History of mathematics the Roman Kingdom and included 356 days plus a leap year every other year. In contrast, the lunar calendar of the Republican era contained 355 days, roughly ten-and-one-fourth days shorter than the solar year, a discrepancy that was solved by adding an extra month into the calendar after the 23rd of February. This calendar was supplanted by the Julian calendar, a solar calendar organized by Julius Caesar (100–44 BC) and devised by Sosigenes of Alexandria to include a leap day every four years in a 365-day cycle. This calendar, which contained an error of 11 minutes and 14 seconds, was
Julian calendar ordinary years in the Roman Republican calendar, from January to December, were: Sacrobosco then thought that Julius Caesar added one day to every month except February, a total of 11 more days to regular months, giving the ordinary Julian year of 365 days. A single leap day could now be added to this extra short February: He then said Augustus changed this, by taking one day from February to add it to Sextilis, and then modifying the alternation of the following months, to: so that the length of "Augustus" (August) would not be shorter than (and therefore inferior to) the