Asolar calendar is acalendar whose dates indicates theseason or almost equivalently the apparentposition of the Sun relative to the stars. TheGregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar.The main other types of calendar arelunar calendar andlunisolar calendar, whose months correspond to cycles ofMoon phases. 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 oldest solar calendars include theJulian calendar and theCoptic calendar. They both have a year of 365 days, which is extended to 366 once every four years, without exception, so have a mean year of 365.25 days.As solar calendars became more accurate, they evolved into two types.
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If the position of the Earth in its orbit around the Sun is reckoned with respect to theEquinox, the point at which the orbit crosses the celestial equator, then its dates accurately indicate theseasons, that is, they are synchronized with thedeclination of the Sun. Such a calendar is called atropical solar calendar[citation needed]..
Every one of these calendars has a year of 365 days, which is occasionally extended by adding an extra day to form aleap year, a method called "intercalation", the inserted day being "intercalary".
TheBaháʼí calendar, another example of a solar calendar, always begins the year on thevernal 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 ofTehran "by means of astronomical computations from reliable sources".[1]
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If the position of the Earth (see above) is reckoned with respect to the fixed stars, then the dates indicate thezodiacal constellation near which the Sun can be found. A calendar of this type is called asidereal solar calendar.[2]
The mean calendar year of such a calendar approximates thesidereal 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.[3]
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TheIslamic calendar is a purelylunar calendar and has a year, whose start drifts through the seasons and so is not a solar calendar.The MayaTzolkin calendar, which follows a 260-day cycle, has no year, therefore it is not a solar calendar.Also, any calendar synchronized only to thesynodic period ofVenus would not be solar.
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Lunisolar calendars may be regarded as solar calendars, although their dates additionally indicate the moon phase. Typical lunisolar calendars have years marked with a whole number of lunar months, so they can not indicate the position of Earth relative to the Sun with the same accuracy as a purely solar calendar.
^Helen R. Jacobus. (2014). Zodiac Calendars in the Dead Sea Scrolls and Their Reception : Ancient Astronomy and Astrology in Early Judaism. Brill
^Leofranc Holford-Strevens. (2005). The History of Time: A Very Short Introduction. OUP Oxford
^Denis Feeney. (2007). Caesar's Calendar : Ancient Time and the Beginnings of History. University of California Press
^Steve Howland. (2024). The EartHeaven Calendar : A white paper presenting a fixed calendar solution for the next 2100 years of this Aquarian Age. Ampers& Studio, Publisher.https://ehc.world
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