Making Connections to
the Physical Environment

Dating the Egyptian Pyramids:
A Problem of Earth-Sun Relations

The solar year being 365 1/4 days in length has made it difficult to synchronize astronomical and civil calendars. For centuries ancient societies looked to the heavens and observed natural phenomena to create their calendars around significant events. The phases of the moon, rising of particular stars and movement of the sun through the sky guided ancient cultures through time. Modern archaeologists must correlate ancient calendars with modern ones to date the rise and fall of ancient cultures, building of structures and other significant events in the past.

The Egyptian Calendar

The ancient Egyptians actually used three calendars,

1. A lunar calendar of alternating twenty-nine and thirty-day months.

2. A civil calendar of 360 days plus five additional days

3. A calendar of 365¼ days based upon the heliacal return of the star Sirius.

The civil calendar of ancient Egypt was used for all official dating and recognized three seasons: Akhet or Inudation, Peret or Sowing and Growing, and Shemu or Harvest. Each season had four 30-day long periods with five additional days after the end of the harvest for a total of 365 days. Because the earth actually revolves around the sun once every 365 1/4 days, the Egyptian civil calendar was constantly losing one day every four years or a whole month over 120 years. As a result the civil seasons rarely coincided with natural seasons. Only once every 1,460 years did the civil and astronomical years occur on the same day. The rising of the star Sirius in the eastern horizon just before daybreak marks the official starting of the new year. The coincidence of New Year's Day and the rising of the star Sirius has been confirmed from the writings of the Roman author Censorius that the Sirius rose on New Year's Day A.D. 139. From this date we can calculate the coincidence occurring in 1317 B.C. and 2773 B.C. The astronomical record of Sirius has been deciphered from hieroglyphic texts which have enabled Egyptologist to correlate the New and Middle Kingdom regnal years and civil dates to our present day calendar.

Dating the Pyramids

Archaeologists have generally believed that the magnificent pyramids at Giza were the work of the Old Kingdom Dynasty 4 in Egypt over the span of 85 years between 2589 and 2504 B.C. Detailed chronologies based on kings lists, regnal dates and the ancient Egyptian civil calendar has set the age of many monuments such as the Great Pyramid.

Sphinx and pyramid from the outskirts of Cairo (Source: FAO 16223 L. Spaventa)

The astronomical records as reported precisely fixes Middle Kingdom dates. But one cannot just count back the reigns of kings into the Old Kingdom, an Intermediate period occur between the Old and Middle Kingdom. The First Intermediate period saw municipalities spring up along the Nile and wars raging between rivals in the north and south. Egyptologists are unsure hold long the First Intermediate period lasted as kings lists are unreliable and the sequence of regnal years and royal monuments is interrupted. The period may have last from 100 to 150 years. The difficulty of dating these structures is made difficult with no observations of astronomical events stretching back into the third millennium B.C. and the early time of pyramid building.

The accurate dating of significant historical events in our ancient past is one difficult indeed. The difference between  civil and natural calendars makes it difficult to date events in the past. To better anchor the dates of major monument building like the pyramids of Giza, scientists have turned to other techniques such as radiocarbon dating. Interpretation of Carbon 14 dates has been problematic in Giza. Even this more accurate method presents problems with establishing dates for these structures (See reference: "Dating the Pyramids").

References:

• Members of David H. Koch Pyramids Radiocarbon Project. 1999, "Dating the Pyramids", Archaeology. pp.26-33. Visited 9 April 2002

• Tehuti Research Foundation, "The Egyptian Triple Calendars", Rediscover Ancient Egypt. http://www.egypt-tehuti.org/articles/triple-calendar.html. Visited 9 April 2002 (No longer available).