Northern Hemisphere marks shortest day as winter solstice arrives

Sunday marks the shortest day of the year in the Northern Hemisphere, signaling the start of astronomical winter. The winter solstice occurs as the Earth’s tilt and its orbit place the Northern Hemisphere farthest from the Sun for the year, yielding the fewest daylight hours. In the Southern Hemisphere, the situation is reversed: Sunday will be the longest day of the year and the start of summer. This year, the winter solstice falls on December 21, though the date can drift between December 20 and 23 in different years.

Technically, the word solstice comes from the Latin sol for sun and stitium, meaning pause or stop. The solstice marks the moment when the tilt toward or away from the Sun is at its maximum. On the winter solstice, the Sun traces its shortest, lowest arc across the sky, producing the shortest day and longest night for the Northern Hemisphere. In the days that follow, daylight grows longer as the Sun climbs higher in the sky, though the calendar days will remain short for a while.

Across cultures, solstices have been observed for millennia with monuments aligned to the Sun’s path at solstice sunrise or sunset. Stonehenge is among the most famous examples, reflecting ancient builders’ attunement to the Sun’s yearly schedule.

Equinoxes, by contrast, mark times when the tilt and orbit align so that the Sun falls on the horizon with equal day and night. The Northern Hemisphere’s autumnal equinox can fall between Sept. 21 and 24, depending on the year, while the vernal or spring equinox can fall between March 19 and 21. These are two different ways to carve up the year. While astronomical seasons depend on how the Earth moves around the Sun, meteorological seasons are defined by the weather. Meteorologists break down the year into three-month seasons based on annual temperature cycles: spring starts on March 1, summer on June 1, fall on Sept. 1 and winter on Dec. 1.