Solstice

• 1. Introduction

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  The solstice is a moment when the Sun appears to freeze in the sky. Twice a year, around the 20th through the 22nd of June and the 20th through the 23rd of December, the Sun reaches its farthest point north or south of the celestial equator and then reverses course. For a few days either side of that turning point, instruments as precise as a sextant fitted with a vernier scale cannot detect the change. The Sun simply appears to stand still.

  The Latin word solstitium captures this idea exactly: sol for sun, and -stitium for stoppage. The Romans were not being poetic. They were describing a real observational fact. From any fixed point on the ground, the Sun's daily path climbs higher in the sky for roughly six months, then descends for the other six. At the peak and the trough of that cycle, the vertical motion of the Sun appears to pause before turning back.

  What does that pause mean for the length of the day? What does it mean for the seasons, and for the people who have watched the sky long enough to know it was coming? Those are the questions this documentary will answer.

• 2. Why the Sun Stands Still

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  Earth's axis is tilted about 23.44 degrees away from a straight line perpendicular to its orbital path. That angle has a name: the obliquity of the ecliptic. Because the axis holds that tilt steady relative to the background of stars, one hemisphere leans toward the Sun for roughly half the year while the other leans away. The two moments when the lean reaches its maximum are the solstices.

  At the June solstice, the Sun sits directly above latitude 23.44 degrees north, a circle around the globe called the Tropic of Cancer. At the December solstice, it shines directly over 23.44 degrees south, the Tropic of Capricorn. Every latitude between those two lines receives the Sun directly overhead exactly twice per year.

  Far from the tropics, the effects are more extreme. During the June solstice, everywhere north of the Arctic Circle, at latitude 66.56 degrees north, the Sun stays above the horizon for a full twenty-four hours. That is the midnight sun, also called polar day. At the same moment, everywhere south of the Antarctic Circle, at 66.56 degrees south, the Sun never climbs above the horizon at all: polar night. The December solstice reverses both conditions.

  The warmest and coldest times of year in temperate regions do not fall exactly on the solstices. They lag by about a month because of what the source calls Earth's thermal inertia: the ground and the oceans take time to absorb or release heat, so the peak of summer heat arrives after the longest day has already passed.

• 3. A Word Frozen in Time

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  Solstice entered English during the Middle English period, carried over from the Latin solstitium that was already in use during the late Roman Republic of the 1st century BC. Pliny used solstitium in his Natural History in a sense close to the modern one.

  Before the Latin term took hold, English speakers used a native word: sunstead, spelled sunstede in older texts. It is a calque, a word-for-word translation of the Latin. Sunstead became rare after the 17th century. A related calque, sunstay, appeared in the 16th century and faded even faster.

  Sunstead is not uniquely English. The same concept appears in other Germanic languages with near-identical forms: sólstaðr in Old Norse and sunnenstat in an older Germanic tradition. Every one of these words tries to capture the same physical observation: the Sun appears to halt before turning back.

  The ancient Greeks expressed the same idea with the term heliostasio, meaning stand of the Sun. When Greek astronomers embedded solstice calculations into their model of celestial navigation, they were working with this concept of apparent standstill as its foundation. The writer Cleomedes, describing the geometry of the zodiacal band, placed the tropical circles, the two latitudes of the solstices, at the outer boundaries of that band. The ecliptic, the Sun's actual path through the sky, ran through the center.

• 4. The Problem of Precision

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  Knowing the solstice is coming is easier than knowing exactly when it arrives. In the days just before and after the solstice, the Sun's change in declination drops below thirty arcseconds per day. That is less than the apparent angular size of the Sun, and the equivalent of just two seconds of right ascension. No gnomon, no astrolabe, no sextant with a vernier can resolve a difference that small from direct observation.

  Greek astronomers almost certainly used an interpolation method to get around this limitation. By recording the Sun's noon declination angle on two days placed symmetrically on either side of the solstice, one before and one after, they could find the midpoint between two matching readings. An interval of forty-five days between the two observations was later identified as the best spacing for achieving up to a quarter-day precision.

  In 2012, the journal DIO published a more precise formula. Observing the Sun's equal altitudes about twenty degrees before and after the summer solstice, it found, allows accuracy of one or two hours with balanced errors. The remaining noise in the result is roughly forty-one hours divided by the number of days in the observation interval, assuming one arcminute of visual sharpness.

