Solstices

The Solstice occurs twice each year (around June 21 and December 22) as the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. The seasons of the year are directly connected to both the solstices and the equinoxes.

The term solstice can also be used in a broader sense, as the day when this occurs. The day of the solstice has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice) for any place other than the equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are June solstice and December solstice, referring to the months of year in which they take place.

At latitudes outside the tropics, the summer solstice marks the day when the sun appears highest in the sky. Within the tropics, the sun appears directly overhead (called the subsolar point) from days to months before the solstice and again after the solstice, which means the subsolar point occurs twice each year.

The word solstice 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 seasonal movement of the Sun's path (as seen from Earth) comes to a stop before reversing direction.

For an observer on the North Pole, the sun reaches the highest position in the sky once a year in June. The day this occurs is called the June solstice day. Similarly, for an observer on the South Pole, the sun reaches the highest position on December solstice day. When it is the summer solstice at one Pole, it is the winter solstice on the other. The sun's westerly motion never ceases as the Earth is continually in rotation. However, the sun's motion in declination comes to a stop at the moment of solstice. In that sense, solstice means "sun-standing".

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