Build a Calendar
Your voyages will require a new understanding of time. Choose a planet and get back its months, balance its leap years by continued fractions, and certify the drift.
I · Choose your world
II · The specification
III · The leap rule
IV · The calendar
DAY = one full sunrise-to-sunrise cycle (the solar day, not the rotation period — on slow-spinning worlds they differ wildly). Leap rules are the convergents of the continued fraction of the year’s fractional day — the same mathematics that turns Earth’s 0.24219-day remainder into 1/4 (the Julian rule), 7/29, 8/33 (the old Persian cycle) and 31/128. Against the mean tropical year, the Gregorian 97/400 drifts about a day in 3,200 years. 31/128 fits that average better, but over such spans the year’s own length changes faster than 31/128 drifts. The Gregorian rule was tuned to the spring-equinox year, which it fits better. “Beats Earth’s Gregorian” means more of the world’s own years pass before its seasons slip a day. Drift figures assume the orbital numbers hold, and stop where the precision of the numbers entered runs out: each figure is taken as good to half a unit in its last decimal place, and whole numbers typed without a decimal point are taken as exact. A few presets carry their own, coarser uncertainty (Pluto’s year is known only to about 50 days), which applies until the figure is edited.