Solstices occur twice a year, when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes. 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, its apparent movement north or south comes to a standstill.

The term solstice can also be used in a wider sense, as the date (day) that such a passage happens. The solstices, together with the equinoxes, are connected with the seasons. In some languages they are considered to start or separate the seasons; in others they are considered to be centre points (in English, in the Northern hemisphere, for example, the period around the June solstice is known as midsummer, and Midsummer's Day is 24 June, about three days after the solstice itself). Similarly 25 December is the start of the Christmas celebration, which was a pagan festival in pre-Christian times, and is the day the sun begins to return to the northern hemisphere.

Names

The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress.

• Summer solstice and winter solstice are the most common names. However, these can be ambiguous since seasons of the northern hemisphere and southern hemisphere are opposites, and the summer solstice of one hemisphere is the winter solstice of the other. These are also known as the 'longest' or 'shortest' days of the year.
• Northern solstice and southern solstice indicate the direction of the sun's apparent movement. The northern solstice is in June on Earth, when the sun is directly over the Tropic of Cancer in the Northern Hemisphere, and the southern solstice is in December, when the sun is directly over the Tropic of Capricorn in the Southern Hemisphere.
• June solstice and December solstice are an alternative to the more common "summer" and "winter" terms, but without the ambiguity as to which hemisphere is the context. They are still not universal, however, as not all people use a solar-based calendar where the solstices occur every year in the same month (as they do not in the Islamic Calendar and Hebrew calendar, for example), and the names are also not useful for other planets (Mars, for example), even though these planets do have seasons.
• First point of Cancer and first point of Capricorn. One disadvantage of these names is that, due to the precession of the equinoxes, the astrological signs where these solstices are located no longer correspond with the actual constellations.
• Taurus solstice and Sagittarius solstice are names that indicate in which constellations the two solstices are currently located. These terms are not widely used, though, and until December 1989 the first solstice was in Gemini, according to official IAU boundaries.
• The Latin names Hibernal solstice (winter), and Aestival solstice (summer) are sometimes used.