A calendar is a system for assigning calendar dates to days. The dates may be based on the perceived motion of astronomical objects. A calendar can also be a physical device (often paper) that illustrates the system (for example, a desktop calendar). The term is also used to indicate a particular set of planned events (for example, court calendar).
A lunar calendar is synchronized to the motion of the Moon (moon phases); an example is the Islamic calendar.
A solar calendar is based on perceived seasonal changes synchronized to the motion of the Sun; an example is the Persian calendar.
A lunisolar calendar is synchronized to the motions of both the Moon and the Sun; an example is the Jewish calendar.
An arbitrary calendar is not synchronized to either the Moon or the Sun; examples are the week and the Julian day used by astronomers.
There are some calendars that appear to be synchronized to the motion of Venus, such as some of the ancient Egyptian calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator.
Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.
Under the Roman Republic, the solar Julian calendar was adopted. It numbers days within months that are longer than the lunar cycle, so it is not convenient for tracking phases of the moon, but it does a better job of tracking the seasons. Each calendar year has 365 days, except every 4th year which is a leap year of 366 days. So the mean calendar year is 365.25 days.
Unfortunately, Earth's tropical year is a little less 365.25 days (it is approximately 365.242 days), so this calendar, too, slowly drifted out of sync with the seasons. For such reasons, the Gregorian calendar was later adopted by most of the West, starting in 1582, and it has since become the world's dominant civic calendar.
Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each moon phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons. It does, however, stay constant with respect to other phenomena, notably tides. A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons.
Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.
Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be done with leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.
Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years.
Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.
Leap Seconds Civil time is occasionally adjusted by one second increments called leap seconds. A detailed explanation of what a second actually is, and why leap seconds are necessary. http://tycho.usno.navy.mil/leapsec.html
Astronomical Time Keeping Extensive descriptions of many astronomical time keeping systems, with information on time zones and Julian day numbers. http://www.maa.mhn.de/Scholar/times.html
Time and Date Lots of time and date related information, such as yearly and monthly calendars, counters, countdown, and the world clock which shows the current time in cities all over the world. http://www.timeanddate.com/
Time and Dates Information on date, time, and time zones from the New York Public Library. http://www.circ.uab.edu/nypldr/1time/index.htm
Lunisolar Calendar Calendar with lunations, from new moon to new moon, instead of months. Moon phase on each day. Chinese, Jewish, and Islamic months. Eclipses, meteors, planets, star charts. Christian, Pagan, Islamic, Hindu, and Jewish holidays. http://www.flash.net/~pburch/lunarcal.html
Blue Moon Myths Second full moon in a month, or third full moon in a single season? http://www.infoplease.com/spot/bluemoon1.html
Maya Calendar 2000 The calendar was the center of Maya life and their greatest cultural achievement. To carry out their astronomical calculations they developed a sophisticated mathematical system. The Maya Calendar website is developing with the intent of providing a complete view of Maya culture. http://mayacalendar.com
Calendar Studies Articles on the Gregorian and Julian calendars, the ISO date format, the Julian day number system, the Maya calendar, the Goddess lunar calendar, the Liberalia Triday Calendar and C functions for date conversion; plus software for calendrical conversion. http://hermetic.nofadz.com/cal_stud.htm
LunarCal - A Perpetual Chinese Lunar Calendar LunarCal is a 160-year perpetual Chinese Lunar Calendar for 1900 to 2060. Chinese festivals are listed and the moon phase is displayed. http://www.geocities.com/lunarcal
The Chinese Calendar The mathematics of the Chinese calendar. Explains the rules for the Chinese calendar. http://www.math.nus.edu.sg/aslaksen/calendar/chinese.shtml
The Islamic Calendar The mathematics of the Islamic calendar in Singapore. http://www.math.nus.edu.sg/aslaksen/calendar/islamic.shtml
Calendars through the Ages History and FAQs of calendars, from ancient Rome to outer space. Including Julian, Gregorian, Jewish, Islamic, Chinese, and Mayan. http://webexhibits.org/calendars/
Annus Novus Decimal Calendar A proposal for a non-sectarian, culturally neutral calendar system. http://www.atlantium.org/calendaran.html
Calendrical Calculations Published by Cambridge University Press. Gives a unified algorithmic presentation of the Gregorian, ISO, Julian, Coptic, Ethiopic, and Islamic civil calendars. http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml
Calendopaedia The Encyclopaedia of Calendars. http://www.geocities.com/calendopaedia/
Millennium Mistake Explains the difference between millenium and year 2000 as the absence of a year zero. http://www.millenniummistake.net/
Rosetta Calendar An online handy tool for converting to and from the Gregorian, Julian and Jewish calendars. http://rosettacalendar.com/
The Calendar, Leap Years and the Year 2000 An explanation of the calendar including the origin of the day, week, month and year. http://chemistry.csudh.edu/oliver/smt310-handouts/calendar.htm
Indian Moons A list of many American Indian tribe's names for months, days, and other calendar related information. http://www.americanindian.net/moons.html
Astro Portal - CalSKY The worldwide interactive online astronomical/space calendar and calculator - for friends of astronomy, as well as astronomers. http://www.calsky.com/
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