Xiazhi occurs at celestial longitude 90°, around June 21-22. The northern hemisphere's longest day; yang energy reaches its annual peak. Classical wisdom warns: at peak yang, yin is born — heat will increase, but the seed of decline is planted.
Tradition: eating cold noodles, dumplings; rituals to thank the heavens for harvest. Climate: hottest days are still ahead — the "three fu" period typically follows.
The twenty-four terms were not fixed all at once, and the two solstices came first. Plant an upright pole on level ground, measure the noon shadow through the year, and the day the shadow is shortest is the summer solstice, a determination needing a ruler and patience, not any calculation. That is why the solstices were settled long before the equinoxes and the four beginnings. By the Western Han the astronomical chapter of the Huainanzi already lists the full set of twenty-four terms by name, so the scheme was complete as a system by then. The gap between old and modern computation shows up here more than anywhere. After the Qing adopted the Shixian calendar, terms were spaced by equal arcs of longitude, which makes the intervals between them unequal; the earlier method divided the year into equal spans of time, and the two can put the solstice about a day apart. Ritual practice was divided as well: classical rites paired the winter solstice with offerings to heaven and the summer solstice with offerings to earth, while popular observance treated the day mainly as a farming and eating occasion, with food customs that vary widely by region.
The line about one yin being born at the summer solstice is not a poetic flourish but a statement about hexagrams. Han-dynasty calendrical Yi learning matched twelve waxing-and-waning hexagrams to the twelve months, assigning wu month the hexagram Gou (姤): five yang lines above and a single yin line newly arrived at the bottom. Its mirror is Fu (復) at the winter solstice, one yang returning. The principle in the Book of Changes that extremes reverse is thereby given a calculable address, since the day of greatest light is also the day the turn begins.
The note that the hottest days are still ahead has a mechanism behind it. Heat accumulates with a lag: daylight shortens from the solstice onward, yet the ground keeps taking in more energy than it loses for weeks, so the peak arrives around Xiaoshu, Dashu and the fu days, not on the solstice itself. Day length is not uniform either. It lengthens with latitude, reaching continuous daylight inside the Arctic Circle, and in the southern hemisphere the same instant is the winter solstice.
In Four Pillars work Xiazhi is a median term in wu month and does not move the month pillar; Mangzhong and Xiaoshu do that. It still carries weight in luck-period and annual analysis. Once the first yin appears, fire is conventionally read as advancing no further: ding fire still commands the month, but its momentum has passed the top. Date selection and fortune reading often use the solstice, paired with its winter counterpart, as one of the two hinges of the year. The distinction to hold on to is that this describes the movement of qi, not the thermometer.
The three pentads are deer shedding their antlers, cicadas beginning to sing, and pinellia sprouting. The first shows the reasoning clearly. Deer were classed as yang animals, so their antlers drop as the first yin appears, while the elaphure, classed as yin, was said to shed at the winter solstice when the first yang returns. This is not zoology but a way of fitting an observed event into the yin-yang frame. Pinellia, banxia (半夏), literally half of summer, is named for sprouting at the season's midpoint, the same logic applied to a plant.
No. The solstice is the day with the longest daylight and the highest noon sun in the northern hemisphere, but surface heat accumulates with a lag. Even as daylight shortens afterwards, the ground keeps absorbing more energy than it radiates away for several more weeks, so temperatures usually peak about a month later, during Xiaoshu, Dashu and the fu days. Tradition treats the solstice as the peak of yang energy, not the peak of heat.
The phrase comes from the Han-dynasty scheme matching hexagrams to months. Wu month, which contains the solstice, is assigned the hexagram Gou: beneath five yang lines a single yin line has newly appeared, signifying that yang has reached its height and the opposite force has begun to stir. Its counterpart is the hexagram Fu in the month of the winter solstice, where one yang returns. It describes a turn in the movement of qi, not the day's weather.
Not directly. The month pillar changes only at the twelve sectional terms, and the solstice is a median term sitting in the middle of wu month. It is still used in analysis: once the first yin appears, fire is conventionally read as past its advance, so although ding fire still commands the month its momentum has turned. In date selection and annual readings the solstice, paired with the winter solstice, often serves as a dividing line between halves of the year.
They are counted from the solstice but do not begin on it. The traditional rule opens the first fu on the third geng day after the summer solstice and the middle fu on the fourth, with the last fu starting on the first geng day after the beginning of autumn. Because geng days recur every ten days while the span from solstice to Liqiu varies, the middle fu runs either ten or twenty days and the whole block lasts thirty or forty days.