![]() ![]() Difficulties have resulted from the fact that these three periods are not commensurable that’s a fancy way of saying that one does not divide evenly into any of the others. The natural units of our calendar are the day, based on the period of rotation of Earth the month, based on the cycle of the Moon’s phases (see later in this chapter) about Earth and the year, based on the period of revolution of Earth about the Sun. Second, to be useful to a large number of people, a calendar must use natural time intervals that everyone can agree on-those defined by the motions of Earth, the Moon, and sometimes even the planets. First, it must keep track of time over the course of long spans, allowing people to anticipate the cycle of the seasons and to honor special religious or personal anniversaries. There are two traditional functions of any calendar. ![]() ![]() Long before the era of digital watches, smartphones, and fitness bands that tell the date, people used calendars to help measure the passage of time. “What’s today’s date?” is one of the most common questions you can ask (usually when signing a document or worrying about whether you should have started studying for your next astronomy exam). Explain the origins of our modern calendar.Understand how calendars varied among different cultures.By the end of this section, you will be able to: ![]()
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