On September 23, 1699 (O.S.), the German council of protestant theologians (Corpus Evangelicorum) at the Reichstag in Regensburg adopted a reform of the Julian calendar that was still employed in most of the protestant states in Germany. The new calendar, popularly known as the Improved Calendar (Verbesserte Calender), would be synchronized with the Gregorian calendar by deleting ten days between February 18 and March 1, 1700, and would be kept in step with the Gregorian calendar by adopting the same leap day rule.
However, there was a subtle difference in the Easter reckoning as, at the recommendation of the astronomer-mathematician Erhard Weigel (1625-1699) from Jena and advocated by his pupil Gottfried Wilhelm von Leibniz (1646-1716) and several other German astronomers and mathematicians, the date of Easter Sunday in the new calendar would no longer be derived from ecclesiastical tables (based on the mean motions of the Sun and the Moon) but from the astronomical times of the spring equinox and the following full moon. The astronomical calculations for determining these dates were to be based upon Johannes Keplers Tabulae Rudolphinae (1627) and the meridian of Tycho Brahes former observatory Uraniborg on Hven.
An additional provision based on an erroneous interpretation of the acts of the First Council of Nicaea of 325 stipulated that if the astronomical Easter Sunday happened to coincide with the first day of the Jewish Passover feast (15 Nisan), Easter would be postponed by a week to the next Sunday.
Proposals to adopt the modified calendar were also sent to the governments and heads of state of other European countries which still (partly or completely) employed the Julian calendar but only Denmark (which then also included present-day Norway), the Netherlands and the protestant cantons of Switzerland decided to adopt the German proposal.
As astronomers rapidly discovered, Easter Sunday dates calculated in this fashion would occasionally differ from the dates obtained from the Gregorian reckoning and that (without taking the Passover postponement rule into account) such occurrences would take place in 1700, 1724, 1744, 1778, 1798, 1802, 1805, 1818, etc. Already in the first year of the calendar reform (1700), the astronomical Easter Sunday would occur a week before the Gregorian Easter Sunday but as it would then coincide with the Jewish Passover feast it was postponed to the next week (thus coinciding with the Gregorian date).
The following table lists all cases between 1700 and 1900 when the date of astronomical Easter Sunday differed from that calculated by the Gregorian Easter rules. The times for the spring equinox and the astronomical Full Moon are calculated from modern luni-solar ephemerides and are referred to the meridian of Tycho Brahes observatory Uraniborg (Hven), which was adopted to be 12° 42' (0h 50.8m) East of Greenwich.
|Year||Spring Equinox||Astronomical Full Moon||Easter Sunday||Jewish
|Date||h m||Date||h m||Weekday||Astron.||Gregor.|
|1700||20 March||15 17||3 April||19 05||Saturday||4 April||11 April||4 April|
|1724||20 March||10 59||8 April||16 21||Saturday||9 April||16 April||8 April|
|1744||20 March||07 13||28 March||09 44||Saturday||29 March||5 April||28 March|
|1778||20 March||13 12||11 April||21 10||Saturday||12 April||19 April||12 April|
|1798||20 March||09 30||31 March||23 34||Saturday||1 April||8 April||1 April|
|1802||21 March||08 39||18 April||03 27||Sunday||25 April||18 April||17 April|
|1805||21 March||01 55||14 April||00 37||Sunday||21 April||14 April||14 April|
|1818||21 March||05 41||22 March||14 59||Sunday||29 March||22 March||21 April|
|1825||20 March||22 16||3 April||07 18||Sunday||10 April||3 April||3 April|
|1829||20 March||21 33||19 April||07 12||Sunday||26 April||19 April||18 April|
|1845||20 March||18 35||23 March||21 10||Sunday||30 March||23 March||22 April|
|1876||20 March||07 01||8 April||20 30||Saturday||9 April||16 April||9 April|
|1900||21 March||02 30||15 April||01 53||Sunday||22 April||15 April||14 April|
Note that in the years 1700, 1778, 1798 and 1876 (indicated with a different colour in the above table), the astronomically calculated dates of Easter Sunday fell on the first day of Jewish Passover (Nisan 15).
In 1724 and 1744, Easter was indeed celebrated a week earlier in the protestant regions of Germany, Switzerland (only in 1724) and Denmark. In the Netherlands, no one seems to have been aware of the fact that the astronomical Easter Sunday date could occasionally differ from the Gregorian Easter Sunday date and during the 18th century and afterwards Easter was always celebrated on the Gregorian dates.
In 1776, two years before the next anomalous astronomical Easter Sunday would occur (though the Passover postponement rule would have resulted in its coincidence with the Gregorian Easter Sunday), the protestant states of Germany finally agreed to adopt the Gregorian Easter reckoning. Together with the catholic states of Germany, who agreed to renounce the papal authorship of the calendar, both protestant as catholic communities in Germany henceforth observed the Common Imperial Calendar (Allgemeiner Reichs-Kalender) which in every detail was identical with the Gregorian calendar.
The following table lists the years between 1700 and 1845 when the improved Easter dates differed from the Gregorian Easter reckoning and the regions where these were observed.
|Improved Easter reckoning observed in||Gregorian
|1724||9 April||Yes||Yes||Yes||||||16 April|
|1744||29 March||||Yes||Yes||Yes(*)||Yes(*)||5 April|
|1802||25 April||||||||Yes||Yes||18 April|
|1805||21 April||||||||Yes||Yes||14 April|
|1818||29 March||||||||Yes||Yes||22 March|
|1825||10 April||||||||No||Yes||3 April|
|1829||26 April||||||||No||Yes||19 April|
|1845||30 March||||||||||Yes||23 March|
(*) Reckoned as 19 March in the calendar of Sweden and Finland (click for more details)