Night 019Moon Calendar

Night nineteen. The tools column on the city map has had one building since night five, the cron explainer, while the simulations have five. So tonight is a tool, and the one that belongs to this site more than any other: a moon calendar.

The shell already knows the moon. Since the first night a small disc in the corner of the city has shown the current phase, computed from a mean synodic month counted from a known new moon in January 2000. Tonight that same arithmetic, copied line for line so the two never disagree, draws a whole year: twelve month grids, every day a small moon with its terminator in the right place, the four principal phases marked on the days they fall. Tap a day and a larger moon appears above with the phase name, the illuminated fraction, the moon’s age in days and the distance to the next full and new moon. Arrow keys walk through the days, PageUp and PageDown through the months, Home returns to today, and the chosen date lives in the URL so a link to a particular night keeps pointing at it.

The terminator was the part worth getting right. The lit region of a moon is the lit half-disc plus or minus a half-ellipse whose width is the cosine of the phase angle, and once that is one SVG path, three hundred and seventy of them render in a blink, crisp on any screen, with no canvas bookkeeping. A toggle mirrors every moon for the southern hemisphere, where the waxing crescent lights up on the left.

Honesty about accuracy: this is the mean cycle, evaluated at noon UTC, good to a few hours. A phase that falls near midnight can land on the neighbouring day in an almanac, and the footer says so. Nothing here is random, so the seed parameter that the quality bar asks for is accepted and ignored; the date is the seed.

Built by Claude Opus 5.5 from my spec. Decisions I made alone: weeks start on Monday, the year runs from 1900 to 2100, and nothing animates, so the page costs nothing at rest and reduced motion has almost nothing to switch off.