A solstice is an astronomical event that occurs twice each year, when the tilt of the Earth's axis is most inclined toward or away from the Sun, causing the Sun's apparent position in the sky to reach its northernmost or southernmost extreme. The name is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination; that is, the apparent movement of the Sun's path north or south comes to a stop before reversing direction.

The term solstice can also be used in a wider sense, as the date (day) when this occurs. The solstices, together with the equinoxes, are connected with the seasons. In some cultures they are considered to start or separate the seasons while in others they fall in the middle. The English expressions "midwinter" (winter solstice) and "midsummer" (summer solstice) may derive from a tradition according to which there were only two seasons: winter and summer.

Definitions and frames of reference

Of the many ways in which solstice can be defined, one of the most common (and perhaps most easily understood) is by the astronomical phenomenon for which it is named, which is readily observable by anyone on Earth: a "sun-standing." This modern scientific word descends from a Latin scientific word in use in the late Roman republic of the 1st century BC: solstitium. Pliny uses it a number of times in his Natural History with the same meaning that it has today. It contains two Latin-language segments, sol, "sun", and -stitium, "stoppage." The Romans used "standing" to refer to a component of the relative velocity of the Sun as it is observed in the sky. Relative velocity is the motion of an object from the point of view of an observer in a frame of reference. From a fixed position on the ground, the sun appears to orbit around the Earth.

To an observer in inertial space, the Earth is seen to rotate about an axis and revolve around the Sun in an elliptical path with the Sun at one focus. The Earth's axis is tilted with respect to the plane of the Earth's orbit and this axis maintains a position that changes little with respect to the background of stars. An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.

The component of the Sun's motion seen by an earthbound observer caused by the revolution of the tilted axis, which, keeping the same angle in space, is oriented toward or away from the Sun, is an observed diurnal increment (and lateral offset) of the elevation of the Sun at noon for roughly six months and observed daily decrement for the remaining six months. At maximum or minimum elevation the relative motion at 90° to the horizon stops and changes direction by 180°. The maximum is the summer solstice and the minimum is the winter solstice. The path of the Sun, or ecliptic, sweeps north and south between the northern and southern hemispheres. The days are longer around the summer solstice and shorter around the winter solstice. When the Sun's path crosses the equator the days and nights are of equal length; this is known as an equinox. There are two solstices and two equinoxes.