Solstica Kalendaro: A proposal for a regular, solstice-anchored calendar faithful to the real lengths of the seasons

The Solstica Kalendaro is a proposal for a perpetual solar calendar of 365 days, with perfectly regular months and anchored to a solstice. Its contribution is to assign each season a different number of days, matched to its real astronomical length. Other proposals have recognised that the seasons are not of equal length, but all of them build the year symmetrically and divide the days equally; this one measures those lengths and searches for the integer allocation that comes closest to them. The year consists of twelve months of twenty-eight days — exactly four weeks each — together with four blocks placed at astronomical markers: two seven-day equinoctial periods, a fourteen-day central period around the 90° solstice, and a single closing day. The year begins at the 270° solstice. The seasons do not last the same. By Kepler's second law the Earth crosses the four quadrants of the ecliptic in 88.975, 92.720, 93.669 and 89.879 days. The transition periods do not belong to a single season: they are shared between the season that ends and the one that begins, and calibrating that split gives each season 89, 93, 93 and 90 days. An exhaustive search shows this to be the optimal integer allocation: no season departs from its real length by more than 0.67 days. The ordinary year comprises 52 uninterrupted weeks plus one day outside the weekly cycle, so that every date always falls on the same weekday. The intercalated day sits at the 90° solstice, where the allocation leaves a deficit. The intercalation rule is the Gregorian 4/100/400, which turns out to be the closest fit among the usual rules for a solstice-anchored calendar, because the solstice-to-solstice year is longer than the mean tropical year that the alternative rules target. The calendar is single and global; only the seasonal labels invert between hemispheres. The seven-day week is preserved intact, with the names and rest days of each tradition. Months and special days take Esperanto names, with no reference to any national or religious tradition. The individual elements of the design have precedents — twenty-eight-day months in Comte, the extra-weekly day in Mastrofini, astronomical anchoring in the French Republican Calendar — and the recognition that the seasons are unequal appears in contemporary proposals as well. What has not been done before is to measure that inequality and derive the structure of the calendar from it. All computations are reproducible: the seasonal lengths are derived from ephemerides, the allocation search is exhaustive, and the calendar conversion is validated by round-tripping every day of forty consecutive years.