for National Geographic News
On Friday, June 20, the summer of 2008 will begin in earnest across the Northern Hemisphere with the longest day of the year. This year is unique in that the solstice has not occurred before June 21 since 1896. The early arrival—albeit only by a minute—is due to a complex quirk of the leap-year calendar.
Before the sun sets on the June solstice, get the facts on why it occurs and how people throughout history have celebrated the event.
—The word solstice's Latin roots mean "sun stands still," an apt description of how the astronomical event appears from Earth.
Since ancient times people have followed the movement of the sun as it rises, crosses the sky, and sets along a path that changes incrementally throughout the year.
For a few days surrounding the solstice, however, our star seems to rise and set at the same locations. It also hovers at the same noontime spot, pausing before its trajectory begins its incremental shift until year's end—the December solstice.
—The "summer solstice" should be called the "June solstice," because it is actually the winter solstice in the Southern Hemisphere. Despite the reversed seasons, the event has long been observed south of the Equator as well.
—Winter and summer occur largely because the planet is tilted on an axis running through the poles at an angle of 23.5 degrees. As the planet orbits the sun, each hemisphere receives varying amounts of light and warmth determined by the direction in which it is tilted: summer when tilted towards the sun and winter when tilted away.
(Read more about Earth's orbit.)
On June 20, 2008, the North Pole will tilt most directly toward the sun, so that the noon sun appears at its highest point in the sky—nearly directly overhead. This is the year's longest day in terms of daylight hours.
(Related story: "In Scandinavia, Solstice Means Fun in the Midnight Sun" [June 21, 2005])
At the same time, in the Southern Hemisphere, the pole is tilted farthest away from the sun, and the June solstice falls in winter, marking the shortest and darkest day of the year.