The Sothic cycle, also called the Canicular period, is a time period lasting 1,461 Egyptian civil years, each with 365 days, or 1,460 Julian years, each averaging 365.25 days. Over this time, the 365-day Egyptian year falls behind the Julian calendar by enough time that its beginning again matches the moment when the star Sirius becomes visible above the horizon on July 19 in the Julian calendar. This cycle is important in Egyptology, especially when studying the Egyptian calendar and its history. Records of how the Egyptian calendar shifted compared to the stars may have led to the creation of more precise calendars, such as the Julian and Alexandrian calendars.

Mechanics

The ancient Egyptian civil year, its holidays, and religious records show that it was created based on the return of the bright star Sirius to the night sky, which was believed to signal the annual flooding of the Nile. However, the civil calendar had exactly 365 days and did not include leap years until 22 BCE. This caused its months to gradually move backward through the solar year by about one day every four years. This shift matched the difference between the civil calendar and the Sothic year, which is about a minute longer than a Julian year. The sidereal year, which lasts 365.25636 days, applies only to stars on the ecliptic (the Sun's apparent path across the sky) that do not move through space. Sirius, however, is located about 40° below the ecliptic, has its own movement through space, and is affected by the wobbling of the celestial equator. These factors make the time between Sirius's first appearances in the sky each year nearly exactly 365.25 days. Over time, the 365-day calendar lost one day relative to the solar and Sothic years every four years. After 1461 Egyptian civil years, or 1460 Julian years, the calendar's drifting day would return to its original position relative to both the solar and Sothic years.

Discovery

This calendar cycle was widely known in ancient times. Censorinus wrote about it in his book De Die Natale in 238 CE. He said the cycle had started 100 years earlier on August 12. In the ninth century, Syncellus summarized the Sothic Cycle in the "Old Egyptian Chronicle." In 1833, Isaac Cullimore, an early Egyptologist and member of the Royal Society, published a study about the cycle. He suggested that Censorinus might have incorrectly stated the cycle’s starting date, and that it was more likely to begin in 136 CE. Cullimore also estimated the cycle was created around 1600 BCE.

In 1904, Eduard Meyer studied Egyptian inscriptions and writings to find records of when Sirius rose at dawn. He found six records that are used to support traditional Egyptian timelines. Censorinus recorded that Sirius rose on the first day of the Egyptian New Year between 139 CE and 142 CE. The actual record refers to July 21, 140 CE, but astronomical calculations show Sirius rose on July 20, 139 CE, according to the Julian calendar. This connects the Egyptian calendar to the Julian calendar. A Julian leap day occurred in 140 CE, so the Egyptian New Year (1 Thoth) fell on July 20, 139 CE, but on July 19 for 140–142 CE. Meyer compared the Egyptian date of Sirius’s rising with the Julian calendar date, counted the number of leap days, and calculated how many years had passed since the cycle began.

To calculate dates astronomically, the location of observation is important because the latitude affects when Sirius rises. Observations were made at Heliopolis (or Memphis, near Cairo), Thebes, and Elephantine (near Aswan). Sirius rose about 8 days later at Cairo compared to Aswan.

Meyer concluded the Egyptian civil calendar was created in 4241 BCE. However, recent research has questioned this claim. Most scholars now either adjust Meyer’s calculation by one Sirius cycle to 19 July 2781 BCE or argue that the document Meyer used does not confirm Sirius’s rising at all.

Chronological interpretation

Three important observations about the heliacal rise of Sirius help determine the timeline of ancient Egypt. The first is an ivory tablet from the reign of Djer, which may show when Sirius first appeared at sunrise on the same day as the start of the Egyptian new year. If this marks the beginning of a Sothic cycle—a 1,460-year pattern where Sirius aligns with the calendar—it would date to about 17 July 2773 BCE. However, this date is later than when Djer ruled, so some experts think the tablet instead connects Sirius’s rise to the lunar calendar, not the solar civil calendar, making it less useful for dating events.

In 2017, Gautschy and others found a new Sothic date from the Old Kingdom and used it to support the Sothic cycle model through a study of star positions.

