Calendars have always been based on the Sun, Earth, and Moon. Ancient people observed that the position of the Sun in the sky changed with the seasons. They also noticed that the stars seemed to change position in the night sky throughout the year. They saw that the Moon went through phases where it appeared to change its shape over the course of about 30 days. All of these astronomical phenomena happened regularly. As a result, people began to organize their lives around these periodic occurrences, thereby forming the basis for calendars.

This undated calendar with Latin text tracks Venus as seen by Earth throughout the year. It depicts an unhappy Sun in the center with rings representing the Zodiac, months, dates, and Venus' phases.

Through the centuries, calendars have changed because of astronomy, politics, and religion. Over time, astronomers learned more about...