Scientists define a solar year based on the movements of the sun, but use the movements of the moon to define a lunar year. Unlike the lunar calendar, most places across the globe use the solar calendar to track the passage of the year. Solar years have a different length to lunar years, and the term “epact” describes this time difference. One epact is 11 days.
The Definition of a Lunar and Solar Year
12 lunar months make up a lunar year. A lunar month can also be defined as the time the moon takes to pass through each of its phases (new moon, half moon and full moon) and return back to its original position. A lunar month takes 29.5 days, although this varies slightly around this average.
We call the time period required for Earth to complete one revolution around the sun a solar year. A solar month means one twelfth of a solar year. Calendar months differ from this, but in practice the differences are minor and exist so we can have a whole number of days in each month.
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The Time Period of Lunar and Solar Years
A lunar year has approximately 354 days. A solar year has 365 days. This leaves an 11-day difference between one solar year and one lunar year, resulting from the difference in their definitions. The term epact describes this specific time difference. Over the course of 33 years, there will be a lag of one year between solar and lunar calendars because of the successive epacts.
Lunar and Solar Calendar Conventions
Most places around the world use a solar calendar rather than a lunar one. However, Muslims and Jews follow a lunar calendar. Islamic calendars, otherwise known as Hijri calendars, are based on the lunar cycle, and its year consists of 12 lunar months. The Hijri calendar plays an important role for religious purposes and the Muslim religious festivals are based on this calendar. A Jewish calendar uses primarily lunar definitions, and each month begins on the new moon, but the years are based on solar years. Chinese calendars are a type of lunisolar calendar, a combination of a lunar calendar and a solar calendar.
Leap Years and Leap Months
Because there is an 11-day difference between a solar year and a lunar year, people using a lunar calendar insert an extra (13th) month into it every three years. In the solar calendar, people add a leap day to the month of February every four years.