In ancient Egypt, the New Year began with the "heliacal rising" (appearing to rise just prior to the Sun) of the star Sirius. It was known as the "Nile Star", or the "Star of Isis" to the early Egyptians. When they saw it rising just before the Sun, they knew that the "Nile Days" were at hand. The star seemed to return just before the Nile River rose, and so announced the coming of floodwaters. This would add to the fertility to the adjacent lands, and people would open the canal gates to irrigate their fields.
Later, this period was called the "Dog Days" by ancient observers in the Mediterranean, who marked the period as the 20 days before to 20 days after the conjunction of Sirius and the Sun. As both stars rise around the same time during this period, some felt that the combination of the brightest luminary of the day (the Sun) and the brightest star of night (Sirius) was responsible for the extreme heat that is experienced during the middle of the summertime.The time of the Sun/Sirius conjunction varies with difference in latitude, and because of the precession of the equinoxes it changes gradually over long periods in all latitudes.