What the Sun governs
The Sun stands for vitality, the direction a life is organised around, and the part of the chart a person is prepared to be identified with in public. In traditional practice it also carries authority, the father in some house systems of reading, and physical stamina. It is not the personality in the everyday sense. The Moon and the rising sign usually describe more of what people notice from outside.
When astrologers say a chart is solar they mean the Sun is angular, in its own sign, or closely aspected by several bodies, so its concerns override the rest. That is a judgement about the whole chart rather than about the sign the Sun occupies.
About a month in each sign
The Sun moves roughly one degree a day and crosses a sign in about thirty days, so everyone born within the same month shares it. That makes the solar sign broad rather than precise. It is genuinely personal in the sense that it is not generational, but it is shared with a very large number of people born the same fortnight.
The boundaries matter. Anyone born within a day of a sign change needs the actual ingress time for their birth year, since the Sun does not enter a sign on the same calendar date every year.
Rulership and dignity
The Sun rules Leo and is exalted in Aries. It is in detriment in Aquarius and in fall in Libra. Unlike the other classical planets it governs a single sign, which it shares with no other body, and it has no nocturnal counterpart of its own.
Dignity here is a statement about ease of expression, not about quality. A Sun in Libra is not weaker than a Sun in Leo; it is working through a sign whose priorities of balance and negotiation sit awkwardly with the solar habit of centring itself.
The Sun never turns retrograde
Retrograde motion is an appearance produced by the Earth overtaking a planet, or being overtaken by it. Since the apparent path of the Sun is a direct consequence of the Earth's own orbit, the Sun cannot appear to reverse. Neither can the Moon. Any chart or app showing a retrograde Sun has an error in it.
What the Sun does have is eclipses and its own annual return, which is the basis of the solar return chart cast for the moment it comes back to its natal degree.
Why the house needs a birth time
The sign describes the manner; the house describes the area of life it is spent on. A Sun in Capricorn in the fourth house is a very different reading from the same Sun in the tenth, and only a birth time separates them, because the houses turn through the whole zodiac in twenty-four hours.
Without a recorded time, read the sign and the aspects and stop there. Rectifying a birth time from events is a specialist procedure, and estimates drawn from a rough time of day are not reliable enough to build a house reading on.
Questions people ask about this
Can the Sun be retrograde?
No. Retrograde motion comes from the Earth's position relative to another planet, and the Sun's apparent path is produced by the Earth's own orbit. The Moon never retrogrades either.
Is my Sun sign the same as my star sign?
Yes. The sign given in newspaper columns is the sign the Sun occupied at birth, which changes about every thirty days.
Which signs are the Sun's dignities?
It rules Leo and is exalted in Aries, in detriment in Aquarius and in fall in Libra.
Do I need a birth time for my Sun?
Not for the sign, unless you were born within a day of a sign change. You do need one for the house.