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 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.
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
