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Release It! by Michael T. Nygard

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Peak Season

In the middle 1500s, a Calabrian doctor named Aloysius Lilius invented a new calendar to fix a bug in the widely used Julian calendar. The Julian calendar had an accumulating drift. After a few hundred years, the official calendar date for the solstice would occur weeks before the actual event. Lilius’s calendar used an elaborate system of corrections and countercorrections to keep the official calendar dates for the equinoxes and solstices close to the astronomical events. Over a 400-year cycle, the calendar dates vary by as much as 2.25 days, but they vary predictably and periodically; overall, the error is cyclic, not cumulative. This calendar, decreed by Pope Gregory XIII, was eventually adopted by all European nations, although ...

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