Gottfried Wilhelm was born in the evening hours of 21 June 1646 in the building of "Großes Fürstenkolleg" on Ritterstraße in Leipzig. The following entry by his father Friedrich Leibniz can be found in the family chronicle: "21. Junii am Sontag 1646. Ist mein Sohn Gottfried Wilhelm, post sextam vespertiam ¼ uff 7 uhr abents zur welt gebohren, im Wassermann."* Two days later, the infant was baptised in St. Nicholas Church.
Nowadays, we celebrate Leibniz's birthday on 1 July. But why?
Time scales are determined by astronomers. Since the beginning of time, people have tried to illustrate the cycle of nature by means of a calendar dividing the year into months, weeks and days. One year is defined as the time it takes the Earth to orbit the Sun. A complete orbit takes exactly 365 days, 5 hours, 48 minutes and 45.252 seconds – a quarter of a day longer than our calendar year. For this reason, Julius Caesar first implemented a periodic leap year in 45 BC. Since this rule was imprecise, the date “21 March” (first day of spring) slowly drifted away from the astronomical event of the spring equinox in the Northern Hemisphere. By the 16th century, the discrepancy in the Julian calendar had added up to 10 days.
This became a problem - especially when setting the dates of moveable religious festivals. In particular, Easter is determined by astronomical events (Easter Sunday = first Sunday after the first full moon following the first day of spring). In 1582, Pope Gregory XIII solved this problem by omitting the days between Thursday 4 October and Friday 15 October. The Gregorian calendar was soon adopted in Catholic Spain and Portugal as well as in most parts of Italy, while the Protestant territories refused to accept it. In the Protestant city of Hannover, Leibniz later joined the much-needed efforts to reform the existing system, promoting an interdenominational solution. Along the way, he identified a large market for printed calendars that were much sought-after among the public. In addition to dates, these calendars contained popular astrological tips - such as the perfect day for a haircut, for weaning infants or for bloodletting. With the construction of the Berlin observatory in 1700 and a "calendar patent" (monopoly-like privilege), he succeeded in acquiring funding for establishing a "Society of Sciences" (today's Berlin-Brandenburg Academy of Sciences and Humanities), which brought together the humanities and natural sciences. During the Diet of Regensburg in 1699 and encouraged by Leibniz's initiative, the Protestant German territories finally reached an agreement and adopted the new calendar, which was quite similar to the Catholic calendar.
Henceforth, the date of Leibniz's birthday was moved by 10 days.
Happy Birthday, Gottfried Wilhelm Leibniz!
* Sunday 21 June 1646. My son Gottfried Wilhelm was born this evening at 6.45 p.m., in the sign of Aquarius.
