Solstice dates — summer & winter

On the June solstice the Sun reaches its most northerly point in the sky, directly overhead at the Tropic of Cancer (23.4° N). For everyone north of that line the Sun climbs higher than on any other day of the year, daylight is longest, and shadows at local noon are shortest. North of the Arctic Circle (66.6° N) the Sun never sets — the “midnight sun.” South of the Antarctic Circle the situation is reversed: 24-hour polar night, the Sun never rises.

Six months later, at the December solstice, the Sun is overhead at the Tropic of Capricorn (23.4° S). The Northern Hemisphere sees its shortest day; the Southern Hemisphere sees its longest. London gets about 7 hours 50 minutes of daylight in December and over 16 hours 30 minutes in June; Sydney's daylight is the inverse. At the equator, the difference between solstice and equinox daylight is a mere 7 minutes.

Ancient builders tracked these extremes with great precision. Stonehenge in England aligns with the June solstice sunrise; the burial chamber at Newgrange in Ireland fills with light at the December solstice sunrise; Chichén Itzá in Mexico, Karnak in Egypt, and Maeshowe in Orkney all encode solstice alignments in their architecture. The solstices are the deepest-rooted calendar markers humans have.

Astronomical definition. A solstice is the instant when the geocentric apparent longitude of the Sun equals 90° (June solstice) or 270° (December solstice). Equivalently, it is when the Sun reaches its maximum or minimum declination — its highest or lowest point on the celestial sphere relative to the equator. Modern ephemerides resolve the moment to the second.

Earth's 23.4° axial tilt. Our planet's rotation axis is tilted 23.44° away from the normal of its orbital plane around the Sun. That tilt is fixed in space (on human timescales — it precesses on a 26,000-year cycle), so as Earth orbits, different hemispheres are leaned toward the Sun at different times. At the June solstice the north pole is tipped 23.4° toward the Sun; at the December solstice it's tipped 23.4° away.

Why the date drifts. The tropical year — the time between two June solstices, say — is 365.2422 days, almost a quarter day longer than the calendar year. So the solstice instant lands about 5 hours 49 minutes later each common year, then jumps back about 18 hours 11 minutes when February has 29 days. Across a four-year cycle the calendar date oscillates between the 20th and the 21st (or the 21st and 22nd). The Gregorian calendar's 400-year leap rule is fine-tuned precisely to keep the solstice from drifting away from late June and late December.

Solstice ≠ hottest day. The summer solstice is the longest day, but rarely the hottest. Oceans and land masses take weeks to absorb and re-radiate solar heat — a thermal lag known as “seasonal lag.” In most temperate climates the warmest period falls 4 to 8 weeks after the June solstice, and the coldest 4 to 8 weeks after the December solstice.