This 2,200-year-old slab bears the world's first mention of leap year
Evidence dating back to 238 B.C. shows that ancient Egyptians recognized the need for a leap year to correct the slow drift of the seasons.
Every four years we add an extra day to the calendar to account for the solar year being roughly 365 and a quarter days, rather than 365. But how long has the leap year actually been around for? At least 2,262 years, according to a spectacular find from the Egyptian desert.
The Tanis Stele is a limestone slab (stele) more than seven feet tall and nearly three feet wide that was discovered in 1866 by a group of German scholars visiting the site of the ancient Egyptian city of Tanis in the Nile Delta. Like the more-famous Rosetta Stone, it bears an inscription in two languages, Egyptian (written in both hieroglyphs and Demotic script) and ancient Greek. The inscription, dated to 238 B.C., records a decree of pharaoh Ptolemy III that follows the standard of the time, including praise for the pharaoh, a description of military campaigns, and the stipulation that a copy of the decree should be erected in every major temple.
What is entirely original about this decree—known as the Canopus Decree for the ancient Egyptian city it was issued in—are its instructions regarding the calendar:
"And so that the seasons should always correspond to the established order of the universe, and that it should not happen that some of the public festivals which take place in winter are celebrated in summer, as the sun changes by one day in the course of four years… (it was resolved) to add from now onwards one festival day in honor of the gods…every four years to the five additional days, before the new year, so that all may now know that the former defect in the arrangement of the seasons…"
Why we need a leap year
The first references to a 365-day calendar, which specify a year of twelve 30-day months and 5 epagomenal days ('monthless days' added to the calendar to make it approximately equal to the solar year) are found in the records of Egypt's Fourth and Fifth Dynasties around 2600 B.C., according to Adrienn Almàsy-Martin, an Egyptologist at the University of Oxford. The inaccuracy this introduces is sufficient to cause a slow drift of the seasons through the calendar.
The ancient Egyptians noticed a celestial coincidence that occurred annually at the same time as the flooding of the Nile—the appearance of Sirius, the brightest star in the sky. In the same way that some constellations are not visible all year, Sirius was not visible to the ancient Egyptians for the same 70 contiguous days of every year because it was too close to the sun. Annually after this absence, Sirius would reappear on the eastern horizon in the dawn sky, rising close to and just before the sun—a phenomenon known as "heliacal rising."
Egyptian civilization depended upon the Nile floods to bring the rich silt that fertilized its farmlands. The reappearance of Sirius, linked as it was to the crucial Nile inundation, and which also occurred at the summer solstice, was keenly observed, and heralded the beginning of the ancient Egyptian new year. By measuring the elapsed time between each annual heliacal rising of Sirius, astronomers eventually realized the solar year was a quarter-day longer than 365 days. While this realization likely happened earlier in history, the Canopus Decree of 238 B.C. is our earliest recorded evidence for the leap year.
Multiple copies of the Canopus Decree would have existed in ancient times, according to Almàsy-Martin, and six complete or fragmented versions of the decree have survived to this day. The two best-preserved examples—from Tanis in 1866 and the site of Kom el-Hisn, in 1881—are in the Egyptian Museum in Cairo. While they were discovered after the decipherment of the Rosetta Stone in 1822, the better-preserved examples of the Canopus Decree feature a greater number of hieroglyphs and their study ended all remaining doubt about the Rosetta's decipherment. For this reason, their inscription is considered second only to the Rosetta's in significance for understanding ancient Egyptian.
We know Ptolemy III's directive in the Canopus Decree to add an additional day to the calendar every four years was ultimately unsuccessful, but not when or why his directions were ignored. It is possible the priests controlling the calendar didn't want to change their traditions, or perhaps they thought the drift of the seasons through the calendar was unnoticeable within a typical 40-year lifetime.
What we do know is that when the Romans annexed Egypt in 30 B.C., the Egyptians were again using a 365-day calendar, and by 22 B.C—a few years after the Egyptian-inspired Julian calendar had been implemented in Rome—Emperor Augustus had reintroduced the leap day back to the Egyptians.
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