CALENDARS

People around the world celebrate New Year’s Day on January 1. But the new year begins at other times, too. The Chinese New Year begins between January 21 and February 19. The Jewish New Year begins in autumn. The Muslim New Year falls 11 days earlier each year than the last. Why? Because there are many ways to arrange a calendar.

WHY WE USE CALENDARS

A calendar is a way of measuring time to help people organize their activities. Calendars divide time into fixed periods, such as days, weeks, months, and years.

Long ago, when people learned to farm, they needed an accurate way to measure time and the changing seasons. They used calendars. Calendars helped them establish the best time to till the soil and plant crops. Calendars told them when the harvest should begin.

The time periods used in calendars are based on movements of the Earth, Moon, and Sun. But a year does not divide evenly into months or days. So people have adjusted their calendars so that certain events, such as a harvest or important holiday, always occur during the right season.

ANCIENT CALENDARS

The first calendars were lunar calendars. They were based on the Moon. One month meant the time from one full moon to the next. That’s about 29.5 days. Some early lunar calendars alternated between months with 29 days and months with 30 days.

Measuring a year was harder. People knew that it equaled about 12 months, but not exactly. For farmers, it was important to measure a year accurately. That required a solar calendar, based on the Sun.

A solar year is the time it takes for Earth to make one trip around the Sun. About 6,000 years ago, the Egyptians became the first people to measure a year as 365 days. But a solar year is just a little longer than that. In fact, it’s about six hours longer.

Over time, these extra hours add up. Solar calendars gradually lose their accuracy. After four years, for example, a solar calendar will be off by about 24 hours, or a full day.

THE JULIAN CALENDAR

In 45 bc, the Roman general Julius Caesar reformed the old Roman lunar calendar. The new calendar became known as the Julian calendar. It had 12 months and was based on the Sun. A year lasted 365 days. Every fourth year a day was added, making the year 366 days long.

These longer years are called leap years. This is because the extra day causes all the days following it to “leap” forward.

OUR MODERN CALENDAR

The Julian calendar was used for hundreds of years. But it still contained a small error. The Julian calendar’s year was 11 minutes and 14 seconds longer than the actual solar year. By the 1500s, the Julian calendar was off by ten days!

In 1582, Pope Gregory XIII introduced a more accurate calendar. To avoid the old error, the new calendar changed the way leap years were added. Most of the world uses the Gregorian calendar today.

WHAT ABOUT WEEKS?

A week is not a natural division of time. It may come from the ancient Hebrew custom of resting every seventh day. The Romans named the days of the week after the Sun, Moon, and planets. The English names for days still show the Roman influence. Sunday and Monday, for example, come from the Roman words for Sun and Moon.