Leap-year arithmetic is revised

Pope Gregory XIII issued the bull Inter gravissimas. It reformed leap-year rules to bring the civil calendar closer to the solar year.

The reform used astronomical calculation and a revised leap-year cycle to limit the drift of the equinox through the calendar.

The calendar illustrates how numerical models become social institutions through political and religious authority.

Catholic states adopted the reform first, while other societies accepted, adapted or resisted it at different times; calendar history was not a single global switch.

In several Catholic territories, Thursday 4 October was followed by Friday 15 October 1582. Ten numbered dates were omitted to realign calendar and season.

Events near the reform may have Old Style and New Style dates. Databases must identify the calendar before comparing dates.

European empires later carried Gregorian dating into colonies, while many communities retained religious and Indigenous calendars alongside it.

Descartes published La Géométrie as an appendix to his Discourse, linking algebraic equations with geometric curves.

The work helped establish methods later called Cartesian geometry, although modern coordinate notation developed gradually.

Publication in French rather than only Latin widened access while still excluding many people from formal scholarly networks.