At each solstice the Sun seems to pause in the sky before turning back
Twice a year the Sun's noon height stops climbing or sinking and reverses. The Romans called that moment solstitium, a sun-stoppage. On the equator, though, the longest day of the year falls not in June but around 23 December, and the hottest weeks elsewhere arrive about a month late.
Earth's spin axis is tilted about 23.44 degrees relative to its orbit, and it keeps pointing the same way in space as we circle the Sun. For half the year the Northern Hemisphere leans sunward; for the other half the south does. The solstices, around 20 to 22 June and 20 to 22 December, are the two moments when that lean has its strongest effect. In June the Sun stands directly overhead at 23.44 degrees north, the Tropic of Cancer; in December it is overhead at the Tropic of Capricorn.
From the ground, that shows up as the noon Sun rising a little higher each day for about six months, then lower for the next six. At the turning point, its drift north or south halts for an instant. The Sun keeps crossing the sky from east to west as Earth turns; only the seasonal climb stops. Beyond the Arctic Circle, at 66.56 degrees north, the June solstice brings a Sun that never sets, while below the Antarctic Circle it never rises. In December the roles flip, and the dark lets polar sea ice grow back.
Because land and sea take time to heat up and cool down, the warmest and coldest periods in temperate zones lag the solstices by about a month. Near the equator the longest and shortest days do not line up with the solstices at all, since the Sun's apparent speed along its path varies through the year.
Naming them is trickier than it sounds. Summer and winter solstice swap meaning between hemispheres, so astronomers often prefer June and December solstice, or northern and southern. Old English had its own word, sunstead, which faded after the 17th century. East Asian calendars treat the solstices as the midpoints of summer and winter, calling them summer's extreme and winter's extreme.
Source: Solstice
