Scientifically speaking, the two solstices in a year are the days during which the earth's tilt towards the sun is the most extreme. For the solstice in June today, the tilt is towards in the sun in the Northern Hemisphere (summer solstice) and away from the sun in the Southern Hemisphere (winter solstice).
For the solstice in December, it's the opposite.
The science aside, most people think of summer solstice as the beginning of summer and the day of the year with the longest period of daylight.
Below are pictures of individuals celebrating and observing the summer (and winter) solstice around the world.
A man uses a camera on a rocky crest filled with astronomical markers during the summer solstice at the Kokino megalithic observatory June 21, 2010. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked as the fourth oldest observatory in the world, according to NASA.ReutersA man uses a camera on a rocky crest filled with astronomical markers during the summer solstice at the Kokino megalithic observatory June 21, 2010. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked as the fourth oldest observatory in the world, according to NASA.ReutersA man uses a camera on a rocky crest filled with astronomical markers during the summer solstice at the Kokino megalithic observatory. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked as the fourth oldest observatory in the world, according to NASA.ReutersA couple celebrates summer solstice at the marker for the summer solstice at the Kokino megalithic observatory June 21, 2011. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked the fourth oldest observatory in the world, according to NASA.ReutersA man celebrates summer solstice at the marker for the summer solstice at the Kokino megalithic observatory June 21, 2011. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked as the fourth oldest observatory in the world, according to NASA.ReutersA man celebrates the summer solstice at the Kokino megalithic observatory June 21, 2008. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked as the fourth oldest observatory in the world, according to NASA.ReutersA man points at the marker for the summer solstice at the Kokino megalithic observatory June 21, 2008. The 3,800-year-old observatory was discovered in 2001 in the northwestern town of Kumanovo, 70 km (43 miles) north from Skopje, and is ranked as the fourth oldest observatory in the world, according to NASAReuters
