Astronomical winter begins Thursday evening, December 21 at 10:27 pm. This is the instant the Sun is directly over the Tropic of Capricorn at 23°26′10.2″ south latitude. This also marks the "longest night", or when the Sun spends the most time below the horizon for everyone north of the equator. The opposite is true for those south of the equator.
That subsolar point has been moving slowly southward each day since the autumnal equinox in September when the Sun was directly over the equator. The Sun will pause briefly before beginning a slow migration northward. It pass over the equator at the March equinox before reaching the Tropic of Cancer at 23°26′10.2″ north latitude in June, marking the beginning of astronomical summer.
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The Sun pauses at the tropics winter and summer (solstice comes from the Latin word sōlstitium or "Sun stand still"), because of the Earth's axial tilt, which is also about 23.5 degrees. That tilt has been measured as far back as 1100 BCE by astronomers and mathematicians in India and China.
Earth's tilt makes the seasons possible. But, like most things in the universe over very long time periods, that tilt, or obliquity, is changing.
Serbian scientist Milutin Milankovitch described these changes in the 1920s finding that, over the last million years, obliquity has varied between 22.1º and 24.5º. The greater the tilt, the greater the seasonal variation, especially near the poles. The lesser the tilt the milder the seasonal changes are.
Earth's axis was last at that maximum tilt about 10,000 years ago. The tropics of cancer and Capricorn were even further from the equator then and are currently moving at a rate of about 15 meters each year. While that does mean a trend toward a milder climate,
The changes are very subtle and happen over very long periods, and not responsible for the global increase in temperatures seen in recent years.