The Ancient Egyptian Calendar

The star-sprinkled Egyptian night sky that not only stuns visitors to Egypt was also studied intensely by special temple priests who soon discovered that the appearance of a star they named sepdet (which we know as Sirius) was associated with the beginning of the Nile flood . This was the start of the world's first calendar, invented over 5000 years ago.
To develop a calendar, you need a regular event that is predictable. And what was more regular and important to the ancient Egyptians than the rise and fall of the River Nile?
The waters started rising around the end of June, and the flood period (achet) lasted until October, covering the land with rich black mud and preparing it for the sowing and growing period (peret). The harvest time (schemu) started at the end of February and ended with the new Nile flood This predictable, ongoing cycle defined the agricultural year.
But there was a problem! The flood came within a range of 80 days with variable intensities .... all in all, not very accurate timing.
Sirius, or Sothis as it was called by the ancient Egyptians, the star who's heliacal rising was in early July 3000 years ago, but due to the wobble of the earth on its axis is now a few weeks later, turned out to be a very reliable predictor of the recurring flood and defined the exact length of the trip of the earth around the sun . (Sirius also revealed the entrance to the tomb of Akhenaton in the novel The Watch Gods by Barbara Wood. )

From their mythology, the Egyptians saw a connection between Sepdet's appearance and the beginning of the Nile flood. They believed the Nile flood was caused by the tears that Isis ( the Great Mother of All Gods and Nature ) shed, after her husband Osiris was murdered by his brother Seth. Sepdet was the cosmic appearance of Isis.
The first new moon following the reappearance of Sirius after it disappeared under the horizon for 70 days was established as the first day of the New Year ( Egypt: wepet senet) and of the achet (flood) period--even if the Nile had not yet started to rise.
The priests also observed there were four moon periods that fit into each of the three seasons --or rather didn't quite fit! The lunar month has 29 days, resulting in "short" or "long" years of 12 or 13 new moons.
It didn't really matter because the appearance of Sirius and the next New Year put the calendar back to baseline.
But, as in our times, this calendar was not accurate enough for the central administration; taxes and other things have to be paid on time. So in the Old Kingdom, a standard calendar with 12 months of 30 days each was introduced . Each month was divided into decades of 10 days.
Because this public calendar with 360 days was too short to coordinate with the agricultural and lunar calendar, five extra days called the heriu renpet were added at the end of the year and celebrated with religious festivities.
With this last calendar reform before Roman times, the ancient Egyptians missed the true length of the solar year by only day. But the missing days added up and the gap between the lunar calendar and the public calendar increased by one day every four years . So, in 1460 years the calendar slipped through a whole year, meaning that in between, according to one calendar, it could be harvest time, although in reality the floodwaters were just receding!
This little "big problem" wasn't solved until Augustus introduced the leap year in Egypt around 30 B.C.
Winter and spring are the best time to watch Sirius. In February, Sirius stands low in the southeast and is the brightest star in the sky ----- go and look ------ and imagine the Egyptian priests doing the same 5000 years ago.