Solar calendar
A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun (or equivalently the apparent position of the sun moving on the celestial sphere).
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Tropical solar calendars
If the position of the earth (or the sun) is reckoned with respect to the equinox, then the dates indicate the season (and so is synchronized to the declination of the sun). Such a calendar is called a tropical solar calendar.
The mean calendar year of such a calendar approximates some form of the tropical year (typically either the mean tropical year or the vernal equinox year).
The following are tropical solar calendars:
- Gregorian calendar
- Julian calendar
- Bahá'í calendar
- Coptic calendar
- Iranian calendar (Jalāli Calendar)
- Malayalam calendar
- Tamil calendar
- Thai solar calendar
Every one of these calendars has a year of 365 days, which is occasionally extended by adding an extra day to form a leap year.
Sidereal solar calendars
If the position of the earth (see above) is reckoned with respect to the fixed stars, then the dates indicate the zodiacal constellation near which the sun can be found. Such a calendar is called a sidereal solar calendar.
The mean calendar year of such a calendar approximates the sidereal year.
The Hindu calendar and Bengali calendar are sidereal solar calendars. They are usually 365 days long, but now and then take an extra day to make a leap year.
Non-solar calendars
Calendars that are not solar calendars, include the Islamic calendar which is a purely lunar calendar and calendars synchronized to the synodic period of Venus or to the heliacal risings of stars.
Lunisolar calendars
Lunisolar calendars may be regarded as solar calendars, although their dates additionally indicate the moon phase. Because a typical lunisolar calendar has a year made up of a whole number of lunar months, it can't indicate the position of Earth on its revolution around the sun as well as a pure solar calendar can.
See also
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