If you have been looking for a page on Sun square Mercury or Sun trine Mercury, you will not find one here, and that is not an oversight. The QSun and EMercury cannot form those aspects in a birth chart. The reason is astronomy, and once you see it, it also tells you something useful about how to read these two in your own chart.
Why Mercury stays close to the Sun
Mercury orbits the Sun inside Earth's orbit. From where we stand, it never gets far from the Sun in the sky. Its greatest distance, called greatest elongation, is about 28 degrees. Most of the time it is closer.
An aspect is a measured angle between two points on the zodiac. A sextile needs about 60 degrees. A square needs 90, a trine 120 and an opposition 180. Mercury simply cannot get that far away. So the sextile, square, trine and opposition between the Sun and Mercury do not exist in natal astrology, and any text that interprets them is working from theory, not from real charts.
You may see software or old books list them for completeness. The interpretation library behind this site says the same thing plainly: the two are always too close for a challenging aspect, and never far enough apart for a sextile or trine.
What aspects are possible
The conjunction. Because Mercury is always within 28 degrees, a conjunction is very common. A large share of people have Mercury within 8 to 10 degrees of the Sun. This is the only major aspect the pair can form, and it has real meaning: your thinking and speech are bound to your sense of self. See Sun conjunct Mercury for the full reading.
The semi-sextile. At its widest, Mercury can come close to 30 degrees from the Sun. Within a tight orb of a degree or two, that counts as a semi-sextile, a minor aspect. It is read as a small, supportive link that helps you express your will in words, a subtle aid to communication rather than a defining trait.
No aspect at all. If Mercury is between roughly 10 and 27 degrees from the Sun, it is usually not in any aspect with it. That is common and perfectly normal. The two still work together, since Mercury is always in the same sign as the Sun or a sign next door, but there is no fusion.
What this means for your chart
Three practical points follow.
First, Mercury's sign is always your Sun sign or one of the two signs on either side. A Sun in Leo person has Mercury in Cancer, Leo or Virgo. That limits the range of contrasts. You will never see a fiery Sun paired with a Mercury in the opposite sign.
Second, since there is no hard aspect between these two, any strain in how you think is coming from somewhere else. If you feel a gap between who you are and how you express yourself, look at aspects to Mercury from Mars, Saturn, Neptune or others, or at the relationship between Mercury's sign and your Sun sign.
Third, distance still carries meaning. Traditional astrology treats Mercury within about 8 degrees of the Sun as combust, strongly colored by the ego, and within about 17 minutes of arc as cazimi, a special state of clarity. Mercury well away from the Sun is sometimes described as more objective, freer to see things from outside the self.
Questions people ask
My chart shows Mercury 27 degrees from the Sun. Is that anything?
It is close to a semi-sextile if your orbs are generous, but many astrologers would call it no aspect. Read it as a Mercury with some independence from the ego.
Can Sun and Mercury make aspects in transits?
In the sky, Mercury still stays near the Sun. But transiting Mercury can square or oppose your natal Sun, since your natal Sun is a fixed point. That is a transit, not a natal aspect.
Does the same limit apply to Venus?
Yes, with a wider range. Venus can reach about 47 degrees from the Sun, so it can make a semi-square as well. See Sun and Venus aspects.
Check your aspects
The Aspects tab in your birth chart lists each aspect with its orb and whether it is applying or separating, so you can see exactly where your Mercury sits. For the general rules on orbs and angles, read aspects explained.