LEAP YEARS

one rule, a cycle of four hundred years and the ninety-seven extra days in it

Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.

Three sizes in one archive — from a sheet on the fridge to a poster on the wall.

Formats: A4 · A3 · A2
What you get: Print-ready PDFs, one per size, with every letterform converted to outlines — nothing to install, nothing to shift.
Subject: maths · everyone · reference · classroom

The rule and the length of the tropical year are from the Astronomical Applications Department of the U.S. Naval Observatory. Every other figure here is computed from the rule itself: the cycle, its days, its weekdays and its thirteenths.

THE CALENDAR REPEATS IN FULL

The 146,097 days of the cycle divide by seven without a remainder — exactly 20,871 weeks. The Gregorian calendar therefore repeats in full every 400 years: the same year 400 years later has the same weekdays on the same dates.

And from that follows something one does not expect of a calendar: the 4,800 thirteenths of the cycle are spread unevenly over the weekdays, and the commonest — 688 times — is Friday. The rarest, 684 times each, are Thursday and Saturday.

On the sheet

The rule and the length of the tropical year are from the Astronomical Applications Department of the U.S. Naval Observatory. Every other figure here is computed from the rule itself: the cycle, its days, its weekdays and its thirteenths.
Colour carries meaning, but meaning never rests on colour alone — the sheet reads in black and white.
Nothing about how it was made: the sheet carries only what the reader needs.

Printing

Three sizes in the archive: A4 — 210×297 mm, A3 — 297×420 mm, A2 — 420×594 mm.
All text is converted to outlines. No fonts to install, and nothing shifts on another machine.
White background and sparing fills — it prints on a home printer without draining the cartridge.
Margins of at least 10 mm: a home printer cannot print to the edge.
No stroke thinner than 0.3 mm, so nothing disappears in print.

Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 7 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.

LEAP YEARS

THE CALENDAR REPEATS IN FULL

On the sheet

Printing

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MULTIPLICATION TABLE

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