Calculate the date and daylight hours for the shortest and darkest day of the year (Winter Solstice, ~December 21–22) across both hemispheres.
The astronomical winter solstice marks the exact moment the Sun is at its lowest altitude and shortest daylight duration of the year.
At the winter solstice, the Northern Hemisphere tilts furthest away from the Sun, causing the shortest day and longest night of the year. From this exact turning point onward, daylight hours incrementally increase each day until the summer solstice.
Within the Arctic Circle (66.5° N), the Sun does not rise above the horizon at all (24 hours of twilight or darkness).
Culturally celebrated for millennia as Yule and Dongzhi, marking the astronomical rebirth of longer days.