Gregorian calendar
DOI: 10.1063/PT.5.031321
On this date in 1582, Pope Gregory XIII (pictured) implemented a reform of the Julian calendar that stopped the drifting of the dates of the equinoxes and solstices. It did so by adjusting the number of leap years, which now occur every 4 years, except on centuries that are divisible by 100, unless they are also divisible by 400 (hence 2000 being a leap year but 1900 and 2100 not). In the countries that adopted the new calendar today, the following day became 15 October. The Gregorian calendar is now the most widely used civil calendar in the world. (Image: 17th century engraving by E. Hulsius, stored in the Smithsonian Institution Libraries.)
Date in History: 4 October 1582
