We check our calendars all the time. While they may appear very simple, in fact people throughout history have struggled to design calendars and measure time. Imagine a calendar that simply listed a string of days, for instance. It would quickly become awkward and bulky, with its unwieldy number of days. Our ancestors understood that time had to be subdivided into smaller units that were easier to manage.

Most calendars are based on astronomical cycles, like the movements of the Sun and the Moon. Religions have also had an influence, depending on their specific needs, rituals and holy days to be celebrated.

The solar calendar

There are three main families of calendars. Solar calendars, like the Gregorian calendar, are used in the West and have been widely adopted around the world. They are good for tracking the seasons. The passage from one season to another is difficult to measure precisely, since temperatures change very slowly and the Sun looks the same from one season to another. Solar calendars are based on the height of the Sun in the sky over the course of the year. Unfortunately, the tropical year (the period between two spring equinoxes) does not work out to be an even number of days: 365 days, 5 hours, 48 minutes and 46.2 seconds, to be exact!

To make it easier to use, the Gregorian calendar normally has 365 days, to which a “leap day” is periodically added to make up for the actual length of the tropical year.

The lunar calendar

The earliest calendars were based on the Moon’s movements, since changes in the appearance of Earth’s natural satellite are easily visible with the naked eye. Once again, though, the average length of the lunar cycle is not an even number. Rather, it works out to 29.53 days. A lunar calendar alternates between months of 29 and 30 days, adding up to a year of about 355 days.

Muslims use a lunar calendar. Since the lunar year is shorter than the tropical year, Ramadan shifts by about ten days a year in relation to the Gregorian calendar.

The lunisolar calendar 

Lastly, there is the so-called lunisolar calendar, which occasionally adds a 13th month to the lunar year to prevent the seasons from shifting too far over time. The Chinese and Hebrew calendars are of this type.

January 1, 2017 marked the start of a new year on the Gregorian calendar. Chinese New Year will arrive on January 28, ushering in the Year of the Rooster. The year 5778 on the Hebrew calendar will begin on September 21, 2017. And Muslims will start the year 1439 of the Hijra on the following day.

Happy New Year to everyone, whenever you celebrate it!

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