Anchored to the solstice
The tropical zodiac is built outward from the March equinox in thirty degree steps, which puts three other boundaries on the year's turning points. Capricorn takes the December one. When the Sun's ecliptic longitude reaches 270 degrees it stands directly over the Tropic of Capricorn, and that instant is both the solstice and the start of the sign. The tropic is named after the sign for exactly this reason, though precession has since moved the Sun's actual position among the stars.
The solstice usually lands on 21 December and sometimes on the 22nd. The variation is a calendar artefact: the tropical year runs about 365.2422 days, so the event slips roughly six hours later each common year and jumps back when a leap day is inserted. The table above gives the moment itself, in UTC, for whichever year you select.
The shortest sign in the zodiac
Earth reaches perihelion, its nearest approach to the Sun, in the first week of January, which falls in the middle of Capricorn. A planet moves fastest when it is closest, so this is the part of the orbit that the Earth covers at maximum speed, and the Sun consequently appears to sweep through Capricorn's thirty degrees more quickly than through any other arc.
In practice that means about 29.5 days, against roughly 31.5 for Cancer directly opposite. The zodiac's arcs are exactly equal and its months are not, and Capricorn is the extreme case. This is one of the plainest checkable facts in the whole system and almost no site that publishes a fixed date range mentions it.
Saturn, earth and the cardinal role
Capricorn is cardinal earth and is ruled by Saturn, which also has Aquarius in the traditional scheme. Cardinal signs open seasons, and this one opens winter in the north at the exact moment the light begins returning. The pairing of the darkest day with a sign concerned with structure, limitation and long horizons is old and deliberate.
Saturn kept Capricorn when the outer planets were discovered, so unlike Scorpio or Aquarius there is no modern rival ruler to weigh up. Where Saturn sits in a given chart is the first thing to look at before reading anything into a Capricorn placement.
Solstice births and the reversed season
Being born on 21 December means the sign depends on the hour. There is no cusp: the Sun is at one longitude at any instant, either just short of 270 degrees or just past it. What decides the matter is a birth time together with the time zone that was legally in force at that place and date, which in many countries was not the zone in force there now.
The ingress happens simultaneously for the whole planet, so the dates are identical in Buenos Aires and Melbourne. What is not identical is the season. In the southern hemisphere the December solstice is midsummer and the longest day, so a sign associated in the north with cold, scarcity and endurance opens the height of the warm season. The mathematics travel; the imagery does not.
Questions people ask about this
Does Capricorn season start on 21 or 22 December?
Whichever date the December solstice falls on that year. It is most often the 21st, occasionally the 22nd, and the instant moves by a few hours from year to year because the tropical year is not a whole number of days. The table above resolves it exactly.
Why is Capricorn the shortest sign?
Because Earth passes perihelion in early January, in the middle of the sign, and travels at its greatest orbital speed there. The Sun crosses the thirty degree arc in about 29.5 days, roughly two days quicker than it crosses Cancer near aphelion in July.
Does Capricorn season have a modern ruler other than Saturn?
No. Saturn rules Capricorn in both traditional and modern practice. The outer planets were assigned to Scorpio, Aquarius and Pisces, leaving Capricorn's classical rulership untouched, which is why Saturn's condition in a chart matters so directly to any Capricorn placement.
Is Capricorn season the same in Australia?
The dates and times are identical, because the Sun enters the sign at one instant everywhere. The season is opposite: the December solstice there is the longest day and the start of summer rather than winter.
Is the Tropic of Capricorn related to the sign?
It is named after it. The line marks the southernmost latitude at which the Sun can pass directly overhead, which happens at the December solstice, the moment the Sun entered Capricorn when the naming was done. Precession has since shifted the stars behind that point, but the tropical sign still begins there.