Sopdet, Sirius and the Egyptian year

The star of the year

New Year's Day was likely tied to the first dawn appearance of Sopdet, around the start of the Nile flood.

The Metropolitan Museum's Egyptian Art department writes that New Year's Day was likely associated (opens in a new tab) at first with the heliacal rising of Sopdet, the brightest star in the night sky, known in Greek as Sothis and in Latin as Sirius: its first appearance before dawn after about seventy days below the horizon. In Egypt that happened at about the same time as the first signs of the yearly Nile flood.

The museum points to three texts: one from about 2500 BC that calls Sopdet “Year”; a New Kingdom inscription of about 1250 BC that mentions the rising of Isis-Sopdet (opens in a new tab) on New Year's morning; and the much later temple at Dendera, which says the years are reckoned from her shining forth.

Keep in mind “Likely” is the museum's word. The link is drawn from these inscriptions, and scholars still disagree (opens in a new tab) about when the civil calendar was first set up.

Sources for this section Janice Kamrin, Telling Time in Ancient Egypt, Heilbrunn Timeline of Art History, The Metropolitan Museum of Art (2017) (opens in a new tab)

A calendar that drifted

Egypt's civil year had 365 days, so Sopdet's return slowly slipped through the calendar.

By at least the middle of the Old Kingdom, about 2450 BC, Egypt had a civil calendar (opens in a new tab) of twelve thirty-day months in three seasons, Inundation, Emergence and Harvest, with five extra days at the end of the year.

Because the true year is a little over 365¼ days, that calendar fell behind (opens in a new tab) by about a quarter of a day each year, so Sopdet's rising and the civil New Year matched only once every 1,460 years. Scholars have tried to use this to date the calendar's beginning, but without agreement.

Sources for this section Janice Kamrin, Telling Time in Ancient Egypt, Heilbrunn Timeline of Art History, The Metropolitan Museum of Art (2017) (opens in a new tab)

Stars that told the hours

Groups of stars called decans marked the hours of the night.

The Egyptian day had twenty-four hours (opens in a new tab), twelve of day and twelve of night, and the night hours were apparently reckoned by the movement of groups of stars, the decans, across the sky. From about 1500 BC sundials, shadow clocks and water clocks were also in use.

The month, the museum adds, was three weeks of ten days (opens in a new tab), and a lunar month began when the waning Moon disappeared; scholars think a calendar following the Moon and Sopdet ran alongside the civil one.

Sources for this section Janice Kamrin, Telling Time in Ancient Egypt, Heilbrunn Timeline of Art History, The Metropolitan Museum of Art (2017) (opens in a new tab)

In the sky today

Sirius still disappears into the Sun's glare each year and reappears at dawn; when depends on where you are.

Sirius is the brightest star in the night sky, 8.6 light-years away (opens in a new tab). Like every star away from the poles it spends part of the year behind the Sun (opens in a new tab), hidden in daylight, and returns first in the dawn sky.

The date of that return depends on your latitude and on the slow wobble of Earth's axis, which takes about 25,771 years (opens in a new tab) to come round, so Egypt's dates do not carry over to other places or to today.

Sources for this section NASA Science, Astronomers Use Hubble to 'Weigh' Dog Star's Companion (2005) (opens in a new tab) · ; NASA Space Place, What Are Constellations? (updated 2022) (opens in a new tab) · ; NASA Science, Milankovitch (Orbital) Cycles and Their Role in Earth's Climate (2020) (opens in a new tab)