Astronomy Picture of the Day . ... Advised by Alexandrian astronomer Sosigenes , Roman dictator Julius Caesar , pictured above in a self-decreed minted coin, created a calender system in 46 BC that contained one leap day every four years. ... This "Julian" calender system was used until the year 1582 when Pope Gregory XIII added that leap days should not occur in years ending in "00" except if divisible by 400, providing a further fine tuning. ... Publications with words: Julius Caesar - Leap Days . ...