A far more important slice of time is the year, a full circuit of the earth round the sun - crucial in human activities because of its influence on seasons and crops.
But the length of a year is exceptionally hard to measure.
This observation of Sirius also enables the Egyptians to become the first people to move from a lunar to a solar calendar.
Compared to anybody else's calendar at the time this is very satisfactory. The priests cannot have failed to notice that every four years Sirius appears one day later.
Familiarity may make the amazing events going on at ground level seem almost ordinary. Two great objects travel through it, one hot and constant, the other cold and changeable.
Plants and animals grow and die, rain falls, rivers flow. In the daytime it is moody; there may be blazing sun, or racing clouds, or darkness followed by thunder and lightning.
By the 1st century BC reform in Rome has become an evident necessity.
The existing calendar is a lunar one with extra months slipped in from to time in an attempt to adjust it.