Keeping Track of Time


You might think that a calendar is only a piece of paper that identifies the days of the week, shows when months start and end, and helps you remember special days like birthdays and holidays. But did you know that there is also a calendar in the sky? The movements of the sun, moon, and stars are linked to the passage of time. Here are some of the ways people in various times and cultures have used this sky calendar to record and order time.

The ancient Chinese calendar is one of the oldest calendars still in use. It is made up of twelve-year cycles, and a name is given to each year in the cycle. The current Chinese year, 4688 (1990 on the Gregorian Calendar), is the Year of the Horse. Can you figure out the name of your birth year?

The Aztecs used a complex calendar based on the movements of the sun, moon, and stars. This calendar had eighteen months of twenty days each, and the days had names like “Crocodile,” “Flower,” and “Earthquake.”

For Hebrews, each new day begins at sunset. Their months are based on the movements of the moon, rather than the sun, and they observe the Sabbath on Saturday, the seventh day of their week.

The Egyptians who lived in the valley of the Nile River had to know when the river was going to flood so that they could save their crops, their property, and their families. They noticed that the flooding occurred every year when the star Sirius rose in the east, so they created a calendar in which a new year began shortly after the appearance of this star.

One of the oldest and most unusual devices believed to have been used for keeping track of the seasons and the years is a circle of huge stones called Stonehenge, which rests on the Salisbury Plain in England. On the longest days of the year (June 21–24), you can watch the sun rise over the Heel Stone and cast a long shadow over the Altar Stone.

When Julius Caesar became the ruler of Rome, the Roman calendar was out of line with the 365 1/4-day solar year. In order to correct the calendar, Julius Caesar ordered that 90 days be added to the year 46 B.C., creating a 445-day year. Can you imagine what a long year that must have been for the Roman people? Julius Caesar also decreed that one day be added to the calendar every four years to make a 366-day leap year. This plan worked for a while, but because the solar year isn’t exactly 365 1/4 days long, eventually the calendar again became out of line with the sun’s position.

In 1582 the calendar was again corrected when Pope Gregory XIII deleted ten days from the year, decreeing that October 5 would become October 15. Today, while many countries still use their ancient calendars for religious events, and scientists adjust the measurement of time by means of an atomic clock, most of the world use the Gregorian calendar to conduct daily affairs.

[illustration] Illustrated by Scott Greer