Yesterday was the summer solstice (in the northern hemisphere), known to humans for thousands of years as the longest day of the year. In ancient times, people celebrated this day as the center point of summer. Some still observe the solstice with ceremonies and prayers, gathering on mountaintops or at spiritual landmarks. Others greet the day with parades, or by simply going outdoors and soaking in the sunshine. Collected here are images of people observing yesterday's solstice in the northern hemisphere -- and in the southern hemisphere, where it marked the darkest day of winter.