Solar calendar

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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 in its orbit around the sun is reckoned with respect to the equinox, the point at which the orbit crosses the celestial equator, then its dates accurately indicate the seasons, that is, they are synchronized with the declination of the sun. Such a calendar is called a tropical solar calendar.

The duration of the mean calendar year of such a calendar approximates some form of the tropical year, usually either the mean tropical year or the vernal equinox year.

The following are tropical solar calendars:

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, a method called "intercalation", the inserted day being "intercalary".

The Zoroastrian calendar is a religious calendar used by adherents of the Zoroastrian faith, and is an approximation of the tropical solar calendar.

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.

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