The shortest end of the zodiac
Every sign covers thirty degrees of ecliptic longitude, so on paper they are identical. In days they are not, because Earth's orbit is an ellipse and Kepler's second law has the planet sweeping equal areas in equal times. Near perihelion, the closest point to the Sun, the Earth moves fastest, and perihelion falls in the first week of January.
Sagittarius is crossed in the run up to that point, so the Sun appears to move through it unusually quickly. Sagittarius, Capricorn and Aquarius are the three short signs at roughly 29.5 days each, against about 31.5 for Cancer and Gemini at the far end of the orbit. The difference between the longest sign and the shortest is close to two whole days, and it is invisible in any list that prints fixed date ranges.
Beginning and ending
The Sun reaches 240 degrees, leaving Scorpio, around 21 or 22 November. It reaches 270 degrees at the December solstice, usually on 21 December, and that is where Capricorn takes over. So Sagittarius is one of the signs that ends on a major astronomical marker without starting on one.
The start date wanders for the ordinary reason. The tropical year is about 365.2422 days long, so each common year the boundary lands about six hours later than the year before, and the leap day resets it. Follow the Scorpio to Sagittarius crossing across a decade and it moves through more than a day. The table above computes both boundaries for whichever year you enter, so you never have to work from an average.
Jupiter, fire and mutability
Sagittarius is mutable fire, ruled by Jupiter, which it shares with Pisces in the traditional scheme. Mutable signs close a season and prepare the change, which fits a sign whose last degree is the solstice itself. The fire is the outgoing, expansive kind rather than the initiating Aries sort or the sustained Leo sort.
Jupiter's involvement is the source of most of what gets said about the sign: expansion, distance, meaning, and a taste for the general over the particular. Where Jupiter sits in an individual chart matters far more to how a Sagittarius placement behaves than the sign description does on its own.
Boundary births and the reversed season
Because the sign ends at a solstice, birthdays on 21 December are among the most commonly misfiled in the zodiac. There is no cusp to fall between: the Sun occupies one longitude at any instant, so a birth an hour before the solstice is Sagittarius and an hour after it is Capricorn. Only a birth time and the correct historical time zone can decide it.
South of the equator the dates are identical, since the ingress happens at one moment for the whole planet, but the season is inverted. Sagittarius season there runs through late spring towards midsummer, ending at the longest day rather than the shortest. The seasonal reasoning behind the tropical zodiac was written in the north, and this is one of the places where that shows most plainly.
Questions people ask about this
Why is Sagittarius shorter than most signs?
Because the Sun is crossed through it while Earth is approaching perihelion in early January, the point in the orbit where the planet travels fastest. The arc is a full thirty degrees like every other sign, but it takes about 29.5 days rather than the 31.5 that Cancer takes near aphelion.
Does Sagittarius season always end on 21 December?
It ends at the December solstice, which in most years falls on 21 December and occasionally on the 22nd. The exact instant shifts by several hours a year because of the leap year cycle, so the calendar date is a consequence rather than a rule.
When does Sagittarius season start?
Around 21 or 22 November, when the Sun leaves Scorpio at 240 degrees of ecliptic longitude. There is no astronomical marker at that point, so the date genuinely varies with the year rather than being anchored to anything nameable.
Is Jupiter still the ruler of Sagittarius?
Yes. Unlike Scorpio, Aquarius and Pisces, Sagittarius kept its classical ruler when the outer planets were discovered. Jupiter rules it in both traditional and modern practice, and shares rulership of Pisces in the older scheme.
I was born on 22 November. Which sign is that?
Either Scorpio or Sagittarius, and the date alone will not tell you. The boundary can fall on the 21st or the 22nd and at any hour. Convert the ingress time in the table above to your birthplace's clock for that year and compare it with your birth time.