Now, the way they used to count time was based on the phases of the moon, which, regularly and predictably, goes through a cycle, starting with a new moon, then to a full moon, and back again to the new moon.
Now this cycle was then used to determine the length of their month.
So, um, one lunar cycle was one Egyptian month, and about four of the months would constitute a season.
Now, 12 of these months was an approximately 354-day year.
So they had a 354-day agricultural calendar that was designed to help them determine when the Nile would inundate the land.
Well, of course it had to be more complicated than that.
The average amount of time between floodings wasn't actually 354 days.
I mean, although it varies, the average was clearly longer than 354 days.
So how did they keep this short calendar in step with the actual flooding of the Nile?
Well, their astronomers had discovered that at a certain time of year the brightest star, Sirius, would disappear.
Actually, it'd be hidden in the glare of the Sun.
And then, a couple of months later, one morning in the eastern sky just before dawn, Sirius would reappear.
And it happened regularly, about every 365 days.
Even more significantly, the reappearance of Sirius would occur around the same time as the Nile's flooding.
And this annual event is called a heliacal rising.
The heliacal rising was a fair indicator of when the Nile would flood.