Calendars

The History of Timekeeping

Course 2 · Chapter 3 · Calendars

In September 1752, the British Isles and their American colonies went to bed on Wednesday 2 September and woke on Thursday 14 September. Eleven days had been removed from the calendar, and the tale of rioters crying "give us our eleven days" has been told ever since, although historians doubt that there were real riots over it (check). Scotland, for its part, had already moved the start of the year to 1 January in 1600 (check), a century and a half before England. This chapter explains why calendars need such repairs, follows the main steps from the Egyptian and Roman calendars to the Julian reform, and shows how and why the Gregorian calendar replaced it.

Three Kinds of Calendar

As Chapter 1 showed, the Moon and the Sun do not fit together neatly. Calendars choose one of three ways of dealing with this.

Lunar

Months follow the Moon, and the year is twelve of them, about 354 days. The seasons drift. The Islamic calendar is the best-known example still in use.

Solar

The year follows the seasons, and months are only a convenient division. The modern civil calendar is of this kind.

Lunisolar

Months follow the Moon but an extra month is added every few years to keep in step with the seasons. The Hebrew calendar and the traditional Chinese calendar work this way (check the details).

From Egypt to Rome

The ancient Egyptians are often credited with a civil calendar of twelve months of thirty days plus five extra days, making 365 (check). It had no leap day, so it slowly slid against the true solar year, by about one day every four years.

The early Roman calendar had a short year, traditionally said to have started in March, with an unreliable system of added months managed by priests. The officials who controlled the insertions could and did use them for political ends, extending or shortening the term of a magistrate (check). By the time of Julius Caesar the calendar had slipped badly out of step with the seasons.

The Julian reform In 46 BC, Julius Caesar, advised by astronomers including Sosigenes of Alexandria, introduced a solar calendar of 365 days with an extra day every fourth year (check the details). The year of transition was exceptionally long, sometimes called the "year of confusion" (check). The Julian calendar took the year as 365.25 days. This was close, but about eleven minutes too long (check the figure), and over centuries the error added up.

Why the Julian Calendar Drifted

The true solar year, from one spring equinox to the next, is nearer 365.2422 days. The Julian calendar's extra 0.0078 days a year amounts to about one day in 128 years. By the sixteenth century the equinox had slipped about ten days from where the Church fathers had placed it in the fourth century (check). That mattered because the date of Easter was tied to the spring equinox, and Easter was drifting later into the year.

The Gregorian Reform

Pope Gregory XIII issued a bull in 1582 reforming the calendar (check the date). The main changes were these.

Change	What it did
Skipping ten days	In the countries that adopted it first, 4 October 1582 was followed by 15 October (check)
New leap rule	A year divisible by 4 is a leap year, except century years, which must also be divisible by 400
Result	An average year of 365.2425 days, off by about a day in three thousand years or so (check)
Easter	The rules for calculating Easter were adjusted to match

Under the new rule, 1900 was not a leap year but 2000 was.

Politics, not astronomy, set the pace The reform was issued by a Pope, so Protestant countries were slow to accept it. Britain and its colonies switched only in 1752, when the gap had grown to eleven days. Russia changed after its revolution in 1918, and Greece in 1923 (check the dates). For a long period, travellers crossing Europe could find the date changed at a border.

Other Quirks We Still Live With

Month names. September to December are named for the numbers seven to ten, a leftover from when the Roman year began in March (check).
July and August. Named for Julius Caesar and Augustus, who gave their names to months (check).
The seven-day week. It does not match any astronomical cycle exactly, and comes from older traditions whose details are debated (check).
The new year. In Britain, the legal year began on 25 March until 1752 in England, and on 1 January in Scotland from 1600 (check).

Reading old dates Documents from the period of change may carry two years, such as "1688/9", or be marked "Old Style" or "New Style". When using old records, always check which calendar the writer was using before comparing dates, since the same event can carry two different dates.

Exercises

Exercise 1

State the Gregorian leap-year rule, and use it to decide whether each of these was or will be a leap year: 1600, 1700, 1900, 2000, 2024, 2100.

→ Solution

Exercise 2

If the Julian calendar gains one day every 128 years, roughly how many days of error would build up between 325 and 1582? Compare your answer with the ten days that the 1582 reform removed.

→ Solution

Exercise 3

Pick a historical event dated in a book or museum label and find out whether its date is given in the Old Style or New Style. Write two sentences on what you found and how the two dates differ.

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Quick Reference

Calendar types: lunar, solar and lunisolar
Julian (46 BC): 365 days plus a leap day every fourth year; year taken as 365.25 days
True year: about 365.2422 days, so the Julian calendar gained a day in about 128 years
Gregorian (1582): ten days dropped; century years leap only if divisible by 400
Britain: changed in 1752, removing eleven days; Scotland moved New Year to 1 January in 1600
Old Style / New Style: always check which one a source uses
Check: reform dates, the Egyptian and Roman details, named advisers, adoption dates by country

Next chapter: The Mechanical Clock, which covers the escapement, the first tower clocks and the monastic need for bells.