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Perhaps the most surprising thing about mathematics is that it is so surprising. The rules which we make up at the beginning seem ordinary and inevitable, but it is impossible to foresee their consequences. These have only been found out by long study, extending over many centuries. Much of our knowledge is due to a comparatively few great mathematicians such as Newton, Euler, Gauss, or Riemann; few careers can have been more satisfying than theirs. They have contributed something to human thought even more lasting than great literature, since it is independent of language.
In N. Rose, Mathematical Maxims and Minims, Raleigh NC: Rome Press, 1988.
Servois' 1813 Perpetual Calendar, with an English Translation
Gergonne, editor of the Annales, attached the following footnote to page 88 of Servois'  article:
Here, Gergonne used the superscript \(j\) to abbreviate the word jours (days).
The process of intercalation refers to the insertion of a leap day in a given year to keep the calendar synchronized with the astronomical year [Richards]. For instance, the Julian calendar had an intercalation of one leap day added every four years, which gave a mean of 365.25 days per year. The Gregorian intercalation consists of a leap day added only in years that are divisible by 4, except that years that are divisible by 100 do not get a leap day unless they are divisible by 400. This intercalation rule gives the Gregorian calendar a mean of 365.24 days per year. However, the Gregorian intercalation is off by 26 seconds per year, so that after about 3000 years, the calendar would be about one day ahead of the astronomical year [Richards].
In his footnote, Gergonne referred to the Persian intercalation, which is eight intercalations in thirty-three years. In other words, every thirty-three years eight leap days would need to be added. This gives an error of 19.45 seconds as opposed to the Gregorian error of 26 seconds [Richards]. This intercalation is attributed to the mathematician Omar Khayyam (1048-1131), who was appointed by Sultan Malik Shah of Khorasan (in Persia) to reform the calendar in about 1079 CE [O’Connor and Robertson]. According to O'Connor and Robertson , Khayyam measured the length of the year to be 365.24219858156 days, which was very precise given that modern astronomical precision allows us to measure the solar year at 365.242190 days.
Petrilli, Jr., Salvatore J., "Servois' 1813 Perpetual Calendar, with an English Translation," Loci (June 2012), DOI: 10.4169/loci003884