JACKSONVILLE, Fla. — December 21 marks the year’s shortest day and the onset of astronomical winter in the Northern Hemisphere, known as the winter solstice.

At 10:03 a.m. EST, Earth’s axis tilts to its furthest angle of 23.5 degrees away from the Sun, causing the Northern Hemisphere to experience its minimal direct sunlight for the year.

During this event, the Sun reaches its lowest altitude in the sky above Northern Hemisphere locales. On this day, it traces its briefest and most southerly arc from dawn to dusk, rising and setting further south than at any other time of the year.

In Jacksonville, this phenomenon results in the Sun setting more to the southwest, prompting those near downtown to gaze south-southeast across the St. Johns River for sunset views. Meanwhile, sunrise along the Atlantic coast appears slightly south of east.

The low angle of the Sun is a key reason winter temperatures are cooler. When sunlight strikes the Earth at a lower angle, its energy is spread over a larger area and must pass through more of the atmosphere, reducing its intensity. Even on sunny days, this limits the amount of heating at the surface.

From a climatological standpoint, meteorological winter began on Dec. 1. Meteorologists use this definition for record-keeping and seasonal comparisons, as the timing of astronomical winter varies slightly each year. It also better aligns with climate data, since the coldest months are typically January and February, rather than March, when astronomical winter officially ends.