"Jim Smith" wrote in message
...
So how do they arrive at those dates? anyone know?

When the Dog Star Sirius cannot be seen from our latitudes as it is not
above the horizon during hours of darkness (ie, in the Celestial Sphere, to
the south of the Sun)

It will become theoretically visible again in our morning skies before dawn
from about mid August (in practice, not until September probably). The mid
August date would apply to for example Egypt where they had active
astronomers many millennia ago

Jack