Lunar calendars and solar calendars

Long ago people noticed that about 12 cycles of the Moon's phases fit into one cycle of seasons: 12 lunar months.

So lunar calendars were made: the day as the smallest unit, a month of about 29.5 days, and a year of 12 lunar months. Anyone could look up and see which phase the Moon was in, so it was an easy, reliable way to keep track of time.

But there's a catch. The seasons repeat every 365 days or so, while a lunar year is only 354 days. Each year the lunar calendar falls about 11 days behind, so the seasons slowly drift into different months.

Farmers needed to know when the seasons would come, to sow and harvest at the right time. That need for a year that stays in step with the seasons led to solar calendars.

The Gregorian calendar, the one most used today, is a solar calendar. Its months are set to add up to 365 days: that's why some months have 30 days, some 31, and February only 28.

But the Earth takes nearly a quarter of a day more than 365 days to go round the Sun. Those extra hours add up to about one whole day every four years.

So every fourth year gets an extra day: a leap year. In the Gregorian calendar, a year divisible by 4 is a leap year, and February has 29 days. (It's fine-tuned too: century years such as 1700, 1800 and 1900 are not leap years, but those divisible by 400, such as 1600 and 2000, are.)