The calendar in use by the modern world is known as the Gregorian Calendar after Pope Gregory XIII (1572-1585) in whose reign it was introduced. "Gregorian" in this context is therefore not the same as in Gregorian Chant, the name given to the plainchant of the Roman church after the reforms of Pope Gregory I (590-604).
The calendar in use before the Gregorian Calendar was the Julian Calendar, named after its instigator, Julius Caesar, in 45BC. Most of Catholic Europe switched to the Gregorian Calendar in 1582 or thereafter, but England did not switch until 1752. There was therefore a period of 170 years when dates in England and continental Europe were out of synchronisation. This meant that contracts between English and Continental entities required dual dating and Easter was usually celebrated on different dates.
In order to explain the shortcomings of the Julian calendar and why the Gregorian reforms were required it is first necessary to understand the relationship between the day and the year. We commonly think of a year as consisting of 365 days, or 365.25 days recalling that every fourth year is a leap year and has an extra day - February 29th. In fact this relationship is not exact and the cumulative errors in the 16 centuries that the Julian Calendar prevailed meant that corrective reforms became necessary.
The day is the time it takes for a night and a day (a period of dark and light) to elapse. A solar day is divided into 24 hours. The passing of day and night is mainly governed by the earth's rotation on its own axis - running approximately between the north and south poles. The earth spins at a speed of one revolution in 23 hours, 56 minutes and 4 seconds (a sidereal day). To understand why the speed of rotation is not 24 hours we need to remember that in addition to spinning on its own axis the earth is also orbiting the sun.
The solar year is the time it takes the earth to orbit the sun. The earth orbits the sun every 365.24219 days - a period which is 11 minutes and 12 seconds shorter than the 365.25 days that are assumed for the four yearly leap year correction required to keep the solar year and the calendar year in step.
If the angle of the earth's rotation (the daily spinning) is compared with the plane in which the earth is orbiting the sun, it is found that the axis of the rotation is offset from the perpendicular by 23.4 degrees. This explains the seasonal variations that are experienced within the course of each complete year. In summer time the northern hemisphere (or "top" of the earth) is tilting towards the sun and therefore experiences warmer weather.
Half a year later during winter it is the southern hemisphere that is tilting towards the sun and experiencing warmer weather. In Australia it is therefore possible to celebrate Christmas on the beach!
Now we can explain why a day is 24 hours long, but the time of the earth's rotation is 3 minutes and 56 seconds less than 24 hours.
First, consider what would happen if the earth were not spinning at all, but simply orbiting the sun. During the course of the year (365.24219 days) the earth would experience one very long period of "night" and "day" as it moved around the sun and the different areas of the earth's surface alternately pointed towards and away from the sun. The earth's annual orbit around the sun therefore contributes to the passing of day and night and has to be factored into the calculation of the earth's speed of spin. The earth's experience of day and night is therefore made up of (i) spinning on its own axis - 23h 56' 4" and (ii) yearly rotation of the earth - 24 hours divided by 365.24219 = 3' 4".
Does this annual "night and day" effect caused by the earth's orbit of the sun add or subtract to the effects of night and day resulting from the earth's spin? Looking down on the earth, towards the north pole, the earth's spin is in an anti-clockwise direction and the earth's orbit around the sun is also anti-clockwise. The earth therefore has to spin slightly faster than once ever 24 hours in order to combat the effect of the sun's contribution to the day.
Since the earth does not orbit the sun in an exact number of days the solar year and the calendar year can only be kept in step with periodic corrections. The solar year is more than 365 days - by almost a quarter of a day - and so it follows that that every four calendar years an extra day is required to make up the difference. If no correction were made the calendar year would come around more quickly than the solar year. This would mean that the "calendar spring" would arrive before the real spring and so it would seem that the warmer weather was later and later each year. Eventually, after one and a half millennia, the seasons would be reversed and the Northern hemisphere would experience cold summers and warm winters!
Since the solar year is in fact fractionally shorter than an extra quarter of a day it also follows that correction every four years is slightly too much. Therefore there are some leap years that are missed out - every century or so (but see below why it is not every century).
The Julian Calendar
The Julian Calendar was introduced by Julius Caesar in 45BC as a refinement to earlier systems. The need for leap years was understood at that time and it was prescribed that every fourth year was to be a leap year. In fact in the early days there was confusion about the leap years and errors were made in its application. It is only since 8AD that the regular pattern of once every four years was maintained.