The second observation clearly refers to a heliacal rising and is linked to the seventh year of Senusret III. This event likely occurred in Itj-Tawy, the capital of the Twelfth Dynasty, placing the dynasty’s timeline from 1963 to 1786 BCE. The Ramses or Turin Papyrus Canon lists the dynasty as 213 years long (1991–1778 BCE), while Parker adjusted this to 206 years (1991–1785 BCE) by using 17 July 1872 BCE as the Sothic date. Before Parker’s research, other scholars used different dates, such as 21 July 1888 BCE or 20 July 1884 BCE, to estimate the dynasty’s length as 213 or 206 years.

The third observation occurred during the reign of Amenhotep I. If the sighting happened in Thebes, his rule would be dated between 1525 and 1504 BCE. However, if the observation was made in Memphis, Heliopolis, or another Delta location, as some experts suggest, the timeline of the 18th Dynasty would need to be extended by about 20 years.

Observational procedure and precession

The Sothic cycle is an example of two cycles with different lengths combining to form a new, longer cycle, called a tertiary cycle. This is calculated using the formula 1/a + 1/b = 1/t, which is also known as half the harmonic mean. In the case of the Sothic cycle, the two cycles involved are the Egyptian civil year and the Sothic year.

The Sothic year is the time it takes for the star Sirius to return to the same position relative to the sun. The length of this time can vary because of axial precession, which is the slow movement of Earth’s axis over time.

The time it takes for a star to complete its yearly path can be measured by observing when it rises to a specific altitude above the horizon at sunrise. This altitude does not need to be the exact point of first visibility. Each year, the star rises about four minutes earlier at sunrise. Eventually, the star will return to the same position relative to sunrise, no matter the chosen altitude. This time is called an observational year. Stars near the ecliptic or the ecliptic meridian usually have observational years close to the sidereal year of 365.2564 days. The ecliptic and the meridian divide the sky into four quadrants. As Earth’s axis slowly wobbles, the observer’s position changes, affecting the observational year. If the axis moves the observer closer to the event, the observational year becomes shorter. If the axis moves the observer away, the observational year becomes longer. These changes depend on which quadrant of the sky the event is observed in.

The Sothic year is notable because its average length was nearly exactly 365.25 days around 4000 BCE, before Egypt was unified. The slow change in this value over time is also significant. If records had been kept during predynastic times, the Sothic rise would have aligned with the same calendar day every 1461 years. By the Middle Kingdom, this number decreased to about 1456 years. The number 1461 could also be maintained if the date of the Sothic rise was adjusted artificially by moving the celebration of the event one day every four years instead of relying on rare observations.

Problems and criticisms

Determining the date when Sirius first appears in the sky at dawn, called the heliacal rise, is difficult. This is because the exact location (latitude) of the observer must be known. Another challenge is the Egyptian calendar, which loses one day every four years. Because of this, the heliacal rise of Sirius might occur on the same day for four years in a row. This makes it hard to determine the exact year of the observation.

Some problems have been raised about using the Sothic cycle to date events. These issues are serious enough to affect the accuracy of the method. First, ancient records of astronomical observations do not always include the name of the pharaoh ruling during the time of the observation. This means Egyptologists must guess the pharaoh's name based on other information. Second, there is no clear information about how the Egyptian civil calendar changed over time. If the calendar had been adjusted even once over thousands of years, the calculations based on the Sothic cycle would no longer be reliable. Other issues, like the lack of mention of the Sothic cycle in ancient Egyptian writings, are not as serious. This could be because the cycle was obvious to Egyptians, or because writings about it were lost or not yet found.

Marc Van de Mieroop, when discussing how to date events, does not mention the Sothic cycle. He states that most historians today believe it is not possible to assign exact dates to events before the 8th century BCE.

Some recent claims suggest that the eruption of the volcano on the island of Thera marks the start of Egypt's Eighteenth Dynasty. This is based on evidence of ash and pumice found in ruins at Avaris, which is linked to the end of the Hyksos period. Scientists who study tree rings (dendrochronologists) estimate the eruption happened around 1626 BCE. This has led some to believe that the Sothic cycle may be incorrect by 50 to 80 years when dating the beginning of the 18th Dynasty. However, claims that the eruption is described on the Tempest Stele of Ahmose I have been challenged by scholars like Peter James.