Calendar
From ArticleWorld
A calendar is a dating structure to name, predict, and keep records of events in time. The most basic unit of a calendar the day, followed by weeks, months and years. Many calendars observe other cycles, such as planetary circuits and phases.
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Calendar types
Calendars can be based on motions of the sun and moon, or on other events. Calendars are also identified by how they are constructed as well as intended usage.
By cycle
Solar calendars are based on the time it takes the Earth to circle the sun, subdivided into days of the year. These days may be calculated individually, or an average can be created. An example of this is our modern 24-hour day, which is an approximation of the 23.934 hours it actually takes for the earth to make a single rotation.
The lunar calendar also uses the sun to measure days, but these days are incorporated into 28-day months (“moons”) instead of years. Because the phases of the moon do not correlate to the solar year, compensations must often be made in the form of added or skipped cycles.
A modern cycle, that of the fiscal year, is the basis of the fiscal calendar, which is designed to ensure that a month in one year can be compared accurately to the same month in other years. The fiscal calendar requires an added week every several years to maintain accuracy.
By organization
A calendar can be “theoretical,” “pragmatic,” or a mix of the two. In a theoretical calendar, the dating system is based on mathematics rather than constant observation of the solar system. A theoretical calendar can be used to make predictions easily, but it is often inaccurate, and this inaccuracy will always increase over time as calculated cycles become unsynchronized with the actual movements of the sun, earth and moon.
A pragmatic calendar does not always observe easily divided or defined cycles. Instead, it is built on the observations of previous astronomical events. This type of calendar is very accurate, but has little or no predictive value.
The modern Gregorian calendar is both theoretical and pragmatic. Initially, as the Julian calendar, it was formulated mathematically to provide an estimation of solar and lunar cycles. Over time, discrepancies arose between predicted and observed dates, and changes based on this observation were made in the 16th century. Today’s Gregorian calendar has irregular months as well as leap years, but it is much more accurate than its predecessor. Mixed calendars are generally accurate as well as useful for predictive purposes.
Calendar uses
The first calendars were created to predict opportune times for planting crops and observing days of spiritual significance. Today, the purpose of the calendar has expanded, and calendars are currently used for observing birthdays, holidays, days of importance to the public and for record-keeping.