Larger version (JPG, 305 KB)

During the year, the position of the Sun as seen from the Earth moves north or south depending on the season. When the Sun changes direction, it seems to stand momentarily still.
The name solstice is derived from the Latin word solstitium (from sol: "sun" and sistere: "stand still"), so solstices are those moments of the year when the Sun reaches its southernmost or northernmost position, at the Tropic of Cancer (23.45 degrees north) or Tropic of Capricorn (23.45 degrees south). This is the moment when the Earth's tilt faces exactly directly towards or away from the Sun.

Graphic (GIF, 40 KB) illustrating summer solstice: The orbit of the Earth defines an ellipse with the Sun at one focus. The Earth moves around its orbit in the direction of the arrows, while spinning about its own axis, which is tilted to the plane of the ecliptic at 23.5 deg. The Earth axis points toward the North Star.

Summer solstice usually occurs on 21/22 June in the northern hemisphere and on 21/22 December in the southern hemisphere. As a curiosity, the Arctic Circle is the imaginary line that marks the latitude above which the sun does not set on the day of the summer solstice and does not rise on the the day of the winter solstice. North of this latitude, periods of continuous daylight or night last up to six months at the North Pole.

Image of this year's summer solstice (JPG, 217 KB) from 21 June 2006 at 06:00 UTC.

The top image shows the distribution of night and day on Earth as seen from Meteosat-8 21 June 2005 at 06:00 UTC. The two top figures are spring and autumn equinoxes (meaning the moment when the Sun passes over the Equator), and below are the two solstices with northern hemisphere summer on the lefthand image, and northern hemisphere winter on the righthand image.

Graphics courtesy of DMI.

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