22 December to 19 January
Capricorn starts at two hundred and seventy degrees of ecliptic longitude, which is the December solstice, and ends at three hundred degrees on 19 or 20 January. The season therefore opens on the day the Sun stops moving south and begins to come back, an event recorded in almanacs and monuments long before it was recorded in ephemerides.
The solstice falls on 21 or 22 December depending on where the year sits in the leap cycle: the Sun needs about a quarter of a day more than the calendar allows, and the shortfall is settled every fourth year.
The name that outlived its stars
The Tropic of Capricorn is the parallel of latitude the Sun stands over at that solstice, and it was named when the solstitial point lay among the stars of Capricornus. Precession has since carried that point into Sagittarius as the constellations are now drawn.
The tropical zodiac is unaffected, because it is defined by the solstices and equinoxes rather than by the stars behind them. Capricorn still begins exactly at the solstice; it is only the backdrop that has moved, and it moves by about one degree every seventy two years.
Cardinal earth under Saturn
Cardinal signs open the quarters, and this one opens the northern winter. Saturn rules it and also rules Aquarius in the traditional scheme, so the two coldest months of the northern year share a ruler.
Mars is exalted at twenty eight degrees of Capricorn, which falls in the last days of the season, in the middle of January. The decans go to Jupiter, Mars and the Sun, which gives the harshest of the signs a notably bright set of decan rulers.
What the range cannot decide
A birth in the day or so either side of the solstice, or of the January boundary, needs the year, the clock time and the birthplace rather than a table, and the answer that comes back is a single sign.
Sidereal astrology shifts the whole range by about twenty four degrees, roughly three weeks, so most tropical Capricorn births are sidereal Sagittarius. And no date at all can supply the Moon's sign, which turns over every couple of days, or the Ascendant, which turns over every two hours.
Questions people ask about this
What are the Capricorn dates?
Approximately 22 December to 19 January. The exact start falls on 21 or 22 December and the end on 19 or 20 January, varying with the year and with the time zone the birth is dated in.
Why do the dates change from year to year?
Because the tropical year is not a whole number of days. The ingress slips about six hours later each year and is pulled back by each leap day, so it oscillates across roughly a day and a half.
When is Capricorn season this year?
Within the range above, at a minute that has to be calculated for the specific year. Anyone born close to either edge needs the actual solar position rather than a table.
Why does Vedic astrology not make me Capricorn?
It measures from the fixed stars rather than from the equinox, and the two references have separated by about twenty four degrees. That moves every sidereal boundary later by a little over three weeks.
Does being born on the first day of Capricorn mean anything special?
It puts the Sun in the first degree of the sign, which matters for aspects and house placement. The tradition does not read it as a weaker or stronger version of the sign itself.