Calendars 101

Solar system with all of its constellations and planets is a one big mystery as far as I concerned. Recently I heard children singing songs about cosmos and they seem to know more than I do in that respect … Well, my noble quest for knowledge started with my recent learning about winter solstice. It was the first time I heard about solar year which somehow got me curious about existing options of yearly count. Essentially, there are three major calendar types: (solar, non solar and lunisolar) and I will try to get myself briefly introduced to each one of them.

Solar calendars are calendars where dates reflect the position of the Earth on its revolution around the sun or , equivalently, the apparent position of the sun moving on the celestial sphere.

  1. Tropical solar calendars

Tropical solar calendar essentially monitors and counts the year based on the position of the Earth on the orbit with respect to the equinox.

Equinox is the point when the orbit crosses the celestial equator / moment in which the plane of the Earth’s equator passes through the center of the Sun. Equinox occurs twice each year, around 20 March and 23 September when the day and night are of approximately equal duration all over the planet. Since the dates of tropical solar calendar are synchronized with the delineation of the sun, they accurately indicate the seasons.

Egyptian calendar was the earliest solar calendar. The following are the current tropical solar calendars:

  • Gregorian calendar
  • Julian calendar
  • Iranian calendar (Jalali calendar)
  • Coptic calendar
  • Indian National calendar (Sakai calendar)
  • Bengali calendar

Each of those calendars has a year of 365 days and occasionally adds extra day to form a leap year. I am not very familiar with most of the calendars listed here; however, in my future posts I will most likely explore if those calendars serve purely civil purpose or have any religious / celebratory connotations as well.

2. Sidereal solar calendars

Sidereal solar calendar calculates the year based on the motion of the Sun through the twelve zodiacal signs rather than through the tropical movement of the Earth. Since the position of the Earth is reckoned with respect to the fixed starts, the dates indicate zodiacal constellation near which the Sun can be found.

Sidereal solar year is 20 min 24.5 s longer than the mean tropical year.

Indian calendars such as Hindu, Bengali, Tamil and Malayalam are all Sidereal solar calendars.

Lunisolar calendars are essentially solar calendars that additionally indicate moon phases.


If the solar year is defined as tropical solar year, then lunisolar  calendar will give indication of the seasons. The Chinese, Hebrew, Cologne, Babylonian and Hellenic lunisolar calendars track more or less tropical solar year.

If the solar year is defined as Sidereal solar year, then the calendar will predict the constellation near which the full moon will occur. Buddhist and Hindu lunisolar calendars would be representative of Sidereal solar year.

Tropical solar calendar by nature and counts roughly 365.25 days per annum. Lunar month is 29.5 days; thus making lunar year approximately 354 days. Since solar year accommodates 12.37 lunations (lunar months), intercalary  month has to be added to lunar calendar every two or three years in order to synchronize calendar year and solar year thus making lunisolar calendar. This essentially means that usually we would observe 12 full moons per year. However, every two to three years, we would have one “extra” full moon which is commonly known as “blue moon”.

Lunar calendars are based on the cycles of the Moon’s phases. In contrast with the practice of lunisolar calendars, lunar phases are recorded in the calendar fully and not brought into alignment with the solar year by means of intercalation. Lunar calendars differ in terms of what they perceive the start of the month to be, however. Some calendars use new , full or crescent moon as the start of the month while other calendars employ detailed calculations.

Islamic calendar (Hajri calendar) would be the representative of lunar calendars known today.

Curiosity led the cat along its glorious life journey. I will let my curiosity lead the way and I will work on risk assessment and safety measures in the meantime. The topic of calendars seems fairly safe so I am feeling brave to explore and examine.


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