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 in the season where the allocation leaves a deficit, which in the current period is the third and places it at the 90° solstice. 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 seasonal lengths vary slowly with the precession of perihelion. The document establishes the recalibration procedure and tabulates it to the year 10000: ten reallocations that move no month, a single structural reform, and a seasonal error that stays below one day across the whole window. The calendar is single and global. Its cardinal points are phenomena valid for the whole planet; 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. AI assistance. This article was drafted and revised in dialogue with Anthropic's Claude, which was also used to generate the figures and to check the arithmetic with assertion-gated scripts. Every bibliographic reference and every computed figure was verified by the author. See the Acknowledgements for detail.