As far back as five thousand years ago, the ancient Egyptians had already established a calendar with 365 days in a solar year. They divided the year into twelve months of thirty days each, and added five additional days at the end of each year. Millennia later, the Danish astronomer Tycho Brahe (1546–1601) determined the exact length of the solar year to a precision of one part in thirty million—an accuracy of one second per year!