More than three thousand years ago, Ancient Egyptians wrote a calendar predicting good or bad luck for the days and weeks of a year. Archaeologists in the 20th century noticed several patterns when deciphering the document, known as the Cairo Calendar. Unfortunately they couldn’t work out whether the patterns meant anything. Recently, a team of scientists from Helsinki decided to revisit this ancient document to see what they could find.
The calendar was divided into 12 months, each with 30 days. Each day had three parts that were described as lucky or unlucky. The researchers were interested in these patterns of good and bad luck. After they had identified one pattern that lined up perfectly with each month, they noticed another one that took 29.5 days to repeat. This matched up with the orbit of the Moon – it’s about 29.5 days between full moons.
The researchers also noticed a third pattern of good luck that repeated every 2.85 days. They looked at every object visible in the Egyptian night sky, and found only one possible cause.
Algol is a star that’s visible in the Northern hemisphere. In 1783, astronomers noticed it regularly dimmed and then returned to full brightness. Algol was declared to be a pair of stars orbiting each other. When one star was in front of the other, it appeared to dim. Unfortunately, Algol dims once every 2.867 days, not every 2.85 days. The difference is only about 20 minutes each cycle, but Egyptian time-keeping and mathematics was so sharp, they should have noticed this over the course of a whole year.
New observations might hold the key to understanding this difference. It turns out Algol is not two stars – it’s actually three stars! Astronomers think one of the stars is losing gas to another one, and that should be slowing down their orbits. We haven’t seen any change over the past 200 years, but maybe this ancient Egyptian calendar is evidence of a slowdown over 3000 years!