What the aspect actually is
A conjunction is two bodies at the same ecliptic longitude, within about eight degrees for the Sun and Moon. Since the Moon moves roughly thirteen degrees a day and the Sun about one, the pair stays inside that orb for a little over a day either side of the exact new moon.
So this aspect is not a rare configuration. Somewhere between five and seven per cent of people have it, which is what you would expect from a window of roughly two days in a twenty nine and a half day cycle. What makes it distinctive is not scarcity but the fact that it removes a tension most charts have.
In practice it also means your Sun sign and Moon sign are the same, or in adjacent signs when the conjunction straddles a boundary. That is the single most visible consequence and the one people notice first.
Will and instinct in the same direction
The traditional reading treats the Sun as conscious purpose and the Moon as the reflex underneath it: what you decide to do against what you reach for without deciding. Most charts put those two in different signs, so there is a permanent low-level negotiation between them.
A conjunction ends the negotiation. Whatever the shared sign is, it gets applied twice, once deliberately and once automatically. The usual description is single-mindedness, and the usual complaint is that there is no second opinion available internally. People with the aspect often report that they know what they want very early and find it hard to explain why, because the wanting and the reasoning are not separable.
The cost is a narrower emotional range in that one direction. A chart with the Sun in Capricorn and the Moon in Pisces has two very different registers to work in. A chart with both in Capricorn has one, used harder.
The new moon and the missing mirror
There is a literal fact behind this aspect that is worth holding onto: at a new moon the Moon is not visible. It sits between us and the Sun, lit on the far side. The old symbolism for the aspect leans on that heavily, and for once the symbolism and the astronomy are describing the same thing.
What tends to be missing is self-reflection in the ordinary sense of seeing yourself from outside. An opposition, born at the full moon, gives you the other point of view whether you asked for it or not. A conjunction does not. People with it usually learn to get that from other people, which is why the aspect is often read alongside the seventh house and whoever is standing in it.
Orb, house and the parts that change the reading
An orb inside one degree is a different thing from an orb of seven. Tight conjunctions read the way the description above suggests. Wide ones, especially those crossing a sign boundary, dilute quickly, and a Sun at twenty nine degrees of Aries with a Moon at four degrees of Taurus has almost nothing of the classic effect.
The house matters more than most sources admit. Both bodies in the tenth is a very different life from both in the twelfth, because the shared drive is aimed at something specific. So does the sign: this aspect in Gemini and this aspect in Scorpio produce people who have almost nothing in common except the structural fact of not arguing with themselves.
The widget above gives you the orb, the shared sign and the house, which is the information the general description cannot supply.
Questions people ask about this
Does Sun conjunct Moon mean I was born on a new moon?
Within a day or two of one, yes. The exact new moon is the moment the two are at the same longitude, and the aspect holds while they are within about eight degrees of that, which covers roughly two days.
Is Sun conjunct Moon a good aspect?
It is neither. A conjunction intensifies whatever it joins rather than helping or hindering, so the answer depends entirely on the sign and house it happens in and on what else touches those degrees.
Does it mean my Sun sign and Moon sign are the same?
Almost always. The exception is a conjunction that straddles a sign boundary, where the Sun is in the last degrees of one sign and the Moon in the first degrees of the next. That version reads noticeably differently.
How common is Sun conjunct Moon?
Roughly one birth in fifteen to twenty, since the eight degree orb covers about two days of a twenty nine and a half day lunar cycle. It is one of the more common major aspects rather than a rarity.
What does a tight orb change?
Everything about how strongly the description applies. Inside one degree the effect is unmistakable to the person concerned. Beyond five it is usually a background note rather than a defining feature.
What is the opposite configuration?
Sun opposite Moon, which means a full moon birth. That chart has the same two bodies in permanent dialogue instead of permanent agreement, and it is the aspect most often contrasted with this one.