What Is a Solstice?

A solstice is an astronomical event that happens twice a year, marking the points of the longest and shortest days of the year.

Quick Answer

A solstice happens twice a year. The summer solstice, around June 21st, is the longest day of the year. The winter solstice, around December 21st, is the shortest day of the year. They are caused by the Earth's tilt on its axis as it orbits the Sun.

The summer solstice (around June 21 in the Northern Hemisphere) has the longest period of daylight, while the winter solstice (around December 21) has the shortest. These events are caused by the Earth's axial tilt of 23.5 degrees.