Egyptian life revolved around the annual flooding of the Nile and from the middle of the third millennium BC the year was already being divided into three seasons – the flooding, the subsiding of the river, and the harvest – each normally of four lunar months duration.
The brightest star in the sky is Sirius. For part of the year the Sun is close to the star, which is hidden by the glare of the Sun. Then, one morning, Sirius reappears. By chance in Egypt this heliacal rising took place around the time when the Nile began to flood.
Twelve months of 29.5 days fell well short (by eleven days) of the average interval between one rising of the river and the next. If Sirius rose during the last eleven days of the twelfth month then one of the three seasons was given a fifth, intercalary month to ensure that Sirius would rise in the twelfth month of the following year.
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