The leap year: keeping time in AD 2016 & AM 5776

This month we have the bonus of an extra day – February 29th – the ‘intercalary’ day which is added to the calendar in a leap year. It’s a reminder that, in a digital age, we still map out our days according to the solar and lunar year, following ancient patterns of marking time.

Solar eclipse [Rare Books quarto QB 585 ]

Solar eclipse [Rare Books quarto QB 585 ]

It was Julius Ceasar who gave us the ‘leap year’ when he reformed the calendar in 46 B.C. His advisers knew that it took roughly 365 ¼ days for the earth to travel once around the sun – but the calendar only allowed for 365. By adding an extra day every four years, the Julian calendar would keep in step with the solar year. In fact, this was an over-estimate, and the calendar began to drift again – until 1582, when Pope Gregory XIII decreed another reform – giving us the Gregorian calendar that we use today. *

Why was the extra day added in February rather than December? – The old Roman year began in March, so it made sense to Ceasar to insert a day towards the end of the year, in February. Although we gain a day on the 29th, for the Romans it was six days before the 1st or Calends of March – 24th February – which was counted twice. They called this day the ‘bis-sextile’ day – literally, the ‘double sixth day’ before the Calends of March; while the leap year was the annus bissextilis.

Frontispiece of Merlinus Liberatus [Rare Books Rosicrucian AY 751]

Frontispiece of Merlinus Liberatus [Rare Books Rosicrucian AY 751]

This is the frontispiece for an Almanack of 1699 from our Special Collections. It is a year book containing information on the “motion of the planets, remarkable conjunctions, lunations, eclipses, meteorological and astrological observations”. The author tells us that 1699 was “the third year after the bissextile” or leap year. Using the Christian era, it was the year “of our blessed saviour’s incarnation 1699” – Anno Domini 1699 – but it was also “from the creation of the World according to the best of History, 5648”.

The latter is the Jewish era, Anno Mundi (“in the year of the world”) or in Hebrew “from the creation of the world”. It is still in use today – AM 5776 began at sunset on 13 September 2015 and will end at sunset on 2 October 2016. Many of the Jewish archive collections at Southampton follow the Jewish era for the numbering of years. The Jewish calendar is a lunar one, adjusted to bring it in line with the solar year by the addition of intercalary months in a complex 19-year cycle of leap years; and note that the Jewish year does not begin on January 1st.

*So although the Gregorian calendar is the dominant civil calendar across much of the world today, it is not universal, and it was not adopted by all countries at the same time: many Catholic states obeyed the papal decree in the sixteenth century, but Great Britain, Ireland (and the dominions) waited until 1752; in Russia it was 1918. For the historian, reading the archives of different states and faiths across the centuries, this is an important consideration: the Julian calendar is currently 13 days behind the Gregorian calendar!

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