Solstice

A solstice is a specific moment in Earth's orbit. It occurs when the Sun reaches its most northerly or southerly position relative to the celestial equator. This event happens twice every year. These dates usually fall around June 20–22 and December 20–22. For most people living away from the equator, these events define the seasons. The June solstice often brings the longest daylight hours, while the December solstice brings the shortest.

The mechanism behind the solstice involves Earth's specific movement through space. Earth orbits the Sun in an elliptical path. However, Earth's axis of rotation is not straight up and down. It is tilted at an angle of approximately 23.44 degrees. This tilt is known as the obliquity of the ecliptic. As Earth orbits the Sun, this axis maintains its orientation in space. This means the tilt always points in the same direction relative to the stars. Because of this tilt, different parts of Earth lean toward the Sun at different times of the year.

This tilt creates distinct stages of solar position. At the June solstice, the subsolar point reaches its furthest north. The subsolar point is the exact location on Earth where the Sun is directly overhead. This point sits at latitude 23.44° north, a place called the Tropic of Cancer. At the December solstice, the subsolar point reaches its furthest south. This occurs at latitude 23.44° south, known as the Tropic of Capricorn. The subsolar point travels between these two extremes, crossing every latitude in between twice every year.

The name "solstice" has deep historical roots. It comes from the Latin words "sol," meaning Sun, and "stitium," meaning stoppage. This describes the apparent motion of the Sun in the sky. From our perspective on Earth, the Sun seems to move north or south throughout the year. At the moment of the solstice, this vertical motion appears to stop for an instant. Then, the Sun's path reverses direction. This is why the term implies a "sun-standing."

Ancient civilizations studied these solar patterns to navigate and understand time. The Ancient Greeks used the term "heliostāsio," which also meant the stand of the Sun. They understood that the Earth was a sphere. They imagined a celestial sphere where stars moved in parallel circles. They observed that the Sun and planets moved along a different path called the ecliptic. This ecliptic is tilted at an angle to the Earth's axis. This explains why the Sun appears to move through the constellations of the Zodiac.

The effects of solstices are most extreme at the Earth's poles. At the North Pole, the Sun reaches its highest annual position during the June solstice. Inside the Arctic Circle, people experience the "midnight sun." This is a period where the Sun stays above the horizon for 24 hours. Conversely, the December solstice brings the "polar night" to the Antarctic Circle. During this time, the Sun does not rise above the horizon at all. This lack of sunlight causes polar sea ice to re-grow every year.

Solstices are part of a larger cycle of solar events. There are two solstices and two equinoxes in a tropical year. An equinox occurs when the Sun's path crosses the equator. During an equinox, day and night are of equal length at most latitudes. While solstices mark the extremes of daylight, they do not immediately cause the hottest or coldest weather. The warmest and coldest periods in temperate regions are often delayed by about one month. This delay is caused by Earth's thermal inertia, which is the way the planet holds onto heat.