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).
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
- 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.
Lunisolar calendars are almost solar calendars, except that their dates additionally indicate the moon phase.
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