Solstice
The Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20, the 22nd of June and 20, the 22nd of December. Earth's axis of rotation is not perpendicular to its orbital plane but currently makes an angle of about 23.44 degrees. This tilt is called the obliquity of the ecliptic. The axis keeps its orientation with respect to an inertial frame of reference as Earth orbits the Sun in an elliptical path. For half the year the Northern Hemisphere is inclined toward the Sun while for the other half year the Southern Hemisphere has this distinction. At maximum elevation the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction. Outside of the tropics the maximum elevation occurs at the summer solstice and the minimum at the winter solstice. When it is the summer solstice at one Pole it is the winter solstice on the other.
The word solstice is derived from the Latin solstitium. It contains two Latin-language morphemes: sol meaning sun and -stitium meaning stoppage. Pliny uses the term a number of times in his Natural History with a similar meaning that it has today. Solstice first entered into English in the Middle English period. An older term in English is its calque sunstead which became rare after the 17th century. A similar English calque of the Latin term is sunstay which was first used in the 16th century and is now also rare. Sunstead is cognate with other terms with the same meaning in other Germanic language such as Sommersonnenwende. The Romans used standing to refer to a component of the relative velocity of the Sun as it is observed in the sky. From a fixed position on the ground the Sun appears to orbit around Earth.
Greek astronomers must have used an approximation method based on interpolation which is still used by some amateurs. This method consists of recording the declination angle at noon during some days before and after the solstice. They tried to find two separate days with the same declination. When those two days are found the halfway time between both noons is estimated solstice time. An interval of 45 days has been postulated as the best one to achieve up to a quarter-day precision. Guo Shoujing designed the gnomon constructed at Gaocheng Astronomical Observatory in 1276. The measurements from Gaocheng determined the length of the year to within one minute of the current measurement. Government organizations like USNO and IMCCE publish the date and time of the solstice. The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. Thus the solstices always occur between June 20 and 22 and between December 20 and 23 in a four-year-long cycle.
At the June solstice the subsolar point is further north than any other time at latitude 23.44 degrees north known as the Tropic of Cancer. Similarly at the December solstice the subsolar point is further south than any other time at latitude 23.44 degrees south known as the Tropic of Capricorn. During the June solstice places on the Arctic Circle will see the Sun just on the horizon during midnight. All places north of it will see the Sun above horizon for 24 hours. That is the midnight sun or midsummer-night sun or polar day. On the other hand places on the Antarctic Circle will see the Sun just on the horizon during midday. All places south of it will not see the Sun above horizon at any time of the day. That is the polar night. The warmest and coldest periods of the year in temperate regions are offset by about one month from the solstices delayed by the earth's thermal inertia.
During the southern or winter solstice Christmas is the most widespread contemporary holiday while Yule Saturnalia Karachun Hanukkah Kwanzaa and Yalda are also celebrated around this time. In East Asian cultures the Dongzhi Festival is celebrated on the winter solstice. For the northern or summer solstice Christian cultures celebrate the feast of St. John from June 23 to 24. Midsummer's Day defined as St. Johns Day by the Christian Church is the 24th of June about three days after the solstice itself. In the southern tip of South America the Mapuche people celebrate We Tripantu a few days after the northern solstice on the 24th of June. Further north the Atacama people formerly celebrated this date with a noise festival to call the Sun back. The Amundsen, Scott South Pole Station celebrates every year on the 21st of June a midwinter party to celebrate that the Sun is at its lowest point and coming back.
Common questions
When do the solstices occur each year?
Two solstices occur annually around the 20th, the 21st, and the 22nd of June and between the 20th and the 23rd of December. The dates vary depending on the time zone but always fall within these ranges in a four-year cycle.
What is the etymology of the word Solstice?
The word solstice derives from the Latin solstitium containing two morphemes: sol meaning sun and -stitium meaning stoppage. Pliny used the term in his Natural History with a similar meaning to its modern usage before it entered English during the Middle English period.
How does Earth's tilt affect the position of the Sun at solstice?
Earth's axis makes an angle of about 23.44 degrees relative to its orbital plane known as the obliquity of the ecliptic. At maximum elevation the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction while the subsolar point reaches latitude 23.44 degrees north or south.
Which cultures celebrate holidays during the winter solstice?
Christmas is the most widespread contemporary holiday celebrated around the winter solstice alongside Yule Saturnalia Karachun Hanukkah Kwanzaa and Yalda. In East Asian cultures the Dongzhi Festival is also celebrated on the winter solstice.
Where can observers see the midnight sun during the June solstice?
During the June solstice places on the Arctic Circle will see the Sun just on the horizon during midnight while all places north of it experience 24 hours of sunlight. This phenomenon is called the midnight sun midsummer-night sun or polar day.