The December solstice marks the sun's southernmost point relative to the celestial equator, producing the shortest day and longest night of the year for the northern hemisphere and opening the astrological season of winter, which the tropical zodiac reckons as running from that solstice to the March equinox. The season opens with Capricorn, the cardinal sign whose beginning is defined by the solstice itself, moves through the fixed sign Aquarius, and closes with the mutable sign Pisces as the season transitions back toward spring, completing the cardinal, fixed, and mutable pattern the tropical zodiac repeats across every seasonal quarter of the year.
The solstice results from the same axial tilt that produces the June solstice's opposite extreme, with the hemisphere tilted away from the sun at this point in the Earth's orbit experiencing its longest night, and, as at every solstice, the sun's apparent daily path along the horizon briefly appears to pause before reversing direction, the phenomenon the word solstice names directly. A number of ancient monuments across different, unconnected cultures, including passage tombs in Ireland and stone structures elsewhere in Neolithic Europe, are aligned to capture sunlight specifically at the winter solstice, evidence that the solstice's significance as the year's darkest turning point was recognized and marked architecturally long before any of the specific named festivals now associated with the season took their current form.
Sources TetrabiblosClaudius Ptolemy with Astronomy in Prehistoric Britain and IrelandClive Ruggles