It's a bit says-on-the-tin clearer in Dutch than 'Solstice' which we get from the Latin
sol + sistere = sun-stand-still
but all over the world the sun appears to be at its largest departure from the equator and, in the Northern Hemisphere, it's downhill all the way from now on as it crosses the equator on the Autumn Equinox to bottom out on December 21st. You can measure the time of the sun's pause above the celestial equator to whatever degree of accuracy your instruments and your patience is capable of. Today the high-point occurs at 1051 GMT. Next year it will be at tea-time 1638hrs. But the year after that the Solstice won't happen on 21st June because we had to insert an extra day - 29 Feb 2016 - between the Solstices so that the seasons don't drift entirely away from the ticking of the days on the calendar. On Leap Years, therefore, the solstice happens (late) on 20th June: in 2016 at 2234hrs. I sort of knew that the solstice wasn't always on the 21st but, until today, I never bothered to find out when and why.