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A solar calendar is a calendar whose dates indicates the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. The main other types of calendar are lunar calendar and lunisolar calendar, whose months correspond to cycles of . The months of the Gregorian calendar do not correspond to cycles of the Moon phase.
The Egyptians appear to have been the first to develop a solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star—Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River. They constructed a calendar of 365 days, consisting of 12 months of 30 days each, with 5 days added at the year’s end. The Egyptians’ failure to account for the extra fraction of a day, however, caused their calendar to drift gradually into error.
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 Baháʼí calendar, another example of a solar calendar, always begins the year on the March equinox and sets its intercalary days so that the following year also begins on the vernal equinox. The moment of the vernal equinox in the northern hemisphere is determined using the location of Tehran "by means of astronomical computations from reliable sources".
The mean calendar year of such a calendar approximates the sidereal year.
Leaping from one lunation to another, but one Sidereal year is the period between two occurrences of the sun, as measured by the stars' solar calendar, which is derived from the Earth's orbit around the sun every 28 years.Leofranc Holford-Strevens. (2005). The History of Time: A Very Short Introduction. OUP Oxford
Indian calendars like the Hindu calendar, Tamil calendar, Bengali calendar (revised) and Malayalam calendar are sidereal solar calendars. The Thai solar calendar when based on the Hindu calendar was also a sidereal calendar. They are calculated on the basis of the apparent motion of the Sun through the twelve zodiacal signs rather than the tropical movement of the Earth.
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