However the real year is not 365.25 days but only 365.24219 days - 11 minutes and 12 seconds shorter. The cumulative effect of this difference to the middle of the sixteenth century can be calculated by multiplying the number of elapsed years by 11 minutes and 12 seconds. Allowing for the early confusion over leap years the result is that by the middle of the 16th century the calendar year was approximately 10 days adrift of the real year. Because the Julian calendar year's compensation was slightly too great it meant that it was ahead of the real year and therefore days had to be "lost" in order to correct the position.
The Gregorian Calendar
In order to correct the cumulative errors in the Julian Calendar, Pope Gregory's papal bull Inter gravissimas of 24th February 1582 prescribed that Thursday 4th October 1582 should be followed by Friday 15th October. This gave continuity in the days of the week but meant that 10 calendar days had been omitted. Provision for not over compensating in the future was made by ruling that each year at the beginning of the new century was not to be a leap year - e.g. 1700, 1800, 1900. On its own, this secondary level of correction would have resulted in a small under-correction and so a third level of refinement was included which prescribed that every fourth century the century leap year rule should not apply - so in fact 1600 and 2000 were leap years after all. Similarly 2400 will be a leap year and every four centuries thereafter. Further correction beyond these measure will not be required for another couple of millennia or so.
It might be thought that Gregory would have wished to lose 11 days rather than 10 since this is the cumulative error from Julian's introduction of his calendar in 45BC. In fact Gregory's objective was to restore the position as at 325 when he believed that the Council of Nicaea had set out the rules for the calculation of Easter.
Believing that this was all popish nonsense, protestant England did not adopt the Gregorian reforms in 1582 and instead remained with the Julian calendar until 1752. In this year Parliament ordered that Wednesday 2nd September 1752 should be followed by Thursday 14th September. Thus 11 days were "lost" from the calendar bringing England in line with much of Continental Europe.
Note that in 1582 Continental Catholic Europe list 10 days and in 1752 England lost 11 days. The difference is because under the Gregorian Calendar 1700 was not a leap year whereas in England, under the Julian calendar, it was. 1600 was not a leap year under either calendar for the reasons described above.
The first day of the year
A further complication for the historian reading dates is that the beginning of the new year has not always been considered 1st January. In England until the introduction of the Gregorian Calendar the new year began on March 25th - the Feast of the Annunciation and a quarter day.
The Tax Year
Until the change in calendar took place in 1752, the tax year coincided with the calendar year; that is the last day of the tax year was also 24 March. Since 11 days were lost in 1752, had the last day of the tax year remained at 24 March 1753 it would have been 11 days short. The government would have received around 3% less tax revenue (corresponding to the lost 11 days) and the populous would have had to pay their tax 11 days sooner than otherwise. It therefore suited both sides to move the end of the tax year by 11 days to 4 April.
According to the Julian calendar, 1800 would have been a leap year (as with every fourth year), but under the Gregorian calendar, as an '00' century year, it was not a leap year. Using similar logic as in 1752/3, the end of the tax year was therefore extended by a further day to 5 April to compensate for not having had the extra day - 29 February.
Had this logic continued, 1900 would have seen the tax year extended to end on 6 April. However, whereas in 1800 the Julian calendar was still within living memory a century later such 'corrections' were no longer considered important.
As the first day of the year, March 25 was one of four quarter days; the days of the year when quarterly accounting took place. To this day much Long Leasehold property in England uses the quarter days as the due day for ground rents and service charges.
The quarter days are:
The first three of these are easy to remember: March has FIVE letters; June has FOUR letter September has NINE letters.
Note that the Scottish system is different. They are called term days:
The Scottish dates were altered by statute in 1990.
Implications for Historians
The change of calendars and the change of reckoning of the first day of the year means historians must beware when dealing with dates.
There are two books which are essential reading for historians interested in understanding calendars and dates.
The Oxford Companion to the Year. An exploration of
calendar customs and time-reckoning
This wonderful book is in two halves. The first section takes each day of the year in turn and describes holidays, anniversaries, customers, literary references etc. The second section gives a history of the calendar and describes how dates are calculated and converted.
A Handbook of Dates for students of British History
This classic is a 1999 revision of the original 1945 publication. It contains explanations of the Old and New styles (Julian and Gregorian) and has many tables of dates for reference.
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