Shortest Night of the Year

The shortest night of the year is the one at the summer solstice: around 21 December in the Southern Hemisphere, around 21 June in the Northern. That word around is the whole problem, because the date swings between two days and depends on the time zone you happen to be standing in. Choose a year and a hemisphere and the page returns the matching summer solstice date, with a copy button next to it.

Inside it runs Jean Meeus's algorithm: a polynomial for the mean solstice plus 24 periodic correction terms, the same approach ephemerides use. Any year from 1000 to 3000 is accepted, so you can check a date quoted in an old document or plan something far ahead. Output is day, month, year and a UTC marker, with no clock time; the month names and the hemisphere selector are in Portuguese (Sul, Norte).

Check the offset before you schedule anything around it. In 2027 the December solstice lands on 22/12 at 02:43 UTC, which is 21/12 at 23:43 in Brasilia: the page shows the 22nd while Brazil lives the 21st. The tool also says nothing about daylight hours, because without a latitude there is no sunrise and no sunset, so an ephemeris calculator with coordinates is the right next stop. The math is done in your browser.