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).
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
- Thai solar calendar
- Malayalam calendar
- Tamil 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 Solar Calendar is a sidereal solar calendar. It is usually 365 days long, but now and then takes an extra day to make a leap year.
Non-solar calendars
Calendars that are not solar calendars, include the Islamic calendar which is a pure lunar calendar and calendars synchronized to the synodic period of Venus or to the heliacal risings of stars.
Lunisolar calendars are almost solar calendars, except that their dates additionally indicate the moon phase.