Definition
The summer and winter solstices are the two days of the year when day and night differ the most in length. In the Northern Hemisphere, the summer solstice is the day with the longest daylight and the shortest night, and the winter solstice is the day with the longest night and the shortest daylight. This happens because Earth circles the sun with its spin axis tilted by 23.5 degrees.
The Magic of a Tilted Earth
Picture Earth not as a top standing perfectly straight, but as a top spinning with a slight tilt. As Earth travels around the sun, its axis stays tilted at about 23.5 degrees. Because of that tilt, the part of Earth that gets sunlight head-on changes depending on where Earth is in its orbit.
When the north end of Earth leans toward the sun the most, that is the summer solstice, the longest day. The sun takes its highest path across the sky and pours the most sunlight of the year onto the Northern Hemisphere. In Seoul, daylight on the summer solstice lasts more than 14 hours and 30 minutes.
When Earth reaches the other side of its orbit, the north end leans away from the sun. That is the winter solstice, the longest night. The sun skims low across the sky, daylight shrinks to around 9 hours and 30 minutes, and the long night goes on.
To Be More Precise: Is the Summer Solstice the Hottest Day?
If the summer solstice is the day with the most sunlight, you would think it should be the hottest day of the year. But the real heat does not arrive at the solstice (around June 21). It comes about a month later, in late July and early August. In the same way, the coldest days come in mid-January, not at the winter solstice (around December 22).
The reason is the same as heating a big pot of water. The water does not boil the instant you turn the stove to high. It boils hard only after the pot and the water have soaked up plenty of heat. Earth's land, oceans, and air also need time to slowly absorb heat and slowly give it back.
It takes a while for the huge amount of solar heat that arrives at the summer solstice to warm up the ground and the seas, so the peak temperatures show up about a month late. In the same way, Earth keeps cooling down after the winter solstice, so the sharpest cold arrives in January.
In the Southern Hemisphere, Christmas Is Midsummer
When the Northern Hemisphere, where we live, reaches its summer solstice, the Southern Hemisphere on the other side of Earth is having the opposite season. While the north leans toward the sun, the south faces away from it. So our summer solstice day is the winter solstice, with the longest night, for people in Australia or Argentina.
On the other hand, when the Northern Hemisphere freezes on its winter solstice, the Southern Hemisphere enjoys its midsummer solstice with the longest day of the year. That is why Santa Claus in Australia can show up in swim trunks and go surfing at Christmas in December.
Near the poles, the effect is even more dramatic. Inside the Arctic Circle, the sun does not set all day around the summer solstice (the midnight sun), and around the winter solstice the sun does not rise at all (the polar night). Thanks to Earth's tilt, places all over the world get very different views of day and night.
Common misconceptions
The summer solstice is when Earth is closest to the sun, and the winter solstice is when it is farthest away.
What changes the length of day and night and the seasons is not distance but the tilt of Earth's axis. In fact, Earth is closer to the sun in January (around the winter solstice) and farther away in July (around the summer solstice).
The summer solstice is the hottest day of the year, and the winter solstice is the coldest.
The ground and air take time to absorb and release heat, so the real heat and cold arrive about a month after the summer and winter solstices.
Where you meet it
In Korea, there is a custom of eating red bean porridge (patjuk) on the winter solstice to drive away bad spirits during the long night.
Nordic countries hold big summer festivals around the summer solstice to celebrate the midnight sun, when the sun does not set even at night.
In one sentence
The longest day (summer solstice) and the longest night (winter solstice) are made by Earth tilting 23.5 degrees as it travels around the sun.