Libra, Taurus with Scorpio, and so on around the wheel.
How does this compare with Sun conjunct Mars?
They are the two ends of the same cycle. At conjunction Mars is behind the Sun, distant and invisible, moving direct and fast. At opposition it is close, bright and retrograde. The tradition reads the first as fusion and the second as dialogue across an axis.