Xiaoshu occurs at celestial longitude 105°, around July 6-8. Heat intensifies; "lesser" because the absolute peak (Dashu) lies two weeks ahead. The "three fu" hot days typically begin.
Tradition: eating mutton soup (羊肉湯), exposing books and clothing to sunlight to prevent moths. Climate: high temperature with afternoon thunderstorms; typhoons begin in southeastern China.
Working out when the fu (伏) days begin is an exercise in the sexagenary cycle rather than in weather. The first fu opens on the third geng day after the summer solstice, the middle fu on the fourth, and the last fu on the first geng day after the beginning of autumn. Since geng days recur every ten days while the span from solstice to Liqiu is not a fixed number of days, the middle fu runs either ten or twenty days, and the whole hot-period block swings between thirty and forty days. Xiaoshu usually falls near the opening of the first fu, but the two are computed separately.
Why geng in particular? The traditional answer is elemental: geng belongs to metal, summer belongs to fire, and metal withdraws before fire. Fu means to lie low or hide, which is read as metal going into concealment, so geng days mark the stage of that retreat. Records of fu days appear as early as the Qin and Han, and later calendars kept the practice and regularised the arithmetic. On the term name itself the Yueling Qishi'er Hou Jijie line of commentary is blunter: shu means heat, heat divides into lesser and greater, and here it has not yet reached its height.
Xiaoshu is a sectional term, so the month pillar turns from wu (午) to wei (未), the last such change of the summer. Wei hides ji earth, ding fire and yi wood, with ji earth governing, so fire begins converting into earth in the relation classical texts call fire generating earth. The branches chen, wei, xu and chou are known as the season-closing months, each said to have earth in command for the eighteen days before the next beginning, and one line of teaching singles out wei as long summer, a damp-earth season of its own. Whether wei is read as fire or as earth differs by school.
The boundary between lesser and greater heat is itself a convention. Old commentaries split them by position within the month, but actual temperature does not cooperate: the southern peak can arrive before Xiaoshu and the northern one after Dashu. Because the middle fu varies in length, even the question of which term is hotter has no fixed answer. Tradition is fairly relaxed about this. The calendar supplies a scale, what the weather does in a given year is a separate matter, and the two were never expected to match point for point.
The lesser in this term's name measures it against Dashu, not against spring or autumn, so it does not amount to not very hot yet: most regions are already in their annual high-temperature stretch by now and the fu days have usually begun. The customs of airing books and clothing in the sun get taken for something the calendar prescribes. They are practical habits that exploit strong midsummer light against mould and insects, timed differently from place to place, and no almanac requires them.
Its pentads are warm winds arriving, crickets moving to the walls, and hawks beginning to strike. No coolness is left in the wind; crickets abandon overheated fields for the shade of walls and eaves; hawks take to circling and hunting in the high air. All three describe creatures repositioning themselves against heat, a matter of what to avoid and what to seize. An older line of transmission through the Book of Rites gives the third pentad as young hawks learning to fly, a different stage of the same observation rather than a contradiction.
By stems and branches, not by temperature. The first fu begins on the third geng day after the summer solstice, the middle fu on the fourth geng day, and the last fu on the first geng day after the beginning of autumn, with the last period fixed at ten days. Because geng days recur every ten days while the interval from solstice to Liqiu varies, the middle fu is either ten or twenty days, giving a total block of thirty or forty days.
No. Xiaoshu is the solar term at 105 degrees of celestial longitude and falls around July 6 to 8 every year. The first fu is counted from the third geng day after the summer solstice, which shifts from year to year. The two often land close together and occasionally almost coincide, but in other years they are several days apart, so the start of the fu days has to be looked up in that year's stem-and-branch calendar.
Yes. Xiaoshu is a sectional term, so the month pillar turns from wu month to wei month at the moment it begins, the last such change of the summer. Wei is governed by ji earth and also hides ding fire and yi wood, so the month command moves from fire in its proper seat toward the handover from fire to earth, changing the basis for judging strength and choosing favourable elements. Births on the day itself need the exact crossing time.
The traditional explanation is elemental. Geng belongs to metal, summer is the season when fire is strongest, and metal retreats in the presence of fire. Fu itself means to lie low or hide, so geng days were chosen to mark the stage at which metal withdraws. It is a piece of calendar reasoning that joins the sexagenary cycle to the five phases. References to fu days appear as early as the Qin and Han, and later calendars standardised the count.
By the calendar's own naming Dashu represents the crest of the heat, and in most years and most regions that holds. Actual temperature, however, depends heavily on the year and the place: southern China can peak before Xiaoshu while northern China sometimes peaks after Dashu. Since the middle fu also varies in length, there is no fixed answer. The solar terms provide a scale; what a particular summer does is a separate question.