Solstices are the times when summer and winter begin. Summer begins on June 20, 21, or 22. Winter begins on December 21 or 22 in North America, Europe, and other places north of the equator. The equator is an imaginary line around the middle of the Earth. In Australia, part of Africa, and other places south of the equator, the seasons are just the opposite. Summer begins on December 21 or 22, and winter begins on June 20, 21, or 22.

Image to left: Ancient monuments such as Stonehenge were probably built to keep track of the solstices. Scientists have found that on the first day of summer or winter, the rising or setting sun lines up with a certain stone or shines through a special opening. Credit: World Book illustration by Richard Hook At the summer solstice, the noontime sun appears higher in the sky than at any other time of the year. It is the longest day of the year. At the winter solstice, the noontime sun appears lower than at any other time of the year. It is the shortest day of the year.

Solstices happen because the Earth's axis is tilted. The axis is an imaginary line through the Earth, from the North Pole to the South Pole. The Earth spins around this axis, just as a classroom globe does. The tilt of the axis makes the appearance of the sun change positions in the sky as the Earth orbits, or moves, around the sun. How to cite this article: To cite this article, World Book recommends the following format: "Solstice." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.