Which Ancient Civilization Invented the 365 Day Calendar?

The invention of the calendar is one of the most significant achievements of human civilization. It allowed people to measure time and plan their activities accordingly. Many ancient civilizations developed their own calendars, but which one of them invented the 365 day calendar that we use today?


The idea of a solar year, or the time it takes for the Earth to orbit around the sun, has been known since ancient times. The ancient Egyptians, for example, observed the annual flooding of the Nile river and developed a lunar calendar that consisted of 12 months of 30 days each, with five additional days at the end of the year. However, this calendar did not align with the solar year and needed to be adjusted periodically.

The Mayan Calendar

Another civilization that developed a complex calendar system was the Mayans. They lived in Central America from around 2000 BCE to 1500 CE and created a calendar that consisted of three separate cycles: the Haab (365 days), Tzolkin (260 days), and Long Count (a continuous count of days from a fixed starting point). The combination of these cycles allowed them to track time accurately and predict astronomical events such as eclipses.

The Persian Calendar

The Persian Empire also had a sophisticated calendar system that was based on astronomical observations. The Zoroastrian priests who were responsible for maintaining this calendar introduced an intercalation system that added an extra month every few years to keep it in sync with the solar year.

The Roman Calendar

The Romans also had a calendar system that was based on lunar phases. However, it was highly inaccurate and needed frequent adjustments. In 45 BCE, Julius Caesar introduced a new calendar system known as the Julian Calendar, which was based on a solar year of 365.25 days and included an extra day every four years (leap year) to account for the extra quarter day.

The Egyptian Calendar

But which ancient civilization invented the 365 day calendar? The answer is the ancient Egyptians.

While they did not have a 365 day calendar in its present form, they did develop a calendar system that consisted of 12 months of 30 days each, with an additional five days at the end of the year. This resulted in a calendar of 365 days, which was remarkably accurate considering that it was developed over 4000 years ago.

The Sothic Cycle

The Egyptians also observed the heliacal rising of Sirius, the brightest star in the sky, which occurred once every solar year. This event marked the beginning of their new year and was used to adjust their calendar system periodically. This cycle, known as the Sothic cycle, lasted approximately 1460 years and was used to determine important dates such as the coronation of pharaohs.


In conclusion, while many ancient civilizations developed their own calendars, it was the ancient Egyptians who invented the 365 day calendar that forms the basis of our modern calendar system. Their system was remarkably accurate and incorporated astronomical observations such as the heliacal rising of Sirius to maintain its accuracy over time. Today, we continue to use their invention to measure time and plan our activities accordingly.