Description: A calendar is a system to reckon time, in particular to identify seasons. Calendars are often laid out in terms of days, weeks, months, and years. It is an abstract system based on the periodic motion of celestial objects (Moon, Sun, stars). Calendars have been used since ancient times. The Moon and the Sun were two objects in the sky which had easily identifiable periodic motions that resulted in identifiable changes in the passing of days, weeks, months, and seasons. Calendars can be based on the monthly lunar cycle (lunar calendars), based on the solar year (solar calendars), or be lunar calendars with additional days added to match the solar year (lunisolar calendars). There are a wonderful variety of calendars throughout the world resulting from local astronomical heritage.

Related Terms:

• Lunar Phase
• Seasons
• Year

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".