Twice a year, the Sun reaches its highest or lowest point in the sky. Those moments are the solstices. The word comes from the Latin solstitium, meaning sun stands still. Around June 21, the Sun is at its northernmost point, marking the summer solstice in the northern hemisphere and the winter solstice in the southern. Around December 21, the reverse happens.

The solstices occur because Earth's axis is tilted by about 23.5 degrees. As Earth orbits the Sun, the tilt points different hemispheres toward or away from the Sun at different times of year. At the solstice, the tilt is at its maximum. The hemisphere tilted toward the Sun gets the most direct sunlight and the longest day. The other hemisphere gets the least.

The solstices have been celebrated for millennia. Stonehenge aligns with the summer solstice sunrise. The Incas built temples to track the winter solstice. Many cultures mark the day with festivals, bonfires, and rituals. The astronomical event is the same, but the meaning varies.

The solstice is not the hottest day of the year, even in the hemisphere tilted toward the Sun. The oceans and atmosphere take time to heat up, so the warmest days usually come a month or two later. The lag is why the seasons feel delayed.