  Guo Shoujing, a Chinese astronomer working in 1276, took a different approach: he built a taller gnomon. He designed a gnomon 12.6 meters tall at the Gaocheng Astronomical Observatory. Measurements from that instrument put the length of the year within one minute of the value used in the Gregorian Calendar, a figure the Gregorian system would not establish officially for another three hundred years.

• 5. Names Across Cultures

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  Summer and winter solstice are the most familiar English names, but they carry a built-in ambiguity: summer in the Northern Hemisphere is winter in the Southern. The Latinate equivalents, estival and hibernal solstice, have the same problem. So do midsummer and midwinter.

  June solstice and December solstice remove the hemisphere ambiguity, though they introduce a different one: not every calendar places the solstice in those same months. The Islamic calendar and the Hebrew calendar, both of which are non-solar, do not fix the solstice to the same month from year to year. Northern solstice and southern solstice sidestep both problems and have the additional advantage of working cleanly for other planets.

  A fourth pair of names, first point of Cancer and first point of Capricorn, traces back to the Roman Classical period. Due to the precession of the equinoxes, the Sun no longer appears in those constellations at the solstice: it now appears in Taurus in June and Sagittarius in December. The astrological names have outlasted their astronomical accuracy.

  East Asian calendars handle the solstice differently. A year is divided into twenty-four solar terms, and the summer solstice is the tenth, called Xiàzhì in Mandarin or Geshi in Japanese. It begins when the Sun reaches the celestial longitude of 90 degrees, around the 21st of June, and ends when the Sun reaches 105 degrees, around the 7th of July. The winter solstice is the twenty-second term, Dongzhi or Toji. The Chinese character at the heart of both words, 至, means extreme, framing the solstices not as midpoints in a drift but as summits.

• 6. Celebrations Around the World

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  At the Aymara New Year on the 21st of June, a celebration takes place at sunrise, when the Sun shines directly through the Gate of the Sun in Tiwanaku, a site in present-day Bolivia. Further south, the Mapuche people of South America celebrate We Tripantu, their New Year, a few days after the northern solstice, on the 24th of June. The Atacama people, living further north, formerly marked the same date with a noise festival intended to call the Sun back.

  The winter solstice is even more heavily overlaid with tradition. Christmas on the 25th of December, Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa, and Yule all cluster around the December solstice. In East Asian cultures, the Dongzhi Festival falls on the winter solstice itself. The feast of St. John, running from the 23rd to the 24th of June, surrounds the June solstice in Christian tradition. Modern Pagans observe Midsummer at the same time, known as Litha among Wiccans.

  In England, the solstices and equinoxes historically marked the traditional quarter days, the dates when rents fell due and meetings were held. Midsummer's Day, identified by the Christian Church with St. John's Day, is set on the 24th of June, about three days after the solstice itself. The 25th of December similarly falls a few days after the December solstice.

  Antarctica holds its own observance. The 21st of June is celebrated as Midwinter Day at stations including McMurdo Station, Palmer Station, Amundsen-Scott South Pole Station, Casey Station, and several British stations: Bird Island, Rothera Research Station, King Edward Point, and Halley VI. The day marks the southern winter solstice, the shortest day at the bottom of the world. In Fremont, Seattle, Washington, the Fremont Solstice Parade takes place every year on the summer solstice.

• 7. Shifting Through the Stars

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  The solstice points do not stay fixed against the background constellations. Because of precession, a slow wobble in Earth's rotational axis, the Sun's position at each solstice drifts gradually from one constellation to the next over thousands of years.

  Using the official IAU constellation boundaries, the northern solstice passed from Leo into Cancer in the year 1458 BC. It moved from Cancer into Gemini around the year 10 BC, and from Gemini into Taurus in December 1989. It is expected to cross into Aries in the year 4609.

  The southern solstice has its own trajectory. It passed from Capricornus into Sagittarius around 130 BC. It is expected to enter Ophiuchus in the year 2269 and Scorpius in the year 3597.

  These shifts explain why the astrological label first point of Cancer no longer corresponds to the actual constellation Cancer, and why the name Tropic of Cancer refers to a latitude where the Sun once stood at the June solstice but no longer does. The sky keeps moving, even when the Sun appears, for a few days each year, to stand perfectly still.